Compression ration question...

[BlkBird]

Fellow 930'ers, No doubt documented somewhere, but what is the CR for a Turbo when it is on full boost? For example, say your car is 8:1 CR off boost and you're running a 1 bar spring. Does that mean that at full boost you have doubled the atmospheric pressure and therefore doubled the CR to 16:1? I'd imagine there would be some losses and you wouldn't really double the air pressure going into the motor but what's the final result going to be?

[930turbo]

Try this link. It has a turbo compression calculator.
http://www.turbofast.com.au/TFcompB.html

[BlkBird]

Holy Toledo!

Anybody know these figures for a 3.3 930?

Bore: Cylinder diameter in inches.

Stroke: Engine stroke in inches.

Engine VE: Engine Volumetric Efficiency in percent. VE changes with engine RPM, a engine at wide open throttle at 3000rpm could have a VE of 90% and at 6000rpm 60%. This is a rough guide. At maximum RPM modified engines may have a better VE, eg: average overhead valve engines 45% to 75%, overhead cam twin valve 65% to 80%, four valves per cylinder 75% to 90%.

Standard Comp: The Compression Ratio of the engine.

Boost Pressure: Boost pressure in pounds per square inch.

[fredmeister]

97.0mm bore, 74.4mm stroke, stock boost pressure = 0.7bar or about 10psi

[BlkBird]

So using Fred's figures and assuming a VE of 90% and a stock compression of 8:1 -> one bar (14.7psi) would yield a 13.6:1 compression ration when on full boost.

Assuming a VE of 80% and a stock compression of 8:1 -> one bar (14.7psi) would yield a 12.2:1 compression ration when on full boost.

Assuming a VE of 70% and a stock compression of 8:1 -> one bar (14.7psi) would yield a 10.8:1 compression ration when on full boost.

I'm not sure what the VE for a 930 is but I suspect we would fall under the category of "maximum RPM modified engines may have a better VE, eg: ... 65% to 80% " which is in line with the 12:1 compression ratio I've heard used when referring to our cars.

Sound about right?

[930turbo]

I backed into a VE of approximately 78% for the 911.

[fredmeister]

Stock compression for a 3.3 is 7.0 to 1.

You know what would be great is for a survey of a few users that have dynoed their cars and see if we could back into the efficiency ratings for each case to get a feeling for what it should be once we average all the results. This is just a guess for now without real data to back it up.

I had my completely stock 3.3 dynoed at 249 hp at the wheels at 5500rpm....
-------For example---------
1) doing the math yields: 249hp * 1/.88(12% loss at drivetrain)= 282hp at the crank.

2) using the chart on the site and assuming intercooler efficiency of 75% and a turbo efficiency of 60%yields:
ENGINE EFFICIENCY of 76%......not too bad.

Anyone else got some results to post?

[BlkBird]

Here's an interesting link showing dyno results for a number of Turbos, including my own. By the way, what does a stock 3.3 look like?

Turbo Guys Comment on this Dyno Sheet

[fredmeister]

A stock turbo is too fast to capture on film at rest.....needs a high speed camera, though I get your point.
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