Compression Test questions
 

I know another compression thread, but I keep reading mixed results that people get on a piston compression tester and it made me a little confused about my results. Keep in mind the valve on mine was broken and wouldn't let the numbers build up when I wasn't holding down the valve.

I read that with the valve pressed in people get 30-35 psi but I was getting 60s.

And with the valve completely taken out people get 100+ but I was at 80-90 psi.

Test was with a used long block.

We going to have another tester tomorrow so we can see the build up numbers, just wanted to see what you think of the results.

The most important thing on a compression test is EVEN BOUNCES, period. If you have a high bounce and two low bounces, that's a broken apex seal.

The number it builds up to is fun, but IMHO that doesn't really tell me anything worthwhile. Idle vacuum on a running engine tells me a lot more about general engine health since it's warmed up and running.

If this is important to you go buy a good compression tester, they're like $30 at a parts store. Heck, I think some parts stores might do a free loaner.

The Mazda tester can give the compression health of each rotor face which is the big difference there. But, again, all you're really looking for with a compression test is even bounces.

Dale

Bounces were even. The tester I had was from a auto parts store haha. Just a little worried because I have never seen so much carbon in my life. Plugs looked quite nasty. Looks like once its in I'm going to clean engine. Found a thread on dumping water in their engines to steam clean it like water injection.

I rarely disagree with Dale but the overall number is important. It gives you an idea of how the car is going to hot start. Anything below 90 is starting to get pretty weak even with even bounces. Is the engine already installed in the car? If so just look at the vacuum that it is pulling and see how well it hot starts once the engine bay is heat soaked. That will tell you more than the compression number will.

Engine is out of car. Just wanted to make sure its good on compression before throwing her in. I thought even bounces are suppose to be in the 30-35 psi range?

Dale was referring to peak compression, ie using the piston tester in its original format. this test doesn't show uneven bounces which will indicate an issue with internal sealing. piston testers only test 1 cylinder at a time so peak will almost always be accurate, with rotary engines you are testing 3 sealed rotor faces so the tester has to be modified.

most tests are done with the schrader removed from the hose end where it meets the engine.

the test holding down the compression release is what i call the "lazy man's test" and really doesn't tell you much if anything at all. 30psi gives a very narrow range for sticky seals to show up during the test.

WOT with cranking speeds near or above 250 RPMs the minimum compression you should see is 85psi on any given rotor face before starting issues will arise.

So this engine is potentially out of spec?

try a different gauge if yours isn't building up each rotor face properly without holding the valve open.