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...

I love going sideways. For me the adrenaline rush of being beyond the limit but still in control is more fun than anything else you can do in a car. Like it or not, oversteer, powerslides, drifting, whatever you want to call it, often isn't the fastest way around a corner, but it is the most fun way around, and from a spectator point of view it's certainly what's memorable and gets the crowds excited.

Despite my love for it, there are some aspects that drive me flippin' mad, some because they affect everyone associated with drifting, and some purely because so many more people, even people who are already part of it, would enjoy things much more if things were a little different...

While I'm not a JDM fanboy by a long shot, one thing the Japanese have pretty much 100% correct is their drift scene, and if we, as drivers, as fans, as organisers, could have the same attitude to it as they do, things would be a lot better.

My problem mostly boils down to peoples reasoning for why a car performs as it when drifting.
In Japan they rightly say drifting is 90% driver skill, 10% car spec, and they embrace this in every aspect, from practice days where it's normal to see low power basic spec cars drifting with absolutely mind blowing skill, to high end competition, where the cars are surprisingly low power and basic compared to UK competition cars, but the action is so much more impressive to watch, and the driver skill is so much more apparent
.
In the UK however we've got a huge tendency, whether it due to over-confidence or simply ignorance, to blame the cars specification for everything. Because of this, massive spec cars being driven in hugely mediocre ways is the norm here, and even newcomers to the drift world have it in their heads that before they even get behind the wheel, a car needs big power, huge amounts of lock, and a hydraulic handbrake.

Well let me tell you, you're wrong. Lower power lower spec cars entering corners at huge speeds using massive Scandinavian flicks, like you see in Japan, is a whole lot more fun both to do and watch, not to mention cheaper, than seeing a big power car doing a skid on the handbrake, then smoking its tyres around the track with just a lazy stab of the throttle.
It's a good job there's no UK versus Japan drift events any more, as if we went by their rules, our top cars would be hugely embarrassed by cars with half the power and specification, but driven by guys who truly push them to their full potential...

​

While it's less common now, as there's countless cars out there proving it's a load of old bollocks, you still see people commenting that they can't/won't/shouldn't/couldn't use a certain turbo because it's a "diesel" turbo, and their car is petrol.

But do "Diesel" turbos exist? Well, maybe back in the day, but in this day and age? No, not really. Turbos that are the wrong wheel/housing spec to suit a certain engine? Yes, 100%, but what fuel the original engine it was designed for is not the issue here.

Frankly almost all turbos, yes, even the most high-end aftermarket ones, have at least 'some' parts originating on a commercial diesel engine, and many, many, consist of parts that almost all OEM on some diesel engine of some description; they simply consist of choice parts of various diesels to make a good spec one for a typical tuned spark ignition engine.

In fact there's tons of OEM turbos that work straight out the box as fantastic performers on tuned petrol engines. A whole crapload of 'famous' BorgWarner S200/S300/S400 turbos people use on some of the worlds top tuned cars are also OEM on various things, in fact a lot of them John Deere (yes, farm machinery!) engines. Yep, awesome spec, even billet wheel, Borgs are fitted to tractors, combines, generators, and more.

And things like the 12cm Holset HX35 is hugely popular in the tuning world, and proven ridiculously capable on tuned petrol engines, despite being fitted to countless diesel 'things', even diggers. Much the same for the 10cm Holset HY35, which is mega popular on things like 1JZ engines in the UK, proven 520bhp+ and 500lbft+ despite hitting full boost under 3000rpm, but they're OEM fitment to various DAF engines among others.

OEM 'diesel' turbos may not be THE best turbos in existence, but the sometimes amazing performance combined with reliability and a low price often makes them THE most value for money.

The problem with 'some' diesel turbos being great for tuned engines is some people go to the polar opposite of the people who think diesel turbos are the car equivalent of Ebola, and instead think they can fit any old diesel turbo to their engine and expect it to perform well. 

WRONG.

Fact is, as good as some are, most of them aren't suitable for performance petrol applications. Most have a FAR too big turbine side to work well on small capacity engines, so unless you know it will work, or actually understand turbo sizing and what turbo you're looking at, don't do it.

Obviously business is business, so big aftermarket turbo sellers will slate "Diesel" turbos 'til the cows come home, even going as far as to tell bare faced lies, as they don't want people to buy them instead of their own, even if many of theirs are barely changed over an OEM diesel unit.

Like it or not, the fact is, there's a LOT more money to be made in OEM than in tuning, which is a tiny % of the turbocharger world, and for that reason the top OEM engines get the best, most high-tech, turbos, long before the general aftermarket. Christ, even normal, low power, 4cyl Mercedes Sprinter vans now run twin compound turbochargers, both with billet compressor wheels. The only things they don't get vs the aftermarket are gimmicky or unreliable things- OEM is built to last... ;)

DON'T GET ME WRONG- The best turbos, the most powerful and fastest spooling turbos, are not OEM, they are aftermarket ones, but you will be paying the premium to get these, and only you can decide if that money is better spent elsewhere (Hint- Usually).
Honestly, aside from very very few, literally mega-bucks, Pro drag units, and of course things like proper WRC/Rallycross/LeMans race turbos like Garrett TR30Rs, they're all based on OEM "Diesel" units to some extent, be it just a turbine wheel and housing, or much more...

Let's face it, standard cars are boring. No matter how fast they are, they have to cater for the 'average user', so they're safe, they're easy, they're, well, a bit too sensible. This is a big reason we tune cars, not just for more performance, but for more feel, more involvement, more excitement. And as the years roll on, the bar of what's considered high performance is ever rising, so it's rare for an 'old' car to be very fast either.

Well, welcome to a rare exception to the rule, the Lotus Carlton, one of the VERY few production cars I've been hugely impressed with, and certainly the only unmodifed car from two and a half decades ago that's impressed me in modern times.

To be honest, most people I know who've driven them don't like them as much as they expected they would, usually moaning about it being geared too long to use the performance, the car not being as fast as they imagined, or the transmission being hard work, feeling old fashioned, but to me that just says they're pussies who can't drive...

First up, the car IS long geared, but not in a way that detracts performance if you know how to drive it, and being in too high a gear is why most think it's not as fast as it is.
It's not a turbo diesel, you can't plant the throttle at 1500rpm  and expect it to rocket off down the road, as the engine loves to rev, that's where the performance is, and thanks to the long gears you're at least one gear down compared to most modern cars.
Unusually, 1st gear is actually usable in performance terms, good for almost 60mph, and things get even longer at higher speeds, so while 6th is a cruising gear, 3rd and 4th is for hard acceleration at motorway speeds, and 5th is for doing hugely, ridiculously, illegal speeds; and yes, it's very, very, easy to see the claimed top speed of 177mph...

In all honesty, and I'm very rarely impressed by a cars performance, I was hugely impressed by how these things go even compared to their modern rivals, and the driving experience is leagues ahead in my book. Cars are about fun, and this thing is fun.
I like rawness that other seem to dislike. The heavy duty 6 speed gearbox is clunky and slow, and the clutch is similarly heavy, but from a driving experience, that just adds to it in my eyes. The total lack of driver aids seem to scare people off too, but I loved it, being able to feel everything and having to do everything, which along with huge acceleration, and the grunt to easily spin the wide rear tyres at motorway speeds, it was the experience you get in a good tuned car, but this was standard.
Having said this, it's not some kind of wild beast, and as long as you're not deliberately being a dick, as long as you have a decent understanding of car control, it just squats down and rockets off down the road far faster than I expected, and in roll races versus most it's much newer, but much heavier, rivals, they're left wondering just beat them.

Considering the standard car was the kind of fun you normally only get from a well tuned car, I'm 100% with some well thought out mods (Standard inlet temp is 60degC so the standard chargecoolers would be first to go!) it was be absolutely ridiculous fun.

If I had to compare it to any more modern car, it wouldn't be a BMW M5 or AMG Mercedes, in fact it wouldn't be any standard car, as this car doesn't feel 'standard'. The Lotus Carlton feels a lot like driving a well tuned Toyota JZX100, and that includes the fact that as well as the stupidly quick acceleration for a big obscure four door, it drives and handles far better than you'd expect, and can be seriously chucked around corners. 

Unfortunately, the cars and their parts are rare and expensive, so that's very unlikely to happen; the one I had was very kindly loaned to us from the Vauxhall museum! And as the picture top right shows (Top left is on my driveway!), they also let Richard Hammond 'test' the very same car for TopGear...

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!

Performance camshafts are wonderful things, and play a massive part in how an engine drives, but one thing they do NOT do when fitted to a factory turbocharged engine, is help the turbo spool up faster.

Some of you reading this might be thinking "Duuh, no shit, tell us something even more obvious" but for years, over a decade in fact, especially in the Japanese car tuning scene in the UK (but it seems to spread elsewhere too; I've heard it a fair bit even with Renault 5 GT Turbo owners lately), I hear people saying this like it's a fact, despite it much more likely to do exactly the opposite! This does seem to be a UK-only thing; overly wild cams on turbo engines aren't common elsewhere in the world that I've noticed. Whether this is better knowledge elsewhere, or less unscrupulous tuners selling cams by telling bare faced lies to convince people it's needed to improve spool, you decide...

Don't believe me? Well, there's enough dyno sheets out there to compare which prove it, but the production car world is a big hint that these magic 'anti-lag' cams are utter bollocks, as all production turbo engines are fitted with much milder cams than the production non-turbo version of that engine. From Fords to Porsches and everything in between, that's a fact. Do you really think they did it to make the cars laggier and have less low down grunt? No.

The fact is, cams are very give and take, and increasing cam duration (and overlap) reduces low down power, not increases it, and instead improves performance higher in the rev range instead.
A well-specced mildly uprated cam may well not noticeably slow spool over a factory turbo camshaft, and may indeed give a much stronger midrange when it's matched well to the turbocharger(s) fitted, but will it actually improve spoolup rpm? No.

In fact, if you over-do the camshaft you can totally RUIN a turbo engines performance, both spool and top end power, and this is something I've experienced a number of times (not from my own choice, I must add!).

I've seen (but wasn't consulted on, as if so I would've told them it was going to be a disaster) a magazine test of some "Fast Road" cams on a VAG 1.8T engine running the stock KKK K03 turbocharger. I knew even these fairly mild cams were far too big for the athmatic little K03, and lo and behold, the car lost a huge amount of performance from low rpm to high, compared to the standard cams.

Another good example was when a frien bought a Renault 5 GT Turbo modified by the previous owner. The engine was a good spec and fitted with a Piper 285 cam, which I know from previous experience is a fantastic cam for the little C1J lump as long as you've got a decent size turbo fitted to it, but this car had a standard Garrett T2 running 15psi boost. While the turbo didn't really spool much slower than standard, the longer duration still reduced lower rpm power compared to the standard cam, and as revs increased the restrictive little turbine of the T2 combined with the increased overlap of the cam (high pre-turbine backpressure and big cams REALLY don't mix!) meant the engine was completely dead on it's ass by 5000rpm- Lower rpm with the standard cam, making for a slower car everywhere in the rpm range.
When this exact same engine was fitted with a turbo of a more suitable size for the cam (A Garrett GT2560R running 23psi), the engine was transformed, and now makes well over twice the power, and revs hard all the way to 7500rpm, while only running 8psi more boost. Didn't spool faster though ;)

"Turbo Cams" are one hell of a big subject, and one I'll be gradually covering on this website in various forms no doubt, but this 'lag thing is the biggest issue...