Can't believe there isn't a "which turbo?" thread as a sticky. As I'm always asked this by builders and as it always crops up here I will give my 2p's worth! .
First of all, this is just a basic guide for VR/R specific "what turbo's?", specific info on turbo speccing I would suggest reading things like this;
and going through all their guides plus many more threads on the net. This is probably for noobs then who don't want to learn that!
I would estimate that around 90% of owners that come to me are usually looking for around the 400-450hp mark. Should you get a turbo that flows that? No! Technically speaking, say a V6 4Motion runs 200lb/ft of torque, then a highly efficient V6, 100% efficient - running a bar of boost - i.e. you have compressed the ambient air - outside air - to double it's original density - ambient pressure which is 14.7 psi, pounds per square inch or 1013mbar is what you feel every day just walking about - is compressed by the turbo - so would run 400lb/ft or 400hp thoretically at double the stock ambient pressure..
Although it would show 1 bar of boost on your gauge, you have essentially taken normal air - a density of 14.7 psi, and squashed it to half it's size, subsequently making a square inch twice as dense - or 29.4psi absolute pressure - i.e 14.7+14.7. That is basically what a turbo does - compresses air!
So, theoretically speaking, if you double the air density, you're doubling the output - 400lb/ft in that example. However, there are many factors as to why your car doesn't achieve that! Efficiency is the reason. There are a number of things which make your car less efficient.
Most of you know that the air has to go into a filter, down pipework, maybe through an air cooler and into the inlet manifold before entering the engine and as such, losses will occur as the air has to battle these restrictions - therefore causing losses or making it less efficient than is ideal. See the fluid dynamics link below.
The turbo itself has it's own efficiency islands too - the "compressor map" as it were and being an air pump can also suffer efficiency losses if not specced correctly/not operating at it's most efficient. The turbine side of the turbo can also have profound differences on things like spool, but if you want to know more specifics, take a look at the links above and below.
Anyway, due to the above factors to consider in terms of "what hp of turbo?", it is why I always say "get a turbo of at least 500hp!" - do some basic math for R32 differences then - 3.2/2.8 x 500 = 571hp as a rough guide.
There is more to it, for example many owners report higher flowing turbo's with the same turbine ar ratio are very, very similar in feel - i.e. the GT30 v GT35R, seem to have very little in terms of spool difference if the ar ratio is the same - i.e. both 0.82 T3.. Lot's of variables in tuning though - it's always a balancing act for end results!
Which actual turbo's then!!!!!!!!!!! I hear you say?
Well, the proven ones are usually the Garrett and Borg-Warner units. You can get cheaper versions, like the B-W S or Garrett T versions but they are rarely chosen. Why? The turbine side of the B-W S can be large - see Holset info below - or the T series aren't as readily available here in the UK although the journal and ball bearing T series are great budget or relatively budget versions which are often enjoyed by those in other markets like the US.
In the UK, possibly EU at least, the best, readily available options are the Garrett GT/GTX and Borg-Warner EFR turbo's - all very advanced and highly proven. You'll be looking at a minimum of £1k+ most likely though for these but they are usually considered the best.
So let's briefly look at the turbine ar rato of these turbo's:
0.63ar/equiv - good low end/spool due to high air velocity at low volumes but can ultimately choke up top - prob best for 4 cyl - ultimate air flow being the problem!
0.82/0.83 ar equiv - preferred choice for road use, a bar of boost at roughly 3500rpms, good flow bottom to top - can do 600hp too!
1+ ar - good for hp chasers, weekend toys etc, big top end but not always favoured for daily's due to poorer gas dynamics at low loads.
Air speed velocity can help at low loads, a smaller orifice will have x amount of air pass through it quicker than a larger one but too high mach numbers up top are less ideal as they can introduce turbulent flow - Google laminar flow. The preferred turbo's mentioned have both T3 ad T4 flanges so these air speeds/traits should be thought about. same single or twin scroll turbo. Almost every owner I have spoken to usually opts for a GT35R T3 0.82 and they have always been happy, but some prefer a little more lag, whether that's the driving style or drivetrain protection, so sometimes big ar ratio's and T4 flanges are chosen.
Outside of these variants then let's see;
Holset are also a favorite of owners with a few I know enjoying them! These are commercial, can be tons about but are usually used on big diesels, as such, check turbine side as they can be HUGE, good scrap yard finds though and cheap if you can get a supplier of them! I know compressor racing on ebay does them now and again and the turbine is specced/changed for petrol led build's with happy buyers - maybe worth a look? Being workhorses on high temp diesels they tend to be bulletproof.
There is also a number of other options - some decent JDM turbo's on the bay, thrust journal bearing mind you in terms of cost to think about comparatively, but well proven in such circles - just remember though that with import duties on any outside UK/EU you're looking at x 1.25 the cost of item plus postage to your door if not bought within UK/EU. Some nice billet items there and a bit cheaper so worth a look, but factor in these additional duties if you get quotes from domestic suppliers and their turbo's. It could be a case of them being nearly the same cost as a dual ball bearing turbo for example!
Precision Turbo's can also be found on the bay, mainly JDM usage here in the UK but many use in the States, not really au fait with them but lot's of big power builds read about - more expensive for me to buy them than the major manufacturers above but supposedly well proven. Both journal and ball bearing options for broad cost variations and spec turbines as above.
Ebay cheapies - you'd be surprised what I can get these for in bulk! Different QC standards I would presume so do some homework - have a 20vt friend who was looking for a 300hp turbo - suggested a T series due to funds and around 12-14 months later he's just bought a BW billet KO4 as his 5th turbo and all others ebay spesh's that didn't hold up!
Things like boost levels, heat levels etc all make a difference and his anti-lag most certainly didn't help! The thing I have learned from manufacturing techniques is that material specifications matter! Therefore, "you pay for what you get!" rings true. Very sensible boost/stresses - these cheapies seem to hold up quite well at times according to some owners, if not - well, read above!
"You pay for what you get" - compressors spinning at hundreds of thousands of revs, 1000 degree exhaust temps etc - materials and manufacturing processes matter then! Oh - re: the stories about some manufacturers now working abroad and the turbo's now made "cheaply" - QC standards can be of an international standard and I would be surprised if a company that has spent heavily on R&D would suddenly tarnish their reps by using inferior materials/QC standards! Maybe cheaper labour outsourcing, but can't see where named/proven brands qualty would drop with that tbh.
GT35R most popular turbo chosen by owners for their daily's by far in my experience - 0.82AR T3 - 400-600hp well proven.
EFR also top spec - keep compressor/turbine similar to above, similar results
Various mid range turbo's, bit of shopping to do, some decent options mentioned - factor in taxes/duty compared with domestic suppliers, depends on your country/laws I suppose
Cheap generic - your risk, material, usage etc - can make big differences, you do pay for what you get in this area though! Some have worked, most haven't in my exp - I would say if you're struggling and on a budget, keep boost very low and not too many heat cycles until you can afford a more expensive one.
T3/T4 etc flange, turbines - basic guide as above.
*** Note: You may also want to factor in your tune, some can be very turbo & fuel specific, some pay more for custom work in this area but that 440cc tune may not be ideal for your GTX42! Just a thought! ;-) ***
Anyway, my 2p from my experience, hope it helps you choose and hope it helps the mods that pull their hair out for being asked this question time and time again!