finding out CR from compression test?

piggdekk · 05-15-2011, 10:24 AM

Hi,
I've got plenty of invoices and papers on my 71 911 T from the 70's all the way to the 90's. The car was sold first time in the US (Oregon) and seriveced and kept there, I believe, until 2006 when it was imported to Europe. However, I have no info about who rebuilt the engine and when. The engine has been rebuilt quite recently anyway, as it looks nice and clean. In a paper from 1984 they claim the engine is a rebuilt 2.4T.
Among all the papers I've got the are 3 different compression tests. One of them is from the 80's. The average was 110. then I've got one from 2003, the average is 140-145. The last one I've got is dated 2006, and the average is 170-175.
The first test looks reasonable for a T engine 110/14=7.8, the second one looks high for a T engine, and I believe the third one is too high for any porsche engine, but I have no experience with compression tests.
Based on Your knowledge, can I estimate which motor I've got from the data I have?
The car is a matching number, so whatever they did they did it on the original engine.
luca

jstobo · 05-16-2011, 09:46 AM

Compression ratio(CR) is defined as the volume of the cylinder at BDC(Bottom Dead center) divided by the volume at TDC(Top Dead Center). It is a characteristic of the cylinder and combustion chamber design. Your pressure readings from a compression test will not indicate the CR of a motor. Too many other variables such as temperature of the motor, intake, leaks from cylinder etc will affect the pressure test. I hope this helps.

Flieger · Flieger's Garage

PV=nRT

piggdekk · 05-16-2011, 01:17 PM

I understand that there are other variables, but leaks will result into a lower compression, and therefore under estimate the CR.
As for PV=nRT I This you're right Flieger. That's where I got it wrong. The compression test maybe resambles an adiabatic compression (not enough time to exchange heat), so the correct formula should be: PV**gamma = constant.
If that is true, than a compression test giving a reading of 170 PSI should correspond to a volume ratio of about 6 or so. Which makes more sense considering that the engine is having some leaks.
luca

Hugh R · 05-16-2011, 01:59 PM

Cold ignoring heat from compression and you could get local air pressure and convert to PSI. Then for example divide 170/14.7 =11.8:1 assuming 14.7 is your ambient air pressure.

Tyson Schmidt · 05-16-2011, 03:21 PM

How do you factor in valve timing?

Some cams don't close the intake valve until the piston is nearly half way up on the compression stroke.

I suppose with a compression test you are getting a dynamic compression ratio versus static.

aadrew10 · aadrew10's Garage

Quote:

Originally Posted by Hugh R

Cold ignoring heat from compression and you could get local air pressure and convert to PSI. Then for example divide 170/14.7 =11.8:1 assuming 14.7 is your ambient air pressure.

You can't ignore the heat generation, it has a significant impact on the pressure reading. If you let the air cool down, you've lost a lot due to leaks anyways. Then add in the above mentioned valve timing issues, and you may as well guess a CR of 10 plus or minus 5.

You could put a tube with the same fitting as the threads of a spark plug. Then, screw the end of the tube into the spark plug hole on a cylinder. Then, as the engine turns over, somehow measure the volume of air that is sucked in and out. You still have the potential valve overlap issues however. If you are careful, you may be able to tell if there are overlap issues by observing when there is suction or positive pressure in relation to crank position. Once all pressure or vacuum disappears, you know a valve has just opened.

As for me, I believe my engine has a compression ratio of SWEET!