leakdown tests ........accurate ??

Hello, I was just wondering how accurate everyone feels about leakdown tests. I have heard various opinions and would like to know for sure what the facts are ? Most say leakdown tests are totally reliable while recently I was reading an old tech article from PANORAMA and Bruce Anderson states that he does not have faith in it for a couple of reasons. He mentioned that he knew of an engine that tested with excellent results(~5%) but had broken rings. I have also heard that the air cooled engines tend to go slightly out of round when cooling down and may show poor results.

Reason I was wondering is that a friend of mine had a compression test done at a PORSCHE dealer when he purchased his 930 turbo. Shortly after, for ****s and giggles, he had a leakdown test done at an independent shop. His compression numbers were all about the same, but the independent shop told him that his leakdown results were poor. He had something like 90%, 8%, 55%, 15%, and so on. His car seems to run quite well and is very quick. What do ya think guys..

Jerry S

My opinion is if the compression result is good, there is no reason to worry about the leak down test especially you said the car runs well. I don't usually trust independent shops unless I know them very well or have a good reference. I think leak down test is better for diagnose the problem than to check the health of the engine.
Andy
'87 Carrera

Are you serious about the " 90%, 8%, 55%, 15%..." figures? I'd be inclined to think either that engine couldn't run (at least not well) or the testers didn't do the job right.

I agree with Bill 90% would give you a dead hole or damm near And 55%is real week I think the car would be down on power A turbo should be between 3%-8%

Yes, they told him that there was one cylinder at around 90 % and actually a couple that were between 40 -55 %. My guess is if these numbers were true that the car would probably not run well at all. He raced and beat the nuts off of a 944 turbo. Also got a chance against a 2000 911 carrera and pulled slightly ahead and maintained the
lead. Does this sound like a very tired turbo ? Thanks for the replies....

Jerry S

i would certainly get another opinion. it's common to get a high leakdown reading and then put the plugs back in, run it, and recheck the "bad" cylinder to find it now reads 6%. (the carbon under the valve trick).