How much boost on an STi?

That's right. It's right in the 14-15 PSI area. You can up that a couple pounds but the turbo is pretty much running at it's full potential. More boost just blows more hot air. There is not a whole lot you can do to increase boost short of replacing the turbo. Then you are looking at race gas and turbo lag. The boys at Subaru knew what they were doing with the STi.

BlueSTi "Scary-Fast"

Agree that 14-15psi is right for stock turbo and boost control. But the stock turbo can make more boost, and race gas and turbo lag is just silly :)

I ran 20-21psi on my VF39 (of course, that tapers to about 16psi toward redline as the turbo runs out of steam). There's plenty more torque (not necessarily power) to be had with increased boost on the stock turbo.

And on my new turbo (APS SR50), I hit 22psi by about 3200RPM or so - quicker spool than stock, and on pump gas of course.

There are huge gains to be had with flow and efficiency mods on a US STi, while keeping a very drivable, pump-gas street car.

I would find it very hard to believe you could get 20 PSI on pump gas with the STi. The thing will knock on 91 octane on a warm day. You get better results with 94. However, it will retard ignition like crazy if you increased the boost much beyond what it is stock on pump gas. You may get better performance because you can get faster spool-up with a bigger turbocharger, but increasing the boost pressure would be dangerous because of detonation.

Where I am I can get 93 (used to get 94, but Sunoco recently stopped selling it). 91 wouldn't be too much different though.

The STi doesn't need to ping, the problem was the stock ECU which tried to advance timing too far. There was a service bulletin to re-flash the original '04 STi ECUs with a less aggressive timing map, which alleviated the pinging.

Regardless, I increased boost through a UTEC piggy-back engine computer, which overrides the stock ECU's timing and fuel values. It was quite capable of running as much boost as the stock turbo could throw at it.

Currently, I run 22-23psi from 3200 RPM to redline, and on race gas, I run around 25-26psi and a lot more timing advance, with no detonation or pinging. The engine handles it like a champ.

I'm sure it would handle it just fine, but the ignition would have to be significantly retarded, which reduces power as well since it will also reduce the efficiency of the burn. So, increasing the boost but retarding the ignition won't really increase power.

you're wrong. well, you're PARTLY right.

also consider that with increased cylinder charge pressure comes faster combustion. you run less timing because of two reasons:

1) indeed it is easier to knock, and you don't want that, _especially_ at high boost pressures

and

2) you don't NEED as much timing to get peak cylinder pressure at 15 degrees ATDC which is the sweet spot for max torque.

you're right in that it is not NECESSARILY true that you will get more power out of the configuration, and indeed you will certainly eventually get to the point where more boost pressure requires so much ignition retard that you do NOT get more power (at this point your only friend is octane really), but nonetheless there are absolutely a lot of gains to be had on a stock STi by *carefully* upping the boost.

jm2c ken