Originally Posted by lstowell
You can deal with the bolt ons that lean you out by bumping injector size a bit--presuming the fuel management cannot handle it with your stock injectors.
As long as you keep the A/F reasonable, flowing more air pretty much equals more power since you would also be flowing more fuel. Counter-intuitively, a good exhaust and air flow can increase fuel efficiency.
Actually, the more air you flow, the richer you want the mixture. The more air you flow, the more you have to cool it because the faster you move it, more heat it will have. It is not a matter of just keeping it reasonable. Sometimes, upgrading hardware elicits more upgrades without having a performance increase.
For example, the stock Subaru Legacy GT came with an IHI-VF-40 turbo and a stock psi of 14.7 and an AFR of 12. It also had stock injectors of 400 cc/min and a fuel pump of 140 L/min. And a small intercooler. With tuning, you could push the car to about 17-18 psi and you maxed out the hardware. On stock equipment but several levels of tuning deep plus exhaust, I made 250 whp / 250 lb-ft and a nice curves on the dyno graph - a nice whopping torque curve that stays essentially flat between 3000 and 7000 (tapers off a little towards the end).
With a bolt-on upgrade to a TD-04-16-G turbo (a slightly larger, yet quicker spooling turbo that offers more air flow across the entire RPM range), you can get to about 16-17 psi before having to upgrade injectors and fuel pump. So technically, you can push more air, but make less power because you are boosting less than before. The turbo in itself is capable of up to 22-23 psi but it is imperative that you get 650-850 cc/min injectors, a Walbro (or equivalent) fuel pump of 255 L/min; a larger intercooler, an electronic boost controller and so on... AFR's are now 10:1 and people start shaking when they see 10.5...I've had my share of problems with injectors and count them 4 fuel pumps so I settled on a de-tuned version, with reliable stock injectors, STI fuel pump and all the other supporting hardware. Five tunes later, 19 psi, 278 whp, 352 wheel lb-ft, AFR 10.1 and 3300 lbs of total weight, it's enough.
Merely pushing more air and watching the AFRs isn't even beginning to describe the challenges of pushing more air, even in a car designed for turbocharging.