As we all know and have experienced, the car world is a hive of bullshit and excuses, as people can never admit anything is their fault, so either blame the car or someone else.
From people blaming poor performance on a misfire when it's actually shit driving or them lying about how quick their car really is, to companies not honouring a warranty by making some kind of excuse to blame it on user-error- Excuses are a 'car thing'.

The inspiration for this mini-feature was hearing one of the most ridiculous excuses I've ever heard, and something that I'd not heard for about 15 years, and even back then it blew my mind someone supposedly respected had the balls to claim this...

Step back to 15 or so years ago, and the first example of this was about the Skyline GT-R RB26DETT engine, and most notably the ceramic turbine wheels the turbos have, which are known to snap off the shaft or shatter to many pieces.

While the above is an unfortunate and common fact, one of the big name tuners of the time used to convince their followers they needed an engine reubild as soon as this happened, as bits of the turbine wheel will have been 'sucked' in to the engine.

Some of you right now might be thinking "Well can it?", and some others are thinking "WTF?", so let's break it down in to a way even a tuning novice can understand...

• The flow of air, or indeed exhaust gas, at the turbine wheel, is going AWAY from the engine. And at full boost it will be a huge amount of airflow, and travelling at 100s of miles per hour. Next stop of the flow path is the exhaust pipe (in fact it's common to find the snapped off turbine wheels stuck in exhausts).
• The turbine wheel is spinning, likely at well over 100,000rpm at the point of fail, in the OPPOSITE direction to the only possible path to the engine (ie up the exhaust manifold, in to the ports, past the valves, and in to the engine).
• The pressure differential (which is a big part of what spins turbos) between the turbine inlet and turbine outlet is huge at time of fail. At least 20psi more on the engine side pushing outwards, pushing everything outwards, exhaust gas, turbine wheels, whatever. Bear in mind 2psi can knock down houses, 4psi can kill people, and 10psi can rip limbs clean off, it's safe to say that will be pushing broken bits of turbine wheel out the exhaust at a rate of knots, and could never allow it to travel through it towards the engine.
• At very least, the fragment(s) would need to do a full 360deg loop the loop through the turbine housing snail, through at least 1x 90deg bend in the manifold, and make it past a small exhaust valve opening and closing 50 times per second, all while somehow magically fighting against huge air speed and pressure travelling in exactly the opposite direction.
  
  Basically, that claim is BOLLOCKS. But countless people fell for it and had expensive rebuilds due to it, despite no documented proof, and all common sense saying it's totally fucking impossible.

So yeah, more utter shit.

The WORST thing about all this kind of crap is though, is that this isn't words of random idiots, the 2nd example wasn't even from a tuner making an excuse either- This is coming from RESPECTED names in the modified car world, 'FAMOUS' names some might say, and because they're "Somebody", people blindly believe them, despite it being insanely far fetched.

My question of it all is, do they BELEIVE what they're saying, or are they LYING? Either way it is REALLY fucking bad considering they're influential, aspirational, whatever the fuck you want to call them, names, in this stupid hobby we all have.

And this is exaclty why I'm hugely skeptical of what even 'Big' names say, partly by being untrustworthy, partly by not knowing as much as you think (or they'd like you to think) they'd know.

It's terrible really, being a skeptic or having trust issues isn't a good thing, and it'd be amazing if this tuning world was just people being honest and helping, but instead the kind of people and the kind of advice in the tuning scene is a big reason why so much of it is fucking awful and so slow progressing.
You have 2 choices, you can either listen to these people and be one of the crowd getting ripped off or having mediocre stuff, or you can take everything you hear with a pinch (ok, a fucking huge road gritter full) of salt, and fact check stuff from even supposed big and trustworthy names, which should, while it's a lot of work, get you a seriously good car.

"Oh Emm Gee, my car does 0-60 in less than 4.5sec, it's almost a supercar, your car is way slower, it's 0-60 is 6sec" says all kinds of clowns with pretty standard 4wd turbo cars, usually to people with cars with far more power; almost like they've never raced a car in their lives.

In fact 0-60mph sells cars- A whole lot of people seem to choose their car by its 0-60, which frankly, is retarded.

The fact is though, when the fuck is the official manufacturer 0-60mph figures relevant in the real world? Pretty much never, that's when.

Even if the situation arose, how many owners are able to hit the factory 0-60 figures? Not many, that's for sure, and for lots of reasons too.

First up, the majority of factory figures are done by GOOD drivers (and truth be told, most people can't drive for shit), after countless attempts, so a typical owner can't usually get within about a second of it. And I mean REAL times too, not a speedo and a stopwatch.

Secondly, how many owners do full on, and I mean REALLY full on, max revs, clutch dumping, launches, especially in transmission destroying 4wd cars? Again, next to none.
I've had hundreds, maybe thousands, of impromptu races off the lights over the years, and the amount of cars I've came up against who's actually done a full-on launch I can probably count on my hands- Most just fanny off the lights gently and then nail the throttle a second later; even highly tuned cars racing to big speeds.
People might like to think they're straight out of Street Outlaws, but more are more like Driving Miss Daisy.

Thirdly, and maybe most importantly in the UK at least- This ain't Fast And The Furious or Street Outlaws, so a VERY small % of races happen from a standing start; it's almost all roll racing, so your 0-60mph time means cock all.

I think some of it, the bit that's not total blinkers/pride in their own shit, is that some people really think 0-60 acceleration corresponds to acceleration at other speeds too, but it doesn't, at all. 0-60mph is ALL about the launch.

I've timed tons of cars over the years due to my work, and it's fair to say, aside from really, truly, fucking ballistic things that also have great startline traction, and that's rare, 0-60mph does not tally up with rolling start acceleration at all.

The old Dragon Performance FD RX7 drag car was a great example of this. Fuck yeah it launched well compared to most cars, but it still was often hampered by it's first 60ft or so, hence doing 160mph by the quarter mile, but still running low 9s, despite some 'slower' cars in the 8s despite only hitting about 150mph, due to being able to launch off the line like a bullet out of a gun.

In fact, I had some timing data from a run where due to wheelspin the 0-60 time was 3.5seconds, which is still ridiculously fast, but 1 whole second slower than the legendary Bugatti Veyron.
But DESPITE being 1sec slower to 60mph, it still got to 150mph two seconds FASTER than a Veyron- That means it did 60-150mph THREE FUCKING SECONDS faster than the 1000bhp 4wd mega bucks Bugatti hypercar.

So in the real world, ie a typical rolling start race, despite the 0-60 time being way way slower, the RX7 would absolutely fucking annihilate the Veyron, which illustrates my point nicely.

It's just as relevant with standard production cars, especially now with so many hot hatches having 4wd and twin clutch DSG boxes; both MASSIVE advantages on the 0-60mph sprint.
Lately all I see is people wanking off about the new Golf R and A45 AMG with their 4wd and twin clutch boxes, and saying they're sooooooo much faster than, say, the new Civic Type-R for example, purely going from the 0-60 times and various "Race" videos on the internet from the likes of TopGear etc.
Fuck yeah they're much faster to 60 from a standing start, as the Civic just wheelspins when launching hard, and like most manual cars, it loses about 0.5sec on the 1st-2nd gearchange, compared to almost no time for a DSG box. BUT...

BUT... Look at the times at the end of the video, the 0-60 and 0-100mph figures they achieved, and then subtract the 0-60 time from the 0-100mph time, to calculate the real world, and relevant, 60-100mph time...
The Civic is actually 0.1sec FASTER from a rolling start, and that's despite the advantage of the DSG box in the Golf.

So there you go, unless you REALLY do standing start racing, and REALLY do zero mechanical sympathy hardcore transmission destroying launches, 0-60mph means fuck all, so if you're gonna do anything with it, use it to subtract from your cars 0-100mph time to see a real world acceleration figure.

Overall... STOP FUCKING QUOTING 0-60 TIMES! YOU SOUND LIKE A MORON!

I love Ford Cosworths, I really do, and a BIG part of the reason they're so good is they were designed from the outset to be Group A homologation specials. Group A rules at the time (ie mid 80s) dictated the standard block, head casting, crank, rods, exhaust manifold, inlet manifold, intercooler, and general turbo size, all have to be standard. And because of that, if you want to be successful in Group A, the parts fitted to the standard production car needed to be capable of pushing some serious power for full race and rally distances without issue, so Cosworth designed the engine for Ford to be VERY strong and tunable.
​
ANYHOW, while I don't think it's ever been officially documented/admitted, being in this industry for so long means I've got to speak to quite a few people 'off the record', and there's one odd thing about the 2wd/RS500 Cosworth engine, that would've been a lot better if Ford would've let Cosworth have their own way, and here's the story...

Basically, Cosworth designed probably the best standard turbo manifold any production car has ever had- A strong, long runner, and fully twin scroll (2+3, 1+4) T3 flange exhaust manifold, proven capable of 600bhp+
But, bizarrely, the turbos fitted to it, the Garrett T3 on the Sierras and 2wd Saffs, and the Garrett T3/4 the RS500 had, are all single scroll, making the fact it's twin scroll, and the huge turbo spooling advantage that brings, totally and utterly pointless.

I'd always wondered why this was the case, it's bizarre. Twin scroll was pretty much unheard of on production cars back then, and would never deliberately had a single scroll turbo fitted, as the entire operation would be a huge waste of time and money.
While it's no issue with the little T3, the RS500 turbo, while good for 550bhp+, isn't exactly a responsive turbo, with good power starting from around 4500rpm upwards, and a twin scroll setup would've been a HUGE advantage- Even a much larger and more powerful turbo would've spooled up faster in twin scroll, making the RS500 even more dominant in motorsport than it was.

So why was a laggy single scroll turbo fitted then, despite a beautifully designed twin scroll manifold being on the engine? Well, company politics, that's why...

Basically, Cosworth had a VERY successful partnership with UK turbo legends Holset, who produced the turbochargers for the other turbocharged race engines Cosworth ran at the time- The 2.6ltr Indy Car lumps used by most the top Indy cars of the 1980s and 1990s- Including the 1987 Indy 500 winning car actually sponsored by Cummins and Holset (Cummins own Holset)...

The story goes that Cosworth fully intended, indeed full developed, as per their other race engines, to use top-spec Holset turbos. Unfortunately, Ford Europe had an agreement with Garrett to use their turbos on all the production cars, so overruled Cosworth and chucked on something 'suitable' from Garrett- The powerful, but unresponsive, single scroll 'RS500 T4'. This, below, is what it could/should have been...

The RS500 Garrett T4 is legendarily "Laggy", in fact you won't see a single article or film that doesn't mention it; even the racers all talk about anticipating the lag and planting their foot down early so the engine over-comes the lag just at the right moment, but thanks to people tuning Cosworth engines in the many years since, it's now well proven going from the twin scroll Holset plan to a single scroll Garrett was a MASSIVE MISTAKE...

Who originally said this shit anyhow? I've seen it attributed to Carrol Shelby, Enzo Ferrari, and Carrol Smith, but did any of them really say it? And if they did, it must have been taken massively out of contact, as frankly, it's a load of rubbish.
Fuck it, as there seems to be no proof who said it, I'm starting a new rumour of who made it up, maybe that'd stop people quoting this shit...

You know though, I'm sure it's a mis-quote, and I think I know where it comes from...

Imagine you had a car with BIG bhp, but a tiny peaky powerband, and a gearbox without close ratios. So every time you changed up a gear, you dropped out the powerband, killing your acceleration. So despite the big peak power number, a lesser tuned car, which was less peaky so didn't drop out the powerband, and no doubt had more low/mid torque, would actually be faster; especially on tighter tracks.
In the old days, where this quote seems to originate from, this would be even more of an issue, especially with heavy "muscle" cars running 4, 3, or even 2 speed gearboxes, which means powerful but peaky cars would easily drop out the powerband and be shit slow compared to torquey but low power cars.
But these days, where you tend to have to either totally fuck up the spec to make it mega peaky, and even peaky engines have closely spaced 5/6/7/8 speed gearboxes, the likelyhood of even a road car being driven properly falling out the powerband is slim, and literally no chance on a well built race car.
That's my theory anyhow- This quote is out-dated and taken out of context.

Anyhow, this quote, almost always used by butthurt turbo diesel owners, low power V6 and V8 owners, and anyone arguing with a Honda owners, just makes people who are trying to be clever just sound clueless, like they've never had a proper race or even driven a performance car in their lives. I mean seriously? One tear up with a lower torque but higher power car would tell them their argument is pretty retarded.

When most people talk about torque, what they really mean is low/midrange grunt, and while it's great (I LOVE torque, it makes cars much more fun to drive, in fact it's why I love big boost turbo engines), and it's certainly something any performance engine, road or race, should aim for the maximum of, providing they've got the traction to make use of it, it's not what wins races, power is.

BHP is just Torque and Revs combined. It's a tricky one to explain in words, it's far easier to experience in reality by driving various cars, but torque is the "Strength" of your engine at a set rpm, but the faster your engine is spinning the bigger effect torque has on performance, so torque+rpm = POWER, which is key to how fast your engine can accelerate (providing it can stay in the powerband though the gears, that is!).

First up on the explanation list, a BMW F1 engine dyno graph (allegedly) from the a 2007 2.4ltr F1 engine...

So, what do we have here? Well, it's about 740bhp, and a pretty tiny 230lb/ft of torque.
So 6lb/ft less than a Golf TDI. And with about 150lb/ft at 6500rpm, it's got about the same torque at that RPM as an EP Civic Type-R, which frankly, isn't a whole lot- People make fun of Type-Rs for being torqueless, but F1 engines are no better.
Next up, a standard BMW 335D, 3ltr twin turbo diesel engine...

The above dyno is at the wheels, so I'm going to add about 30bhp/30lbft to these numbers, which about tallies up with the official 282bhp and 428lb/ft figures.
Either way, say peak torque is 428lb/ft, that's 200lb/ft more than a Formula Fucking One engine. And a remapped one is something upwards of 500lb/ft if I remember right, over DOUBLE what one of the modern V8 era F1 engines were.

Does that mean Formula 1 teams are stupid and they should've just used a 335D engine with a £150 plug in remap and be shitloads faster? Fuck no, as only retards think that.

Fuck, even comparing like for like cars, 335i vs 335d, and the 335i is plenty faster, despite "Only" having 20bhp more than the diesel, and about 130lb/ft LESS. Basically, it's not true.

The above picture is about the predictable thing every "Torque wins races" person says next... "But, but, but, WRC cars only have 300bhp but like 700lbt/ft, and they're mega fast, they win races" YES, but they've not got '300bhp' by choice- That's all they can manage because the rules dictated an inlet restrictor to prevent power going higher.
With no option of more power, no issues with traction due to 4wd and sticky tyres, AND a style of motorsport that involves a lot of very slow corners where instant momentary punch of acceleration is a big advantage, you'd be out of your mind not to go for maximum torque to go with the limited bhp you're stuck with.
But for next year the rules in WRC are changing once again, inlet restrictors bigger than ever, allowing more than 400bhp if I remember right. So will they still go for maximum possible power, if it's "Torque that wins races"? Fuck yes they will, as it's power that's the no1 performance enhancer.

Another argument by these people are "So, if torque isn't important, why do drag cars run big V8s?". Err, mostly as they're the easiest to get POWER from you clowns. The above car is Larry Larson's S10 10ltr twin turbo V8 pickup truck. Yeah it's got fucking shitloads of torque, but the reason it runs 5sec quarters at over 240mph is because the bloody thing has upwards of 3000bhp!
Torque is a BAD thing for him, and because of this he deliberately launches at just 8psi of boost, as at full boost it'd make so much torque it'd just smoke the tyres. In fact it's only ramped up to full boost (50psi!) by most the way down the track and he's already doing over 180mph! "Low RPM V8 Grunt" is literally no issue on fast drag cars either- Larry's car never sees under 8000rpm for the whole run after 1st gear!
So once again, power wins races, and in fact, torque can slow you down if you've got more than your tyres can handle...

Why do so many really, really, fast drag cars run centrifugal superchargers, despite turbos giving far more power, and positive displacement superchargers give far more torque? Because turbos and positive displacement superchargers give TOO MUCH torque for the grip, meaning too much wheelspin for any given power level, making them slower overall. Centrifugal chargers increase boost linearly with rpm, giving LOTS of power, almost as much as turbos, but adding only a little extra torque, especially at low and midrange rpm, so they are FAST due to the big power, but still don't wheelspin due to the lower torque.

It's the same reason a lot of the fastest FWD track cars run centrifugal chargers too- FWD is grip limited, and they need big power to "win races", but don't want too much torque or it overpowers the tyres, and thanks to the power delivery of the centrifugal charger they can use more throttle more of the time without it being wasted in wheelspin.

It's the same reason many production turbo cars, and most the fastest turbo race cars, run lower boost in lower gears and lower rpm, compared to in higher revs and rpm- To limit torque so they've got more traction, making them far faster overall.

It's also why naturally aspirated 2wd rally cars are often as fast, or faster, than the 4wd turbo rally cars, on grippy dry tarmac surfaces (where 4wd was no advantage), despite having the same power and often well over 400lb/ft LESS torque- Because its POWER that's the number one performance enhancer, not torque. This happened a lot in the late 90s, with the 4wd turbo WRC cars vs the N/A FWD F2 Maxi Kit cars...

Fuck no! Torque is awesome! Generally, I fucking hate cars with no torque! Surely, if you've any experience of driving at all, you know torque is great, but despite all this, it's not the key need for pure performance.
Low/midrange torque makes for a nicer, easier, car to drive, especially in slow/fast/slow/fast/slow driving, saving you constantly rowing the gearbox to stay at high rpm where the power is. It's why things like Type-R Hondas need driving hard and dropping gears constantly to be fast, and a Golf TDI just needs the throttle planting in almost any gear to go fast- But ultimately, the Honda, with more power, despite loads less torque, is the fastest if both are driven on the limit.
I drive like a dick and love drifting, so LOTS of torque to smoke the tyres is great for me too. I actually find a lot of drift car setups bizarre, as they ideally want torque but the engine isn't tuned/specced for it.
And regardless of drifting, I like BIG boost, as boost = torque, and torque = fun to drive, BUT if I wanted to go as fast as possible, while I'd want the MOST torque my tyres could handle- I'd not want more than that, as it'd be pointless.

So overall...
TORQUE = GOOD.
TORQUE IF YOU GOT THE GRIP FOR IT = FAST
BUT FAST? = POWERRRRRRRR

RWhen it comes to modifications of turbocharged engines, one of the first things that come to mind with a lot of people are blow off valves, or dump valves, depending what you like to call them (same thing!).
They've been around since at least the 1970s on race cars (though pretty much disappeared on turbocharged works race cars after the early/mid 90s...), factory fit parts on most turbocharged petrol engines from the mid 80s onwards, and from around the mid-90s onwards aftermarket ones became THE thing to have- It was, and to some extent still is, the first engine mod people do to a turbo car.
But what DO they do? What are they REALLY for? Well this is where the confusion/bullshit comes in, not only thanks to the internet, but thanks to the fact most the things said about them to sell them, in the past at least, were lies too.
Christ, the "Blowoff Valve" Wikipedia page even talks total and utter shit about what they do, and why, and how, and whatnot...

THINGS THAT PEOPLE SAY DUMP VALVES DO...

"Reduce turbo lag"
No. They increase it if anything. Turbo shaft speed drops far more with a BOV fitted than without. That's their true purpose, to prevent surge and overspeeding when you shut the throttle. Honestly, anyone with a turbo speed sensor will have seen this, and I've done tests with timing equipment a few times now, that shows, even if it's not noticeable seat of the pants, it makes it worse, not better. This IS fractional though, don't expect a big, or even noticeable difference on most engines- Really depends on the application.
​
"Prevent your compressor wheel from slowing/stalling (or going backwards)"
No, they actually SLOW a turbo down, that's their actual job, they're a safety thing, to prevent overspeed and surge on a closed throttle, which in some extreme situations can damage the turbo. Honestly, watch a turbo with a turbo speed sensor fitted, the RPM drops far far more with a BOV than without. The 'spinning backward' thing people say is fucking retarded btw.
It's the AIRFLOW that stalls, not the turbo, something I'll explain further down...
​
"Help prevent compressor surge"
Yep, they do this, but only off-throttle compressor surge, which isn't that damaging or sustained to turbos, as there's no load on the turbine off-throttle. It's on-throttle surge that can be hugely damaging, and it does nothing to help that. "Certain" turbo manufacturers and their badly mis-matched compressor/turbine combos are the main cause of on-throttle surge...

"Help stop turbo damage due to (whatever)"
Well yes, this is the real reason they're fitted, and can be useful for this, but their need is often hugely over-stated, and how they help prevent it is ass-backwards too. The usual bullshit myth is they help prevent turbos stalling, and the sudden slowing of the turbo is what damages them. NO. They help SLOW the turbo, as without them turbos can overspeed when the throttle is shut, and surge all over the shop, and in certain applications that can damage the turbo.

"Help MAF sensor equipped cars run right (recirc ones ONLY)"
While it's indeed true many engines with MAF sensors run like shit off-throttle if you fit a vent to atmo dump valve, the common info that you HAVE to run a recirc with a MAF is bollocks too in everything I've ever experienced.
Running no BOVs at all is absolutely fine almost always.
​It's possible there's a car where the MAF sensor does shit itself, but so far I've not found it. I've heard a whole lot of rumours, I was told by countless people without a shadow of a doubt it'd cause the 2.7TT (ie S4) Audi engine to run like shit and so on if removed, but I tried it, and it's fine- Turned out, as ever, none of these people had ever tried, just 'heard' it did.
EDIT- Now I think back, I do recall a time when it made the MAF go crazy for a second, but that was a BIG aftermarket turbo on BIG boost on a small engine, and that was the only time it happened. Generally, no issue at all.

"Makes that cool chatter noise"
I know most of you know better, but it's rare to see a video of a car with decent turbo chatter without at least one comment saying it's the BOV making that noise, or asking what BOV it is as they want one that makes that noise.
IT'S NOT! HAVING NO BOV AT ALL IS WHAT HELPS MAKES THE NOISE!
Fuck, when I was 18 (1998!) even I at first thought the same, as MAGAZINES TOLD ME SO. In fact these "expert magazines" told me that it was HKS SQV valves did that noise, and though they were about £350 back then, and I wanted that noise SO BAD I would've paid that.
In fact, after a tuned MR2 Turbo came past and chattered like fuck one day I rang Torque Developments (they were the only HKS dealer back then) to order one. THANKFULLY, the sales persons tone of voice and choice of words made me very suspicious, while also accidentally giving me some hints that helped me research what REALLY made the noise (No internet back then!), so I thankfully didn't get skanked out of £350...


THINGS DUMP VALVES DO THAT PEOPLE DON'T TALK ABOUT...

"Help suppress noise"
The main reason they're fitted to OEM vehicles is as noise suppression devices, in fact that's the exact name they're given in many official workshop manuals. The noise I mean is turbo chatter, which is a lot harder to silence with an airbox than the ptschhh of a dump valve. We might like chatter noises and induction noises, but your average Joe new car buyer doesn't. In fact back in the day I worked at a dealership and an Impreza STi we had in on a P/X that we then sold was returned to us by the customer due to "Funny noises" it was making, as it had slight chatter as it had an induction kit on it.

"Help slow the car when in limp home mode"
A lot of new cars also use a system that holds the BOV(s) open, preventing boost/power being made in case of an issue that puts the car in limp home mode. Useful as an OEM.

"Leak like bastards, causing underboosting and turbo wear"
I know they're not meant to do this, but this is a major and common issue, and often impossible to detect unless looking for it. A dump valve is another potential leak point, and split diaphragms and/or weak springs means they leak surprisingly often, especially OEM and cheapo ones, and even if your turbo is still making full boost, a leak can cause a slower boost rise, and a turbo (and therefore entire engine) working far harder.

Confused? Don't be. I'll clear your mind (a little) in this next bit...

Ok, here goes. First, let me introduce you to Mr Compressor Map, with some basic annotations via the amazing medium of MS Paint...

As you may or may not know, the horisontal axis of the map is airflow in lb/min, the vertical is pressure in bar (1.00 being atmo, anything above being 'boost'), and anything within those lines drawn on the map is the 'safe' area that turbo is happy enough to run in constantly, in this case, up to about 2.4bar of boost and 49lb/min.

The three dots I've added show three different situations.

ON BOOST- 1.7bar, 42lb/min airflow, well within the map, it's loving life.

SUDDEN CLOSED THROTTLE WITH A BOV- BOV opens, so pre-throttle pressure rapidly drops to zero, while an open BOV means airflow is less but exists, and therefore the compressor drops to a very low speed point on the map. It's still safe and happy.
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SUDDEN CLOSED THROTTLE WITH NO BOV- Throttle closes, so airflow drops to practically zero, while pre-throttle pressure momentarily spikes, putting the compressor in to somewhere on the top left of the comp map (ie lots of boost pressure, but fuck all airflow), well in the surge zone, which, if there were speed lines shown, would be a shitload of rpm too.

Ok, this is where the THEORY vs REALITY thing comes in to tuned cars yet again.

There is NO DOUBT the sole use of a vent to atmo BOV is to help prevent turbo damage via compressor surge and overspeed, as that deffo, deffo, does no good for performance.

In THEORY then, running one is a very wise move indeed.

But in REALITY, how many people genuinely have issues when running without one, even with big boost and hard use over long periods? Well, not fucking many at all.

The issue people seem to forget is severe surge and overspeed happens for a TINY amount of time on a turbocharged engine when you close the throttle.
It's not like a belt driven supercharger, or a gas turbine, or when surge happens ON THROTTLE. That is fucking continuous until the power is shut down, as the compressor is being FORCED to operate at that condition- THAT can be bloody disastrous in some applications as there's huge forces turning it regardless, hammering the shit out of the compressor.
But when you close the throttle on a turbo engine you are stopping power to the engine, and in turn stopping practically all power to the turbine wheel, which is what turns your compressor, so once the slight momentum from being off throttle is gone, there's no danger.

In all honesty, in my 18 odd years of messing about with turbo cars, working around and with turbo cars, testing and timing turbo cars, and reading every worthwhile turbo-related thing on the internet, I've personally seen TWO instances I think 'probably' were no-BOV related turbo fails (both on the exact same type of (uncommon) turbo with a known weak shaft, both at over 30psi), and heard of a few others on huge turbo mega boost V8 drag cars where the turbo failed at the end of the run just after they got off the throttle.

All the other times I've seen turbo fails, it's been other issues entirely. And that's thing, though a turbo CAN fail due to this, it's usually broken due to some other reason long beforehand!

On my own personal cars, even running 1.5-2bar boost for long periods on loads of cars over the years with no BOV I've had no issues, and know countless others doing the same. Christ, I've not ran one on anything since the year 2000 now I think back. And by then I'd experienced plenty of split/leaking BOVs, and have since on other cars too.

Ignoring my own experiences, and the fact I love the sound of turbo chatter, let's look at things logically...

Instances of people talking about their turbos failing that's honestly likely attributed to no BOV- VERY FEW.

Instances of people with fuck all 1st hand experience of the subject telling you to run one for whatever reason- FUCKING LOADS.

% of people with turbo fails that do run BOVs and dont run BOVs- About the same, as frankly, turbos tend to fail for various other reasons before the fraction of a second of surge and overspeed would kill it.

% of turbocharged works race cars since the early '90s that run BOVs- Very very few. Even in the 80s and early 90s they were rarely seen on a lot of applications. If they could improve the odds of ending the race, even slightly, and not cause other, much more serious, downsides, would they run them? Of course they would. Having said this, they use some good, strong, expensive, turbos on works race engines...

Neither, there's no simple answer- I'm saying read what I've told you and make your own mind up- I'm not taking responsibility for your decisions.

For me, personally, no, I never run one. Ran tons of cars at mega boost, hard used, all kinds of turbos, for years, never and issue. Loads of mates do the same, even on fancy big bucks ball bearing turbos, no issue. 
My old R32 Skyline I built, what, 4+ years ago? 1.5bar+ boost, 470bhp, from a journal bearing Holset-based turbo I bought from Compressor Racing (their RS341 model- Like a slightly improved HY35). No dump valve fitted of course, used hard as fuck by me, and hard as fuck by the owners since, track use, drift use, top speed, all sorts- Turbo is still going strong.

Does this make it gospel though? No.

Would I still not run one if I happened to own a turbo with a KNOWN issue of shaft snapping or wheels exploding with not running one? No, I'm not fucking stupid.

Would I risk not running one on some crazy expensive turbo I couldn't afford to fix? No, as I'd not buy one in the first place. Should you? Well, read the above, do your own research, and make your own choice...

Remember though- Only listen to people with NO motive either way (ie they don't sell turbos or BOVs), only listen to people with proven experience on the matter, and remember, someone saying "Well I've got a BOV and my turbo hasn't ever broken" is like me saying I've got a magic stone that protects me from Gorillas and that's why I was never attacked by Harambe...

Stav

Rear mount turbos are pretty new to the tuning scene as 'tuning' goes, having only becoming well known in the last decade or so, and even now are relatively unheard of in the UK. Therefore almost every time they're spotted they're always treated with amazement, both in good and bad ways...
​
The problem is, people's opinions of them tend to be polar opposites of each other.

MOST people, due to it being totally against everything they know, instantly say it will be laggy as fuck, won't spool, will be shit, and so on and so forth.
They're wrong if it's done right.

On the other hand, you got people, again have no actual experience of it, but love the idea and have seen a few on the internet, who tell you rear mounts are amazing, the best way to turbo, as good as a front mount, and so on.
They're also wrong.

Basically, rear mounts are neither shit or amazing, though they're perfectly possible to be shit if done incorrectly, just like a front mount turbo can be. What they are is simply another option if a front mount isn't viable for some reason, and done right can work fantastically well, often far better than most front mount setups, which are often sub-optimal, even when a 'pro' has designed them...

If ALL ELSE WAS EQUAL, a front mounted turbocharger IS BETTER, no iff's or but's, it is. But things never are equal with turbocharging, and there's a multitude of reasons a rear mount setup is useful in some applications, and if done right, they can perform as good or better than a typical front mount setup.

The E55 AMG rear mount I helped build for Compressor Racing was a perfect example of the above. Just like a whole lot of cars, there's next to no room in the bay for a turbo or two, so even IF it's possible, you'll likely be spending a few grand on custom manifolds, even before anything else. And then you've still got to find room for the intercooler, and then deal with the heat management issues for all of that too.
The only other option for the E55 is a supercharger conversion, that costs at least £7000 plus fitting, and makes about 500bhp at 9psi boost, about the limit of the stock (albeit modified) ECU, as we found ourselves.
The E55 rear mount turbo we built was on full boost by 3000rpm (despite a turbo capable of 1000bhp!), and the only time we put it on the dyno it made 465lb/ft at just 4.5psi and due to boost control issues we were only at 2psi by peak power but it still made 440bhp- About 90bhp more than standard.
In the end we sorted out the fuel and boost control issues and got it safely at 9psi. It was never dyno'd at this level, but it WAS timed with Racelogic timing equipment, the proper pro stuff magazines and companies use, not some crappy app or a stopwatch, and the results, despite horrendous wheelspin, were pretty impressive to say the least...
60-100mph time was 4.4seconds at best (backed up by a number of 4.5s runs).
To put that in to perspective, the later, best part of 500bhp, factory supercharged version of the E55 AMG does 60-100mph in 5.5seconds, over a second slower, and even a supercar like the Ferrari F360 is 4.6sec!
And it even made a pretty fun drift car, despite being a huge, heavy, rusty old boat, as these pics show...

And how much did the full rear mount conversion on the E55 cost to do, inc all the non-turbo related bits, such as the uprated fueling parts and so on? Under £2500. 
You'd not even get a pair of custom manifolds made for a front mounted turbo setup on the same car made for that, and even if you did, will be spool a 1000bhp capable turbo much sooner than 3000rpm? Would you even want it to? Probably not for both accounts, and it'd be highly unlikely to make much more power at the same boost either.

While it's MOSTLY pure lack of room that makes a rear mount particularly tempting, it's also been done, even with very high end race cars, for weight distribution reasons, and the simple fact that, done right, it isn't a big disadvantage...

So does this mean we should ALL have rear mount turbo cars? FUCK NO! If you've the room to easily do a front mount turbo setup that's not a complete fucking bodge or cost an absolute fortune, you may as well just do that. As said already, if all things are equal, it will spool faster and make more power than a rear mount. It might not spool loads faster and make loads more power, but still, really no point for a rear mount unless it makes things a lot easier for whatever reason.

BUT as it often does make life easier, I'm sure most are you are thinking "Ok, but say if I do want to go rear mount, how the FUCK do I do it so it's not the laggy mess the naysayers say? How DO I make it as close to as good as a front mount as possible?"
Well, I better tell you what I know and have experienced...

MANIFOLDS- Use standard cast log ones, or short runner tubular ones at very most. For fastest spool you want to minimise exhaust length and maximise heat retention, and regardless of turbo position, long runner exhaust manifolds have very little effect on peak power on turbo cars anyhow.

PRE-TURBO EXHAUST DIAMETER- This is one so many people get massively wrong, and hugely over-do. Treat all your pre-turbo exhaust like you would your manifold on a front mount system, so with a T3 flange and even 600bhp+ a single 2.5in pipe is more than enough, and a single 2in or 2.25in is plenty for lesser power cars. Even with big V8s running over 140mph in the quarter mile, a single 2.5in pipe leading to a T4 flange turbo is well proven. Overly big exhaust diameter will drastically slow spool, and is the biggest killer of performance on rear mount setups.

PRE-TURBO EXHAUST LENGTH- To minimise heat loss, shorter is better, so if you can actually have a mid-mount turbo rather than rear, even better. Just behind the front seats? Instead of passenger seat? It's all been done...

PRE-TURBO CATS- I've not experienced this one first hand, but while basic theory says they'd be bad for power and spool, they're fitted to a great number of very powerful but emissions legal rear mount cars. It's possible the reason they're not the spool killer you might imagine is because post-cat exhaust temp is incredibly high (it's part of their job), which will help maximise exhaust energy and heat at the turbine. But I've never tested this so that's speculation...

BOOST PIPE LENGTH- As discussed in THIS feature, boost pipe length isn't actually a big deal at all. Don't make it longer than it needs to be, but of all the things to fuck up spool on a rear mount, this is a minor one.
One thing you might want to take in to account, though on smaller engine applications (or just using bigger boost pipes) it's likely not an issue, is the instant off-boost punch at very low rpm or just as you open the throttle. On small capacity cars there's not a lot unless there's boost anyhow, but on the AMG it had a huge V8, and once it was rear mount turbo, sucking through inlet pipes 10 times as long but about half the diameter of standard, it lost a little of this punch. You'd not notice unless you drove it back to back, and even then I only really noticed as I wanted maximum punch for drifting, but it was there.
I had a solution though, I fitted this flapper valve taken from a Volvo boat engine, allowing the engine an alternative way to breath in N/A mode...

INTERCOOLING- Yes, compressor outlet temp is lower in a rear mount. Yes, the long boost piping allows for more heat loss. But regardless, the inlet temps aren't that much lower, so if you've got room at all, you'd be mad not to fit an intercooler or chargecooler.

POST TURBO EXHAUST- As explained in THIS feature, bigger is better, and with rear mounts needing all the help they can get, as big, and as short, a post turbine exhaust, the better. Even with a 2.5in pre-turbo exhaust, noticeably faster spool has been achieved by going from a 3in to 4in post-turbo exhaust. As a rule of thumb, your exhaust diameter should be AT LEAST 1.5 times your turbine wheel exducer diameter for best spool...

HEAT RETENTION- This is a big part of spool for any turbo, and a HUGE issue on rear mounts. Basically, the hotter the exhaust gas is by the time it gets to the turbo, the better your spool is. So heat wrap ALL of the exhaust, front to rear, double layered if you can, and of course, put a turbine blanket around the turbo too. The difference between heat wrapped and not is very noticeable.

TURBO POSITION- Under the car is most common, and nothing wrong with that at all, as long as ground clearance isn't an issue. On the Merc we went boot mounted, which personally I prefer for packaging reasons. When we did so we got no fucking end of people saying how can it breath in there, we need to cut holes to let air in, etc etc etc.
Nah, not really.
Boots are FAR from air tight, thousands of small holes and gaps which overall make up to far bigger than a typical 3-4in inlet of an airbox, and the boot itself works as a giant airbox.
No matter how many times we said this, people still said it, so in the end we drilled some holes in the boot to see if it made any difference at all.
Nope, not a fucking sausage, exactly as expected.

​WASTEGATE POSITION- The "Normal" rear mount position is right by the turbine inlet, but IMO they've decided that with zero testing and just their guess. IMO it's wrong. For me, the closer to the engine the better. The WW2 planes and the top drag cars have the wastegates just after the manifold collector. And I'd be more inclined to listen to war winning aircraft and cars that run 190mph in the quarter than some random other person who's probably not tested either way.

OIL FEED AND RETURN- While you certainly can have a stand-alone oil system for a rear mount turbo, it seemed like added complication for little benefit to me, so on the ones I've had dealings with, we've always just fed the engine oil the traditional way- An oil feed pipe direct from the engine.
Oil return is a bit different though. You NEED an electric scavenge pump so suck and pump the oil back to the engine, it won't go on it's own, you'll just end up with a smokey fucked mess. One bonus is, you don't have to return to the sump like on a conventional gravity oil return, anywhere will do- On the E55 we returned to one of the cam covers! 

TURBO SPEC- This is the biggie. Compressor side, well, it should be the same as you'd choose front mounted for the power you want. Turbine, well, smaller improves spool, but also increases backpressure which kills power, and personally I'd not be going much smaller than I would front mounted, maybe not any, and just concentrate on optimising the setup as per above. Just be conservative on turbine size, sensible, as you can't get away with overkill as easy as front, but the exhaust gas still needs to come out, and having a tiny turbine to try and get good spool is why so many rear mounts make shit power for their boost level.
Of course, there's OTHER options to allow a massive turbo without the risk of killing spool.
On the E55 AMG we did for Compressor Racing we used a variable geometry turbine turbo, a Holset HX55V VGT unit in fact, a flippin' huge 1000bhp capable beast. VGT turbos, while not the magic solution to all our woes, have the ability to spool like a small turbine, and flow like a large turbine, so are very useful for rear mounts.
The issue is big VGT turbos are rare as hell in the UK in good condition, and the poxy little ones used on diesel cars are useless, so it's not an easy solution.
Another idea is twin rear mount turbos, compound style, one small for spool, one big for power. I'm currently building a rear mount setup like this now.
Finally, and one I've never seen tested, but I'd like to try one day, is the effectiveness of twin scroll. I don't think it will be a magic solution, but for cars with a very easily done twin scroll setup, mostly inline sixes and twin rotors, I'd be inclined to run two small pipes to the turbo, one to each scroll...

AND THAT'S THE BASICS FOR YOU...
There's a whole lot of bullshit, incorrect 'facts', guesses, and downright lies about rear mounts, but hopefully what I've told you will set a few things straight, and to be honest is plenty enough, if you choose to follow my advice, for a pretty well performing rear mount setup.
BUT DON'T FORGET! Don't do it if there's no good reason to...

OH! Forgot to mention something that's actually a BIG plus point, as childish as it may be... IT SOUNDS FUCKING MENTAL. All you can hear is turbo spooling up and down, super loud chatter and spool and everything else. Sounds like your car has a jet engine rather than a normal engine, which is amusing. Click HERE to see a short vid of the AMG, inside and out, to see what I mean...

​And of course, it looks cool too...

This is the never ending myth that comes up almost every time someone asks about fitting big intercoolers on forums and so on. You can be assured a knowitall, who no doubt has zero experience of it or he'd not say it in the first place, will tell you long boost pipes and/or massive intercoolers will make your car laggy, especially if you have a small turbo.

Well, let me say, that's TOTAL BOLLOCKS.

In THEORY, and it is fucking basic, primary school, narrow minded, short sighted, theory, the longer the boost pipes, and the bigger the intercooler, the longer it takes the 'boost' to travel from the turbo to the engine, and the longer it takes the 'boost' to pressurise it all before it pressurised the engine.

In REALITY, airflow out the turbo travels at fucking HUNDREDS of miles per hour, and airflow in to the engine consists of HUNDREDS of litres per fucking second, so how in God's name do you really think adding even 10ft and 10litres (and that's massively unlikely) to the pre-throttle intake length/volume will make a car laggy is beyond me.

In reality, and I've tried it over the years with really bloody extreme examples, with poxy little turbos and ridiculously big intercoolers and pipe lengths, it never ever feels 'laggier' or 'less responsive' or 'slower boosting' or any other way people want to say it, and usually (Imprezas are a great example as they have insanely long intercooler pipe lengths and massive front mounts) they actually are noticeably MORE responsive and have MORE low down power than before, as low intake temps are far more effective for performance increases than any theoretical response improvements.

Almost all top mount setups, and most v-mount setups too, usually have pretty horrendous inlet temps compared to a decent front mount, and with no real response improvements, giving up lower inlet temps just to shorten boost piping is a terrible plan in reality, which brings me neatly on to this car...

What we have here is an MR2, with a mid mounted 2ltr (2.1 maybe, can't remember and it doesn't matter) 4cyl engine. But the turbo is REAR mounted, a good couple of feet from the engine. That alone is enough to make Pistonheads forum nobheads cry 'LAAAAAAG' in some dweeby middle class voice. BUT hang on, where's the intercooler? Well, it's at the FRONT! Yes, this mid engine car, with a rear mount turbo, has a front mounted intercooler, and at least twenty foot of boost pipes connecting it all up!
Seriously, I've literally NO doubt if this car was created by some random lad in this day and age, not only would it get fucking slaughtered on whatever forum he made the mistake of posting it on, but it'd end up going viral on Facebook etc etc as the hordes of internet 'experts' ripped the absolute shit out of it for being a ridiculous stupid idea, despite none of them having any experience at all with anything even close to it.
I can picture the unfunny memes about it now...

I'm sure, even now, even after everything I've wrote, a lot of people are looking at the pics above and thinking "Fuck me, that must be horribly laggy and useless".
Well, no, no it's not, in fact the above car won the GT300 JGTC (Ie Japanese Touring/GT championship) in 1999, vs RX7s, S15 Silvias, Porsche 911s, Imprezas, and more. 
I'm not 100% as I've not found enough detail to be sure, but the same setup in different livery might have won the 1998 championship too (MR2s with the same drivers did), and I think the 2002 GT300 winning MR-S (below) looks like it might've been a similar setup too, but can't find enough pics to be 100% sure...

Turbos are amazing things, they're a big reason the vast majority of the worlds fastest cars in almost every discipline are as good as they are, and for us tuning obsessed idiots it means we can go a whole shitload fastest for our money than we could if they didn't exist.
BUT, and this is something a lot of people either underestimate or seem to be totally oblivious to- They're not fucking magic! You can't just throw a turbo at a pile of crap and expect it to work as well as proven results elsewhere, you need to do things properly...

Some people are just plain stupid, and I mean the ones who wonder why they've not instantly got the same 800bhp another engine with the same turbo has, despite the other engine being of a totally different spec, running twice the boost pressure as them, or not even being the same fucking engine. But forgetting people who shouldn't be allowed near a turbo in the first place, it's the smaller, but almost as drastically important changes, that most seem to miss. There are literally hundreds, but this is some of the most common...

PORT MATCHING- Christ, the amount of people I see who copy someone elses turbo and manifold spec, spend well over a grand on some fancy 8Boost/Part-Race tubular job, but just slap them together and think that's going to work great is unreal.
Look at the pics at the top of the page- Just because something has a T2/T3/T4 bolt pattern flange DOES NOT mean the port sizes are the same, in fact they're almost always far, FAR smaller than the manifold ones. And that flippin' massive flat step between the manifold and turbine is a killer for flow, and a good way of buggering up how fast your turbo spools, and if you're anywhere near it's limit, even what peak power is coming out of it.
A lot of BorgWarner 'T4' flange turbos have typical T3 sized ports, so the step is huge, and to be honest even most T3s, especially things like twin scroll Holset ones, the ports are far smaller. It's an OEM thing too, look at the top middle pic, that's a Skyline GT-R turbo, R32, R33, and R34 ones are all like that- T2 flange, but the port only goes 2/3 the way along compared to a typical manifold/gasket, leaving a 5cm by 1cm flat plate for the exhaust gasses to smash in to, causing all kinds of flow destroying turbulence.
Do what do you do? Well, you port match it. You don't have to be an engineering genius, it doesn't need to be perfect, and you certainly don't need to open up the entire port, but if you can use a Dremel, die grinder, even some files, and turn these big flat edges in to smooth transitions from the manifold port size to the turbine port size, you can drastically improve flow and performance.

TWIN SCROLL TURBOS-
Fair play, twin scroll turbo setups can be damn good. A good example of how good they are is a little 2ltr engine with a big Holset HX35 12cm on it. Even using the standard Cosworth YB 2wd exhaust manifold (that big cast iron rusty thing centre bottom of the pics above), which is twin scroll, a HX35 12cm (a big, journal bearing, cast wheel, 600bhp+ proven turbo, not exactly a new design either) is well proven to hit 25psi boost by just 3500rpm. Same results on Mitsubishi 4G63s with twin scroll setups.
Awesome when you consider a single scroll setup with a similar size turbo, even fancy BB core ones with £1500+ price tags, tend to not hit full howl until upwards of 5000rpm- That's a whole different world of drivability.

The thing is, twin scroll manifolds are expensive and relatively uncommon, so a lot of people can't run them, certainly not as easily as they can find badass twin scroll turbos. That's not the end of the world, as while a lot of people ask me if it's possible, I can assure you you can 100% run a twin scroll turbo on a single scroll manifold. Of course it won't spool like a twin scroll setup, but done right it's as good as the single scroll version, so yeah, go for it- Thousands of people do with huge success.

BUT, just like the port matching mentioned earlier, the divider on the turbine inlet flange is a big ol' turbulance creating flat plate when ran on a single scroll manifold- So again, smooth it off rather than just bolting it on.
TBF you really don't need to fully knife edge it like the one pic at the top, that's a whole lot of work and super thin metal is asking for it to break off and take your turbine wheel with it, but massaging it from a flat plate to a nice smooth curve is well worth the 15min work it will take to do.

POXY LITTLE DOWNPIPES-
​Just like I explained in THIS feature the other day, the first few feet of exhaust after the turbine wheel is by FAR the most important part, especially for spoolup. Unfortunately people often don't realise this, so fit god awful 2.5in downpipes despite the turbos being 600bhp+ beasts, and then they wonder why their spoolup sucks ball.

Some make the excuse of "Well, the turbo outlet is 2.5in, so why would I need bigger?" Because reasons, that's why. Honestly, it makes a huge difference, even swapping to a big downpipe on a 2ltr engine fitted with little 350bhp turbos can make a big difference to spool, never mind turbos twice the size.

To be honest, to get the best out of a lot of turbos even the standard turbine outlet needs modifying. Good examples of this are most Holset HX35s and HY35s, which have a 2.5in outlet which is cast as part of the housing for the first 4inches or so. People have seen VERY noticeable improvements in spool, over 500rpm, by chopping off the back section and welding a 3-4in v-band right from as close to the turbine wheel as possible.

OIL FEED AND RETURN-
I'll finish this off with a non-performance one, but one I get asked all the time, and that's oil feed and return sizes.
​I've lost count of the times I've seen people buy a brand new turbo, fit it, it smokes its tits off, and they remove it crying that it's broken already and it's completely shit. No mate, it's you that's shit.
Of course some turbos are faulty and oil is pissing past the seals through no fault of your own, but that's pretty rare as the oil seals are a design that just don't wear or break easily.
90% of the time it's because whoever has fitted it has fitted too big a feed, or most commonly, a comedy small oil return.

"Oh I fitted a dash12 billet anodised alloy amazing one I found on eBay, it must be fine" they say. Until you ask how big the internal diameter is. And they don't know. And then they look, and it's 10mm or something, hence the oil can't get out fast enough, and it's forced past the bearings instead. The fact is, oil returns need to be BIG to work well, most turbo manufacturers specify 19mm internal diameter minimum, and changing a little pipe to a big one usually stops the smoke. So bin the fancy looking but restrictive aero fitting and just use a normal but big metal and rubber return line that actually works properly- It's not as if anyone can see it anyhow...

Oil feed is similar, but a bit more complicated- Too big and you can get oil flooding past the seals again, but too small and you'll starve it of oil and kill the bearings in no time. Why it's complex is all turbos are different and engines have wildly differing oil pressures.
Almost all manufacturers state a maximum oil pressure, but as this is measured at the turbo inlet, almost nobody checks this, so just guess- often wrongly.
A rule of thumb I tend to have luck with is non-BB turbos use a -3 line with no restrictor, the more common -4 can be too much for some, which leads people to use restrictors, and they they fit one way too small and run the bearings dry. Then cry a lot and blame the turbo even though it's their fault 100%.
BB turbos always in my experience have manufacturer recommendation on oil feed and restrictor size, so that's rarely an issue unless you can't read.
But most of all- Make sure the bloody oil return is big enough, has no kinks, and ends above the sump oil level. A great flowing oil return can usually counteract a slightly overkill oil feed in my experience...

Exhaust backpressure on turbo engines is one of the most misunderstood aspects in tuning to many people, but it's pretty simple- It's not good.
On non-turbo engines backpressure is a single issue, the pressure in the exhaust system, but on a turbo engine it's actually two issues, pressure in the post-turbo exhaust system, and pre-turbine backpressure in the exhaust manifold.

A lot of people still think turbo engines 'need' backpressure or they lose low down power, but that's total crap, in fact the opposite is true- The BEST post-turbo exhaust system would be no exhaust system at all. Bigger really is better for both spoolup and power, especially directly after the turbine wheel, and it's why fitting a big downpipe to a turbo engine makes a massive difference to spool especially, much more than the rest of the system; in fact some new very high performance production cars turbo have amazingly big turbine outlet elbows for this very reason.

I'm not one for going in to the science of why things are how they are, as in the real world it usually doesn't matter, but a big part of why a turbine spools is the pressure drop between the inlet and outlet, the bigger the pressure drop, the better for spool, so having as little as possible in your post-turbo exhaust is what you want.

But post-turbine backpressure is only part of the story, pre-turbine backpressure is one of the biggest, yet totally invisible, killers of tuned turbo engines, and is also a massive reason certain engine setups don't make the power they're hoped to either.
​
Pre-turbine backpressure is inevitable, it's just a turbo engine thing, one of the things that spools your turbo, and a worthwhile exchange for acceptably fast spoolup for your application. Unfortunately, too small a turbine, be it the wheel, housing, or both, can cause too much backpressure, which in turn means exhaust gas struggles to leave the cylinders, huge EGTs, higher cylinder temps, diluted fuel/air mix in the cylinders, and so on, which in turns means hugely more susceptible to engine-killing det, not to mention general reliability issues and power restriction.

Some well specced, huge turbo, drag engines, actually do have lower backpressure than boost pressure, and this 'holy grail' of scenarios actually makes for an engine that makes amazing power and hugely det-resistant. Unfortunately, these sort of setups just won't spool fast enough for 95% of applications, so tend to be found on high-end drag engines only.

In the real world, most engines can be expected to have up to twice the backpressure as boost, but more than that, and some engines do have a LOT more than that due to peoples obsessions with tiny turbine housings (and a great many badly specced turbos with overly small turbine wheels compared to compressor size), which causes a lot of reliability and performance issues, and with no way to know what backpressure they've got, most don't even realise this is the main culprit!

You often find that if your pre-turbine backpressre is more than about twice what the boost pressure is (and some engine setups can be 3 times or more!), changing to a bigger turbine housing, or ideally a bigger turbine wheel, will not only drop the backpressure, but hugely increases power and reliability per psi boost, and actually not slowing your spoolup at all despite the bigger turbine; in fact in some situations it speeds it up as it was so strangled beforehand.

So yeah, BIG exhaust is good, a backpressure gauge is good to see if your turbine is strangling your engine, and don't be afraid of a bigger turbine- It may well transform your engines performance in every way...

This is another thing I see all too often, and that's people obsessing over creating a cold air feed for their turbocharged engine, and usually at the same time neglecting other things that have a far bigger effect on performance.
It's all well and good having a fully enclosed cold air feed to your turbo, but if it's too small, and in most instances it is, it doesn't matter how cold the air is, your car will perform worse in every way than if you had a totally nonrestrictive air feed, even if it sucked in full engine bay temp air.

While a cold air feed is a bonus on a turbo car, it's really, truly, not a big deal, and not something I'd exchange for even the slightest restriction. Unlike with N/A, the air goes through the turbo compressor, heating it up hugely, regardless of how cool it is, and then, providing you've got a good setup, it goes through a highly efficient intercooler and exits at a very low temp indeed; way below 40degC. 
If 1deg increase in compressor inlet temp equaled 1deg increase in intercooler outlet temp, it would seem more useful, but unfortunately it doesn't work like that.

Also, while under-bonnet air temps are high when static or on an engine dyno, in the real world, ie when you're flying down the road under full load, the engine bay air is moving so fast that any one portion of air is under the bonnet for such a tiny amount of time they don't get a chance to get hot, so, well, they're not...
​
Finally, and perhaps most importantly, is any restriction. While you can't see or feel it until it's SO bad it's actually sucking your inlet pipe shut, trust me, fit a vac gauge to 80% of turbo inlet pipes and there will be a restriction. Any restriction at all decreases compressor efficiency, which in turn increases compressor outlet temps, totally negating the usefulness of any 'cold air feed', not to mention needing more 'power' on the turbine side to spin the compressor, which will increase pre-turbine backpressure, which then is bad for temps, performance, and reliability.

BASICALLY, while cold air is always, always good, it comes a big big big second place behind having a massive and nonrestrictive air feed.
​Huge air filters and 4in inlet pipes for all!