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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Stock 3.0 has 6.5/1 compression ratio, 3.3 is 7/1, 3.5 w 100mm pistons is 7/1 or 7.5/1 depending on which P&C is used, a 3.4 Turbo P&C with 3.6 crank can be around 8/1 CR. Thus, the 3.0 can safely run the most boost before detonation might occure. 1 bar boost is equal to about 4 points of compession increase. Thus, a 7/1 CR motor at 1 bar is running an effecitive CR of near 11/1.

Then there is the fuel system limitation:

390-400chp is reported to be the limit for a stock system as to fuel delivery. 12/1 can be ideal if the CIS is in good shape, it is delivering ballanced fuel, and or it is mostly street work.

With the Andial fueler you can get about 1 point higher AFR or about 8-10% more fuel and support maybe up to about 450chp on a street motor.

There are other factors at work. Timing, intercooler effectiveness, ambent temp, Turbo efficency and capacaty.

Having said that a well built 3.5 with an efficent turbo should make more like 450chp at .9 bar.

There is one well respected builder here that on his 3.5, twin plug, SC cam, build sets boost at .7 bar and it makes 450chp.

He sets the boost at .7 bar because that is the fuel delivery limit and that is after he has shimed the fuel dist pressure and modified the metering arm pivot for more fuel. With this he can bump compression to 8/1 for a bit better low end.

Hope this is a little clearer.

Originally Posted by gumba View Post
The statements above are a little confusing(at least to me). I'm running a 3.5 twin plug with stock CIS, stock WUR, SC cams, Mahle P&C at 7:1 compression. The motor dynoed at 380 crank h.p. at .09 boost, f/r was 12:1 to 6500 rpm. I was told I could occasionally run it up to 1.1 or 1.2 boost with no problems, which I have. The car came with a Andial fuel enrichement system which we removed. This is a street car.

Not to get off topic, I also feel your pain. I had 2 well know shops build my 3.4 race motor. The first time it lasted less than 2K miles, the second time less than 1,000 miles. Both failures were due to lousy work.
Old 06-13-2010, 07:19 AM
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