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...