I know the flames will be forthcoming but...

What kind of power difference will 3-5 PSI make?

I've asked this on the appropriate manufactures forum and no one would chat with me about it.

The scenerio is I'm running 14psi on my low setting of my WRX and 15.5 on the high(DSBC). my fuel cut off is 18.5, so I've thought about clicking it up to 18 for the track setting (high setting). I didn't know if I would see enough power increase to really go through the hassle of adjusting the boost controller.

 

unless you've built the engine a bit i'd stay away from 18 psi. i read somewhere that a guy was running 18 psi on the stock turbo and blew his engine pretty quick.

 

From what I have "read and heard" is ~10 WHP per 1 TUNED PSI

so baisc num,bers would be 30-50 WHP. Again, this is with the proper fuel, exhaust, etc..

Hope that helps

 

thanks for your info. The fuel cut off for peak pressure is 18.5

I assume that there is a margin of safety for the engine built in by subaru. I dont think they would set the cut off above the detonation point of the engine. I'm not advocating the practice of driving the car at max boost either. I was just curious.

 

How much power per psi totally depends on the turbo and it's efficiency range. It's really not a linear thing.

Ex. On my girlfriend's Integra, we gained 56 whp at 3 psi. But when we went from 3 psi to 5 psi on the wastegate, we only gained 11 whp for that 2 psi difference. In a few weeks, I'll find out what difference there is from 5 psi to 7 psi. So for the first 3 psi of boost on that car, we gained ~19whp per lb boost, but only 5.5 whp per lb boost from 3-5.

 

ok good, can the "efficiency" be termed as the effectiveness to maintain a certain PSI throughout the RPM range? for example a turbo that can maintain 16psi up to the redline would have a 100% efficiency after it builds up peak boost? In the case of your G-F's integra, is that a fuel thing or a wastegate issue?

 

Well, efficiency is more how effective the turbo is at converting exhaust gas to compressing intake air and how much it is heated in the process. Tiny turbos for the most part can hold 20psi as well as a bigger one but they'll heat the air a lot more in the process. This is b/c the smaller size requires more exhaust energy to flow higher psi and the turbos are spinning really high rpms to push that psi. A larger turbo can move more air at lower rpm w/ less heating and choking of the exhaust.

In the Integra's case, the hp increase from the extra 2 psi was offset by additional heat from pushing the turbo harder so there weren't very large gains. The two runs were done on different days w/ different conditions and fuel tuning so they aren't that comparable. The hp gains per psi really depends on where the turbo is setup to be efficient. It has to do w/ the shaping of the turbine and compressor blades, their operating ranges etc...basically need to know a lot about fluid dynamics to answer it accurately. Reading compressor maps sucks but it's the only way to know the efficiency island for your particular turbo unless you're willing to experiment w/ your car on a dyno all day.

 

typical numbers are 12-15 hp per 1 psi, possibly more if more tuneing is done.