compression and leak test
 

I have a 1976 911S 2.7L CIS. I recently did a compression test and a leak test. For each of the cyclinders, I got a range of 115-130 PSI for the compression test. When I did the leak test at 100 PSI I got a range from 10% -35%. I don't understand how the compression test could be close to the spec, and the leak test can be so out of whack. The Bruce Anderson book says that a good engine should leak test around 3%-5%. Any advice on whether I need to completly replace the P&C's or whether I can get away with just replacing the rings. I was hoping to wait until I "blew" the engine to do a complete the rebuild.

I'm not sure that the compression test came out all that great for you. The last time my SC was tested by Andial, it came out to 150 per cyl, exactly. You are getting almost 15% variation per cyl., which seems like it may be ok, but not great. 115 also seems a little low, but gauges vary.

Also, I don't think that there is necessarily a correlation between a compression test and leakdown tests. In other words, you could get uniform numbers across the cyls on a compression test, but still have a bad leakdown test. That just means the engine has worn down uniformly!

I'm sure others will have some comments on this.

Brian

You said you wanted to wait until you "blew your engine" before doing a rebuild. Waiting for a catastrophic failure is an expensive propistion. If your engine is old an tiered do not wait for a major failure before thinking about a rebuild.

I suggest you read the thread on 'Leakdown and compression on 930' !!! I echo the concern of others about your comment about waiting for the engine to 'blow' ... was that a frivolous comment or a 'sane' opininion of your true intentions?

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Warren Hall
1973 911S Targa

I was hasty is saying that I was waiting for the engine to blow to do a complete rebuild. It would probably be better to ask whether the leak down numbers that I have received warrented a rebuild. If the compression test is borderline and obviously the leakdown test is not even close to the numbers I should be getting, does this tell me that I need a rebuild now, or can I wait. Can I get away with just a re-ring job. I realize that waiting for a major failure within the engine can make my rebuild more costly. I have only been working on the car for around a year and half, and I'm looking for people that have more experience to guide me with some input. The mechanic that I work with recommends a rebuild based on these tests, but I would like some verification that this is a common conclusion. I have also read that the way that the compression comes up during the test can provide valuable information. I not sure, but I think that if the compression is low in a cylinder, if you put some oil in the cylinder and redo the test and the compression come up, this means that the rings are bad.

Assuming your cylinders are Alusil, it is unlikely you can successfully re-ring them. Since valve guide wear is the major problem with 2.7 engines, I suggest a top-end rebuild as soon as the budget permits. Used RS 2.7 (Nikasil, 8.5 comp. ratio) pistons & cylinders are also a possibility.

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Warren Hall
1973 911S Targa