Compression Tests

[murnmill]

What would be an acceptable compression variance (+/- psi) for a 1989 Mariah? thanks!

[haugy]

If I remember right, and I probably don't, I wanna say the range you would like to be in is 120-140 PSI for an engine of that age.

It's 8.5:1 ratio.

[suprasam]

I think you want to stay within 10% but not sure it works the same on a standard motor versus a 2stroke.....I always thought it was compared different

[ts6mnewbie]

I am looking at a 1990 comp right now with a questionable variance myself. The PCM manual does't say what the compression psi
should be but it does say that if the lowest reading is within 75% of the highest it is ok. That seems like a lot to me. The only
other info I found was that allegedly these motors can still run good at as low as 90 psi. The boat that I'm looking at is between
100-125 psi. Still waiting to test it on the water.....

[michael hunter]

Those readings are not bad. Can you post each cylinders reading and how the test was run? How many hours are on the engine?

[murnmill]

the 1989 Mariah I'm interested in reads;

Compression is:
#1 145
#2 150
#3 150
#4 145
#5 135
#6 145
#7 145
#8 145

All within 10%. Not sure why they are so high. It has the 330 hp PCM. Higher HP = Higher PSI??? idk.