Acceptable compression test results.

[apegs]

Buying a used boat soon, I had the guy take it in for a compressiin test. Wondering whats average or acceptable numbers. They fluctuate between 110 and 120 through sux hours. What does this mean?

[gritsnham]

I remember going through this when getting my boat last year but don't remember the specifics. It depends on motor size and model I believe and they should be relatively close (within 10% of each other I believe).

I would do some googling and remember a boat forum having some good threads on this...otherwise get some info from the mechanic you paid to test it.

What rig ya looking at?

[TR21HP]

apegs,

10-15 psi is an acceptable range for cylinder comparison. Very few motors can run under 100 psi so the 110 seems a little low, by low I mean there is an amount of wear on that cylinder. This could be attributed to a number of things but I seem to recall that 120-135 psi is preferrable as far as compression results. I would ask a marine mechanic about these numbers relating to the type of motor you are looking at as suggested by gritsnham.

[apegs]

I remember going through this when getting my boat last year but don't remember the specifics. It depends on motor size and model I believe and they should be relatively close (within 10% of each other I believe).

I would do some googling and remember a boat forum having some good threads on this...otherwise get some info from the mechanic you paid to test it.

What rig ya looking at?

Its a 01 triton with a 150 hp mercury xr6

[TR21HP]

Apegs,

Specs on that motor range from 110-135 psi. The lower the compression usually the more wear on that cylinder. As I said before very few 150's run on less than 100 psi so that would be a concern if I were buying it.

[panolo]

I run a repair center and usually on something 10 years old like that 120#'s is fine. There are a few things that can skew a compression test like fluid in the cylinders, carbon, etc. But as others have stated motors usually don't run well when under a 100#'s or there is more than 15#'s difference between cylinders.

If you have access to a small camera it's pretty easy to slide it into the cylinder holes and slowly rotate the motor. If you see scratching on the walls of the cylinders, hot spots on the center of the piston, no cross hatch left on the cylinder walls it is probably going to need a rebuild.

Good luck!

[panolo]

I should add also a big thing with the XR6 is that it tends to be cold blooded. Make sure it has a good primer bulb and main line from tank to motor. Also any good mechanic can richen it up a bit for starting. Mercury seemed to be very lean with the fuel at start up.

[gritsnham]

Looked over the marine report I got on my boat for my 150hp yamaha ox66 1999. Results had all cylinders between 125-130. You are going towards the lower range it seems but all are within close range of each other.

I would get as much info from that mechanic as you can...would imagine they have a lot of experience with these motors.