Compression Ratio on the Mahle 3.4?

[Jgordon]

I'm trying to give Steve Wong an idea of the specs of the engine so that he can write a chip for me.

Can anyone tell me the compression ratio of the Mahle 3.4 system on a factory 3.2 crank and rods?

Thanks.

[asphaltgambler]

I think there two different static ratios available for the 98mm.

[Carrera3.5L]

Originally posted by Jgordon
I'm trying to give Steve Wong an idea of the specs of the engine so that he can write a chip for me.

Can anyone tell me the compression ratio of the Mahle 3.4 system on a factory 3.2 crank and rods?

Thanks.

There are many other factors involved that help determine the measured compression ratio of the motor. You simply can't take what Mahle says as gospel because they are just providing a baseline. You may be considering twin-ignition and head surfacing will be required at the very least, both of which will have an effect on the outcome.

FWIW, the DME domed set is rated at 9.8:1 and the mfi/carb version is rated at 10.3:1. Your professional engine builder should/will measure once the machine work is completed and the motor begins to be assembled.

There are machining methods available to "adjust" the compression ratio if you or your builder are not happy with the measured outcome.

Do a search on my screen name, plenty of info to chew over the past 6-9 months.

Ralph

[Jgordon]

Yep, once the machine work is done, we're taking measurements and dynoing it, providing steve with a more exact set of data (AFR info, CR, etc.) to refine the program and bring us closer to perfection. But that 9.8 to 1 ratio is what I was looking for, to help us get started and have a program for the engine to use on the dyno.

Thanks very much, I've been enjoying reading your exploits and will continue to do so.

[wholberg]

Once the engine is assembled and a starter is hooked up, run a dynamic compression test and divide the result for each cylinder by 14.7, that will give you the exact compression ratio for each cylinder. Average all six for the engine compression ratio. A compression ration of 9.8:1 should give compression results of 9.8 times 14.7 = 144 psi.

I suspect that the result will be a little low if the compression rings are not broken in, so you might recheck at 1000 miles or so, just to verify the compression.

If you want to be really accurate, you can divide by the atmospheric pressure on the day you run the test.

One atmosphere at sea level averages 14.7 psi = 29.92 inches of mercury = 760 mm of mercury = 760 torr = 1013.25 millibar. Your local weather service will report barometric pressure in one of these units.

[Jgordon]

Wow. I'll see how they measure it, and humbly suggest some of this stuff.

Thanks again.

[wholberg]

Not to beat a dead horse, but something that I found interesting is that in Denver, atmospheric pressure is about 83% of sea level, so 14.7 psi times 0.83 times 9.8 equals 119.52 psi for a compression test, which would be equal to 144 psi at sea level.

So 120 psi compression in Denver would be great, but if I had 120 psi in a compression test here in NC, I'de start planning to tear down the engine.

Of course, if you have boost, all bets are off!