Early 911 - What compression test values should you expect?

[Jim Richards]

I was talking to a seller about his car ('72 911T) and he said his engine compression numbers were ~120 +/- 2. Is this good, bad, or just so-so? Thanks!

[JackT]

my test were 185 psi to 190 psi.

My 82 911SC has 96,000 miles on it.

[jpnovak]

Jim,

I think that you take the normal atmospheric pressure (14.7 psi) and multiply it by the c.r. (8.5:1 for 72 T) to get 125 psi. I would think that 120 +/- 2 psi sounds good. Nice and even numbers. Was the engine hot or cold for the test? usually tests are done at operating temp.

Jamie

[Hladun]

For people that are obsessed with their cars, it's amazing how little information or consensus there is on compression tests. However, the values are affected by CR, cams, altitude, engine temperature and how the test is actually done. The only thing I've ever seen from Porsche is that they should be essentially equal.

When I bought my 78SC I got around 120 psi readings. I've driven the car 20K miles since with no oil consumption or running problems. DrewT's readings are high and would indicate high CR pistons or some other factor. The best I could gather is that 150 or so is a good number.

[rvanderpyl]

Don't have the spec book for 72, but the 69 book lists 9-11 kp/cm2 for the T to S engines, or 128-156 lbs/in with a max deviation of 1.5 kp.cm2 ( 21.33 ) between cylinders with engine oil at 60c.

Later spec books don't list a compression value, just a deviation.

[marcesq]

Jim:

get a leak down test as well if you are serious. That combined with the compression test results hedges your bets against problems in the near future.

See you Sunday.

[Early_S_Man]

The '72 USA MFI engine only had a 8.0:1 compression ratio, and the ROW carbed engine only had 7.5:1 compression ratio! The reason there isn't a consensus among early owners is the wide disparity in compression ... from 7.5:1 all the way to 9.8:1! And, not every owner has tested the compression, personally!

The 120 psi median is probably fine for a 31-year old engine with a low compression ratio. The small deviation is good, and the '72-'73 spec book only gives the deviation figure cited above.

[DavidPorter]

I got 130 - 135 on my '72T with an old Snap-On screw-in tester. Unfortunately that was on four cylinders. The others were 10 and 25.

Waiting for The Book.

Hi,

reading through the posts above,

so is a 120 median reading Ok for the 3.0 1978 911sc?

Provided all the cylinders are all equal ( or say +/- 1 or 2 )

what are the overall limits, how low or high before there's a major problem?

Thanks

[Wayne 962]

Here's a tech article on compression tests, and why you can't compare numbers to other owners:

http://www.pelicanparts.com/techarticles/mult_Engine_Rebuild/mult_engine_rebuild-2.htm

-Wayne