Compression Check Numbers ?

[SHOBound]

What kind of numbers should I look for when doing a compression check ? From my experience, each cylinder should be within 5-7 points of eachother, 10 is bad, but not life threatening, correct ?

<small>[ March 01, 2004, 12:42 AM: Message edited by: SHOBound ]</small>

 

[DHMag]

never heard of "points".....but standard is each cylinder within 15% +/- of its neighboring cylinder. wanna get critical, use 10%. good compression numbers are 220-230. lowest ive seen is 160

 

[sdpatt]

220-230 would be incredible compression numbers for the SHO engine. I have seen numbers from 190-210 psig being in the healthy range for the 9.8:1 compression SHO motor.

In practice, the outcome will vary by what method of attachment you have to the spark plug hole. The longer the tubing (greater volume) from the plug threads to the pressure gauge, the lower the indicated nubmers will be. The volume of the tubing will add to the combustion chamber volume and effectively reduce the compression ratio. You want to have the smallest possible volume of tubing outside the combustion chamber.

The numbers a measured on my very seasoned engine around 260,000 miles were in the 185-190 psig range. That is rather amazing ring and valve sealing performance. Kudos to Yamaha.

 

[SHOBound]

When I mean points, I mean the numbers the compression guage puts up, sorry, I should have been more clear. My boat has a 9.25:1 comp ratio engine and it shows between 157 and 162 on all cylinders, except one at 138 which is the ring I ground down too much on my low gap rings.... Oh well still runs fine, I just wanted to make sure all my cylinders werent holding around 130 psi in my SHO and me being happy they are all the same. Thanks guys !