924Board.org

Aussie931 · Joined: 28 Aug 2006 Posts: 55 Location: Canberra

Hi,
Does anyone know what the compression reading is supposed to be for a 931 with standard 7.5:1 pistons. The Haynes manual says around 150-160 but this value is obviously for the 924NA. I did my own calcs by converting 7.5 atmospheres to PSI and got 112PSI but just wanted to double check that this is correct.

thanks.

Smoothie · Posted: Tue Aug 29, 2006 1:15 am Post subject:

My engine's comp ratio is 8.0:1 and doing the math (8 x 14.7) gives 117.6, but the average from my last comp test was 147psi. Even using the highest stock 924na ratio, 9.3:1, (9.3 x 14.7 = 136.7) doesn't give you 150-160, so apparently you can't go by the calculation alone.
Results of my last comp test and specs from the Porsche Tech Specs booklet are here - http://www.924board.org/viewtopic.php?t=18382
One more thread with test info - http://www.924board.org/viewtopic.php?t=19232
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

-nick · Joined: 16 Nov 2002 Posts: 2699 Location: Cambridge, MA

The multiple is typically anything from 16 to 21 times your compression ratio. Even at the speeds your starter can turn, the incoming air has enough momentum to keep filling the cylinders even as the piston is on its way back up during the compression stroke (the intake valve is still open for some of this rotation). That is what makes static compression in psi higher than just atmospheric times the ratio.

From my notes, an 8.5:1 motor should be around 155-160psi, 8.0:1 is around 140-150, and 7.5:1 around 125-135. This is just a rough guide though. The multiple is somewhere around 18 times the compression ratio for our motors.

Khal · Posted: Tue Aug 29, 2006 2:50 am Post subject:

Here's the results from mine, haven't checked it since we rebuilt the head...
_________________
'80 924 Turbo

Aussie931 · Joined: 28 Aug 2006 Posts: 55 Location: Canberra

Hi all,

Thanks alot for the info. I did a compression test using two different gauges and the results were 110, 110, 112, 111 psi with the first and 150, 150, 152, 152 with the second. I'm guessing that the 2nd gauge is the more accurate of the two. May have to get another for a third opinion.

thanks.

_________________
1979 Series 1 924 turbo