Leak Down vs Compression tests
 

I've had leak down test on an 84 911 with 135K miles that I am looking to purchase (my first 911) . The results were 7% 5% 5% 6% 10% and 5%. Should I have a compression test done also? If so, what would a compression test show that a leak down would not show? At this point I am only interested inthetest to make sure I am not getting into something with serious engine trouble. Thanks.

Check out this article:

http://www.pelicanparts.com/techarticles/mult_Engine_Rebuild/mult_engine_rebuild-1.htm

-Wayne

Wayne,

Thanks for for your response. I've already read part one and part two of the article which were very good. I also purchased a copy of your book on 101 projects. I understand that a leak down test is a static test and a compression test is dynamic. I also understand that most everyone reccomends getting both tests done. What I don't understand is, what problems could a compressions test point to that a leak down could not. I was thinking of something like a bent valve stem. Thank you.

A leak down only checks the very top of the cylinder (and at a small fraction of the operating pressure). A cylinder that was scored/scratched part way down but not at the top might test fine with a leakdown. Good cylinders can have bad leakdowns and visa versa.
-Chris

Leak down tests and compression tests will often give different results. Leakdown tests are useful to determine where a problem lies - compression tests are good for checking the overall health, including cam timing, etc. I would get both tests done...

-Wayne

Anytime I see a leakdown greater than 5% I start looking around for problems.

Thanks for the feedback. I went ahead and had the compresssion test done. Here are the results: (compression/leakdown)

Cyl 1: 165# 7%
Cyl 2: 172# 5%
Cyl 3: 175# 5%
Cyl 4: 175# 6%
Cyl 5: 175# 10%
Cyl 6: 173# 5%

Based on the results I went ahead and purchased the car!

arigato,

Those are good numbers.

Take your new 911 out for some spirited driving and re-test. I’ll bet the numbers improve and even out. You will now have a good base-line for continued monitoring of your engine’s condition. With the right tools this is an easy DIY regular procedure.

Enjoy!

BTW, welcome to Pelican.

Best,
Grady

Grady,

Thanks. Fortuantely I bought the car 130 miles from home so I had a good chance to test it out on some turns and highway. What a car! I was blown away by the handling in the turns. On the highway it was just purring along at 80 mph. Very addictive.