Compression Test
 

Greetings all,
I was wondering if you guys could help me out. I had a compression test done on my car when I got it - about 10,000 miles ago. These are going to be some stupid questions - but please bear with me - I'm now an expert on 915 transmissions and am shifting my focus :)
1. The numbers were: 180, 180, 175, 180, 175, 180. What the heck does this mean??
2. Is a Compression Test the same thing thats referred to as a leak down?
3. If these were the numbers 10,000 miles ago - would you think that things have changed drastically since then?

I think thats about all the knucklehead questions for now.

Mike

1. It means your engine is fine.

2. Compression measures the air pressure the cylinders create as they cycle. A leak down is the opposite, it puts compressed air into the cylinder and measures leakage.

3. Not likely, has your engine had a life changing experience in the last 10,000 miles? Just drive it.

Those numbers, of course, indicate the psi in each cylinder and the ones you posted look good. The main thing with compression numbers is the difference between cylinders--typically should no more than a 10% difference, max, between cylinders.

This is different than leak down numbers which is probably a better indication of engine soundness than a plain compression test. Leak down involves pressurizing each cylinder with air and measuring the percentage of air lost. The results are expressed in percentages. These numbers will give clues to valves, valve guides, piston ring conditions and general health of the engine. You should have this test done by a knowledgable Porsche mechanic.

As far as having the test done 10,000 miles ago, you shouldn't see much change but remember a new test even on the same day as the previous test might produce different absolute numbers--no biggie as long as the relative differences are within specs, and the absolute numbers are not way different than the first test.

Geehrte Herren,
I thank you for the plain language explanation. Now a leak down - how much does something like this normally run? As I said, I have nw become an expert on transmission rebuilding - and after regearing, replacing the clutch, replacing the rear axles - my pocket book is considerably lighter. I have some concerns about oil useage - but in addition to the mentioned repairs, I also fixed all the possibilities for oil leaks - while I was in there. So, hopefully the oil consumption will have improved. But from my reading - a leakdown would be the best indicator. Thanks so much - again.

Mike

Leakdown should be around two hours labour.

But if you do not plan to spend the money on a rebuild, why bother? Drive it.

I got it. For peace of mind I guess. Probably a better plan would be to monitor oil consumption now after all the work I have done - and then if I still have my doubts - well then go ahead and have the leakdown done.

Put a few thousand miles on it, monitor your oil usage and report back here.

What engine/car is this? A 915 could be a on a 2.7, a 3.0, a 3.2 ....

Jawohl - da hast du recht :)
Its an 84 911 cab. Now it has 55k on it - but I have my doubts about this. I've been following Christien's thread - both reading it and executing. My repair was a bit different. While I had to replace synchros and normal wear items - I also went ahead and regeared it. Right now everything is sitting sparkling clean in my gaage. I just have not had the time to reinstall the motor/tranny. Thats Friday's project. Hopefully I'll be making the same Whooooooo Hooooooo report as Christien real soon.

I believe that 3.2 was susceptible to premature valve guide wear. That would be my guess if your oil consumption is high. Unless you are spewing white smoke when you idle or accelerate I would just enjoy the car and monitor consumption. Oil is still cheap enough to burn.

Regardless, you own the greatest body with the best engine Porsche ever built, or ever will build. Until I upgrade to a G50 car.

:D And it will now be even faster - with the new ratios!!! I am so excited - I can hardly wait! so, you really do think that its a great motor - huh? After doing extensive work to it - I have been thoroughly impressed - though I don't have anything to compare it to. Before tearing into the tranny - I knew how to change the oil! But the thought processes behind the construction and the overall quality - wow!

155,000 happy miles on my car.

Thats great,
Before I did the work, I got white smoke on start up - but that was after it had been sitting for quite a while. And with the tranny going bad I really did not drive it a whole lot. Normally -I got zero smoke. As I said, can't wait to start driving this machine again - I mean really driving it. Then I'll see what my oil use is.

Mike, those compression test results have a very specific and very important meaning that you must heed. They mean your engine is plenty healthy. They mean you should drive the heck out of this car and find something else to worry about.

Great! As I have mentioned, I have just gotten done sinking about 6k into a regeared, rebuilt tranny, a new clutch, new rear axles and cv's, all repaired oil seals and hoses. Before I began this pressing repair, like I said, I hadnt driven the car a whole lot - because the synchros were going bad. My oil consumption was questionabe - but then again, I hadn't driven it a whole lot - so 1) I questioned the reading and 2) all the oil seals/hoses hadn't been touched. So, hence my question about the compression as well as its meaning. I can't wait to get that thing going again (tomorrow :)) and "drive the heck out of it" Like it was meant to be driven. Before I reinstall the motor, I'll post some pics of the work I have just done! Thanks so much for the input.

Mike