Leakdown and compression on 930
 

I had a prepurchase inspection done at a
factory Porsche dealer and they came up with
110 ibs across the board. I understand that
turbo static compression is lower therefore
the compression values will also be lowerthan
a 911SC. A couple months later at a different
shop (independent scabby place) I had a valve
adjustment and leakdown test done. They told
me the following 45%,18%,50%,8%,18%,8%.
Frankly, I am not sure I believe these guys.
Is it possible that they are being straight
up and yet my car has good compression or is
there an attempt to take me for a ride ?
Also, the car idles smoothly, has average oil
consumption, and has lots of power. Anybody??

Frankly, without calling into question their honesty or veracity, I would question their competence, as a leakdown test can be subject to human error that will give readings all over the place. If the shop is questionable for ANY reason whatsoever, don't go back! And, a leakdown test takes significant time, perhaps the better part of an hour, so I feel that if they 'talked' you into the test, or did it without you asking for it, they ARE after your wallet, pure and simple! Without knowing the history, mileage, and other information about your car and engine, I have doubts whether I could say one way or the other whether your engine needs a top-end rebuild at this time, but I doubt that it does based on your statement of a smooth idle. It would behoove you to do a bit of extra work, i.e., do your own valve adjustments, AND start keeping a paper record of the valve clearance measurement that you find at each valve and cylinder! It is normal for the clearance to 'open' up slightly between adjustments a thousandth or two on all 911 engines ... and my STRONG recommendation would be to raise a red flag immediately if you ever find any deviation from that tendency, as a 'closing up' of the clearance could be the sign of a valve that has overheated and 'stretched' a bit! A clearance that opens up more than normal could be burned valve, or have a carbon build-up on the seat that could lead quickly to a burned or 'dropped' valve, and I don't think you want to pay for a 'dropped' valve if you can avoid it!

What condition are the sparkplugs in? They are a very good indicator of the health of your engine. A check with a Porsche dealer mechanic might be worth your trouble. I suggest you email Bruce Anderson at Bruce1485@aol.com with as much information as possible and ask him about what to be watching for on your engine and about the advisability of a top-end rebuild before a major problem shows up. Good Luck!

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Warren Hall
1973 911S Targa

[This message has been edited by Early_S_Man (edited 02-08-2000).]

My car is a 1981 930 with 60,000 miles
(100,000 kms). The engine is quiet for a
911 based engine. The car has been in Canada
all of its life with the emissions removed.
If leakdown figures were that high, would one
notice the way the car runs. I would think
that it may not run smooth. For reference,
I am contemplating doing my own leakdown test
am would like to verify that z1 is TDC for
cylinder 1 and 4. Correct ?

Z1 is the TDC on cylinder 1 for the spark timing on THAT cylinder alone, and if you use the distributor rotor as an indicator you can make two more marks on the pulley, 120 degrees apart, with yellow paint or red fingernail polish. The TDC for each cylinder as indicated by the distributor rotor for aech cylinder, and valve clearance should be verified that both valves ARE closed, before trying the leakdown test. In addition the transmission should be put in top gear and handbrake applied so the engine cannot move! Any piston movement CAN affect the leakdown reading! And, believe me, 100 psi against a piston at TDC can move the pistons in an older engine!

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Warren Hall
1973 911S Targa

Can someone tell me that when doing a
compression test, should all of the spark
plugs be removed to let the momentum help
spin the motor faster or does it matter.
What do you think about 110 lbs of compression
for the turbo motor. DOES it sound okay and
what is a healthy range for turbos(actual #s)

Yes, all of the sparkplugs are supposed to be out during a compression test, and the engine is supposed to be warmed up to normal temp. ... NOT COLD! One other thing, the throttle is supposed to be wide open during the compression test, a detail some people forget.

I have no experience with 930 turbo engines, and don't know the exact compression ratio for your engine, but considering the low nominal range used on turbo engines, 6.5 to 7.5 ... I would think anything from 110 to 150 psi average (statistical mean) with no individual readings deviating more than 5% from that mean figure would indicate to me a perfectly healthy engine from the compression standpoint. If you are really concerned about the health of your engine, have you considered oil analysis? Again, an email to Bruce Anderson wouldn't hurt.

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Warren Hall
1973 911S Targa

[This message has been edited by Early_S_Man (edited 02-08-2000).]

After some detonation (yikes!) a leakdown test was done on my 87 930 with 26,000 miles. The test was done on a warm engine. All six cylinders were between 2 and 4 percent leakage. Yes, I was relieved. Your figures seem very high and weird to me; I would have another shop do another test. Let us know the results!

Vic
87 930

I just had a leakdown and compression test done on my 87 930 (1103K miles). The compression was 130 in all cylinders and the leakdowsn was between 7-10%. The engine runs nice and smooth.