I've searched some and couldn't get straightforward answers on what good numbers should be for engine compression. I just bought a '93 FD, and the compression test was done at RP Performance at the request of the dealer when it was put up for sale. Here's the numbers:

F: 8.3, 8.7, 8.3
R: 8.2, 7.9, 8.0

Are these good? If not, then what would be good numbers? I'm asking because I know a rebuild will be in order in the future--just depends how long. In that line of thinking--what kind of numbers should alert me to an engine rebuild? And is compression testing the most accurate indicator of engine condition?

And here's a super noob question: I've seen engine compression numbers in psi and numbers like the above. What are the units of measurement? If anyone can point me to an engine compression FAQ (don't really understand it), it'd be greatly appreciated.

Thanks in advance

 

Those seem fine. They are probably in kg/cm2 by the way. Usually, anything above 7 kg/cm2 is in decent condition. Perfect is around 9-9.5ish. The more important part is that there are no outliers in your numbers; they are all pretty close together, which shows that you don't have any damage on any of the rotor faces.

Taken from Rob Robinette's site:
"All compression above 7 kgf/cm2, 100 psi is good (no rebuild needed). Anything below 5.5 is likely to break down when driving the car home. You should not see differences of more than 1 kg between chambers and 1.5 between rotors for a good engine. Unless it has been partially rebuilt, where you can have strange values like 9 front and 6 rear."

 

Thanks for the info! I even have Rob's site bookmarked...guess I shoulda checked there. :duh:

 

The tester gives results in kg/cm^2. Most of us are accustomed to reading results in psi. I believe the conversion factor is 14.22 to go from metric to psi.

For reference, 85psi is mazda's lowest acceptable compression limit, but in reality turbo rotaries have a good way of turning anything below 95psi into zero within just a few months. 100psi and up is healthy, 110 and up is great, 120 and up is excellent. The highest test I have ever seen (within proper test parameters) on a rotary was 130psi.

Also there should be no more than 20psi difference between faces/chambers, really you dont want to see more than 10.

Your numbers look great, btw.

 

Thanks, man. Definitely good to hear from an engine rebuilder. Much appreciated info on compression numbers, too.

 

Perfect is 8.5 all around, so your engine test essentially new. Excellent numbers!

 

Essentially new? That's pretty cool. Thanks again!

 

Front: 8.4 - 8.4 - 8.3

Rear: 8.1 - 7.9 - 8.1

On my motor. Tested on 11-21-05

 

I wonder if it's common for the middle rear number (sorry, don't know what that is technically is called) to be the lowest. I see Yoshi's at 7.9 like mine, being the lowest of the group.

 

Which faces fall in which position on the tester chart is totally random and irrelevant.