**** Work in progress! ****
I get asked this prickly question all the time, so I thought I'd record my thoughts on the subject. Please note that this only refers to turbos for SR20's and I'll try to keep it to only examples that I've had personal experience with. There's enough secondhand internet wisdom out there already!
Power figures will be quoted in RWKW (Rear Wheel Kilowatts) as these are the numbers I use on the dyno. To convert to horsepower-at-the-flywheel just multiply by 1.7. This converts from RWKW to horsepower and allows for a 20% loss between the rollers and the flywheel. 20% is used by Dyno Dynamics for drivetrain losses plus tyre-to-roller losses and seems to be pretty close to reality.
With turbo sizing the first thing you need to do is work out how much power you want. Be realistic. These days it really is a matter of choosing a number and then building a list that will give you that number. Assuming you have the funds to do so of course! Bear in mind that whatever power level you aspire to, you'll need supporting mods. Big power will need not only large injectors but also a suitable fuel pump and usually an expensive clutch. Big power usually relies on big boost and with this comes a more highly stressed engine, so you also need to invest in better monitoring for things like fuel pressure/Wideband AFR as well as the usual gauges for boost, oil pressure, coolant temperature etc.
T28
Aaaaah - the mighty T28. Such a good little turbo. Such an underrated turbo. Probably still the best value for money turbo for an SR20. Doesn't matter if it's the plain bearing sort (as found on ADM S14/15) or the ball bearing one. Both work great. BB core is slightly better in spool-up and durability. "T28" is actually a misnomer. "T28" refers to a family of Garrett turbos and what we all call a "T28" isn't even part of that family! Garrett call them a "GT2560R". Whatever - we're talking about the turbo that came stock on the S14 and S15 SR20 here.
These little gems are good for about 15psi on an SR20. From the factory S14/15's run from 9 to about 11psi. But this can be turned up a bit with no harmful effects. But generally they won't make much more than 14 or 15psi. You'll get lots more "up front" when boost first comes up (they'll happily make 20psi for a moment) but they always drop back to 14 or 15psi at higher RPM. I usually try to set the boost up so it doesn't spike when boost comes up - just a nice solid 14 or 15psi all the way. They'll usually make full boost at 3400 - 3600 rpm and 190 to 200rwkw with generous area under the curve. This makes them a particularly good choice for light cars like Datto 1600's.
For S13/14 engines you can stick with stock injectors and AFM as long as you don't push things too hard but both will be running close to the line. So it pays to upgrade to at least 480cc injectors from the S15 and fit an R34 "pink label" AFM. Most go straight to 550cc injectors and Z32/R35 AFM which gives you lots of headroom for later if you decide to go for more power.
For S15 you already have a T28 and 480cc injectors (unless it's an auto - these have 370cc injectors. sad face) so it's just a matter of turning up the boost and tuning to suit. Easy.
