Compression test results, need help

[Eddmond]

I bought my 1969 911 coupe twelve years ago. The car is quite fast and I got the reciept for the engine work.
2.4 case, head work, "S" valves, solex cams, webers, headers and a sport muffler. The paper work shows the part numbers for
2.7 RS pistons and cylinders. I've read that this 2.7 conversion gives me a 8.5:1 compression ratio.

Recently I was worried about loose head studs and I decided to take a compression check. The results are: 167 168 170 171 171 168. I don't know what the relation is between compression ratios and compression test results.

So, my question is, being that my compression test results are 170 average is it possible that I don't have the RS pistons?

This puzzles me because I've seen compression tests results on other threads and they are in the 120 range for older 911's.

Please educate me.

Thanks.

[ttweed]

Eddmond-

Don't worry, be happy. Those are excellent results and indicate that your engine is in good shape. The absolute value of the reading is not as important as consistency across all the cylinders.

There can be variations in the calibration of the gauges used in different tests, as well as differences in the combustion chamber pressures caused by the cam's profile. You have Solex cams instead of "S" cams, you say, so I would expect higher pressures due to the lower duration and overlap. If the valves are open for longer on the compression stroke, there is less time for the pressure to build in the cylinder.

TT

[RC70SCoupe]

I have thought of having a compression check on my motor.
Its a 2.7RS Spec motor with MFI.
I know that consistency is important. What should the number range be?

[ttweed]

If all the cylinders were within 5% of each other, I would be very happy. 10% would be OK. Leakdown in the same sort of range would pretty much insure the engine is healthy internally.

TT

[Eddmond]

Thanks Tom!
Your help is Greatly Appreciated.
Regards, Edmond

[GRMPer]

With much trepidation, I tested our 69E this afternoon. Sitting for 10 years, runs well...but you never know.

So, they all wound up being between 170 and 178....woohoo!

Per