what should compression readings be 69 911s?

[blucille]

looking at a '69 911 S, 2.0 orig motor, car has all documentation since 1974, and no rebuilds, compression is within 5% of each other, but seems consistently low, it is possible low compression pistons were installed before 1974...compression readings 4-5 years ago gave the same results as today....so, what numbers should I look for???

-Lewis

[kenikh]

My '69 911S just had a compression/leakdown on its PPI. Motor is relatively fresh and we got ~125 psi w/ 6% - 7% leak down across all cylinders.

[silverc4s]

compression readings can be low on high compression, high performance engines because of valve timing overlap.
Check leak down too, and if all cylinders are good, there is probably not a problem..

[A.T.]

Hey Lewis

I recently had my '69 S engine rebuilt. After some mileage the compression values came out to 162-165 psi and the leak down values were 2-2.5%. This is according to my mechanic who did the rebuild. Everything was to original spec. Does this sound about right to the experts?

A.T.

[kenikh]

I would have never imagine you could psi readings that high on an 'S' cam. I know my motor is about 15K miles old and I was getting the numbers quoted above.

[A.T.]

Kenikh,

My numbers were in Canadian psi so if you figure with the exchange your numbers are probably bang on.

To be honest, I have a vague idea what these numbers mean but no idea if my numbers are in the ball park or totally unbelievable. The compression ratio should be 9.9:1. Again, no idea what that means. Can some one give us the short explanation?

Thanks

A.T.

[kenikh]

LOL. That actually ran through my mind, but I try to keep the Canadian jokes to a minimum. Too many Canadian friends to get off lightly!

CR and psi are only vaguely related. CR is the ratio that the intake charge is compressed on the compression stroke, while the psi is the actual maximum pressure created in the chamber ( I believe it is gereated at TDC? ). Even if you had a hole in your piston, CR would still be the same (since it is a calculation) while you would generate no pressure (zero psi) since the cylinder would no longer be sealed.

Did I miss anything?