Compression and leakdown tests

[vicv]

I'm looking for either a 911 or 930 (1986 to 1989) and want to get one with a good motor. I therefore want to get a compression and leakdown test performed before I buy. What is a good compression number and a good percentage for the leakdown test. Do you know of any other tests or warning signs I should be aware of. This is my first time buying a porsche and can use some help. -- Thanks

[ianc]

Generally, compressions should be in the 175 region, but this figure can vary widely based on many factors, including the particular gauge that's used, whether the engine is hot or cold, the altitude, etc. Rather than looking for an absolute value, you should look to see that all values are relatively equal.

Leakdowns will also vary somewhat; I've heard of acceptable values being between 5 and 15%, and these are also dependent on the specific gauge, temperature, etc.

Be aware that some engine problems (most notably valve guides) won't show in either of these tests, so the condition of plugs and oil consumption will be your guide here.

Buy Bruce Anderson's or Wayne's book. Both give excellent advice on buying 911's. Do your research and don't jump into it lightly. Good luck!

ianc

[Noah930]

Due to the much lower compression ratio of the turbo motor, the compression numbers will be quite a bit lower than the normally-aspirated ones. Somewhere in the 120-130 range, I think. Again, compression numbers may vary greatly depending on whether or not the wrench who did it actually knows the proper procedure or not. For starters, the motor's supposed to be warm. I had to ask the reputed Porsche specialist who did my PPI to repeat the test due to the initial numbers being ridiculously low (yet purportedly "spot on," per the mechanic).

[Rude74]

Can you do a compression test on a wrecked car that is cold and has no oil in it? Is it worth the trouble?

[HaroldMHedge]

There is always some oil left in the engine so don't crank it for a long time.

I was thinking of purchasing an engine which had been pulled from a wreck. We cranked the engine using a battery to jump the starter. Pressure was normal on 4 cylinders, low on one and zero on the last one. Needless to say I did not buy that engine.

I was looking for consistent numbers between the cylinders not a specific number.

[Coastr]

Originally Posted by vicv

I'm looking for either a 911 or 930 (1986 to 1989) and want to get one with a good motor. I therefore want to get a compression and leakdown test performed before I buy. What is a good compression number and a good percentage for the leakdown test. Do you know of any other tests or warning signs I should be aware of. This is my first time buying a porsche and can use some help. -- Thanks

Pay a shop to pull the valve covers and check the head studs. It will cost you oil and a gasket kit & time, but will avoid getting a very costly top end (or more) rebuild.

Check the front suspension pan for rust, both inside and underneath. Pull out the spare wheel and check the metal all along the front. Batteries overcharge and leak and rust it out.

As noted valve guides are a weak spot on the 3.2 motors in the 911 so ask some questions about oil consumption and see what answer you get. There really is no static test for that, save for looking around an owners garage for empty oil containers of 20w50. You could check for lots of blue smoke at idle or after trailing throttle, but lack of smoke does not mean everything is OK.

Try and listen to a few 911 engines so you get an idea of what sounds normal, so when you listen to a car you can tell one with some strange noises. Also check the gearshift if a 915 as 1st-2nd can be very touchy.

[85911]

For your reference, I performed a compression and leak down test on my '85 911 in 2015 with 67,353 miles on it and here are the results. The car runs well and the plugs look good.

Compression Leak Down
#1 175 2.2%
#2 180 0.0%
#3 180 2.2%
#4 177 3.3%
#5 180 1.1%
#6 180 2.2%