Question about compression

[Emerger]

I hope this is in the right forum.

Is cylinder compression universal among outboards, or how much does it vary from manufacturer, type, etc.? Does anyone know of a comprehensive list of cylinder compressions for different outboards? I see a lot of ads for used outboards stating compression of XX in each cylider, but it's difficult to tell if it's good, bad, or indifferent. Thanks.

 

[Paul Moir]

Re: Question about compression

Johnson and Evinrude never published any compression specs to the best of my knowlege. For older outboards they said all cylinders must be within 10% of each other. I think on some later ones they extended it to 15%.

There was a reason for this. The only thing that varies as much as compression testers themselves are the methods employed when taking a reading. Hot, cold, number of compression strokes, how fast it's cranked, altitude, how much residual oil is in the cylinder all have major affects. Even throttle position and barometric pressure make for a slight change. (Although admittedly you would need to be taking the compression test during a hurricane for it to make more than a psi or two of difference. )

And to make matters even worse, you can not even compare similair outboards. The big V6 J/E engines tested new around 120 psi some years and around 80 psi other years. Even a slight change in port timing has a dramatic affect on "static" compression.

 

[Emerger]

Re: Question about compression

Thanks for the help. I understood that the compression should be equal (+/- %) among the cylinders, but I've seen equal compression advertised from 80 to 155 psi.

Again, thanks for clarifying.

 

[steelespike]

Re: Question about compression

Generally speaking you look for at least 100 PSI and within 10% cylinder to cylinder.For some reason some manufacturers don't release their numbers just the 10%
figure.I've found tune up figures from 115 to 165 psi.
I would say generally that newer smaller motors 7-50 hp would be in the range of 100-140psi.And larger motors in the range of 125-170 PSI.And it is important to be even cylinder to cylinder.I've seen smaller old timers run fine at 60 PSI.

 

[tashasdaddy]

Re: Question about compression

agree with Spike. the closer they are the better they run.