What is a good compression for the 15 Mercury A and the 15 OMC A motors? I have ben told that many guys do not consider compression very important. I disagree and think that compression does have an affect on acceleration not necesarily top end. Anyway I am trying to establish a base line to follow when considering a re-ring or bore job on a block. Any advice would be helpful. Also does anyone have any idea what the rpm range should be on the straights and corners to achieve optimum performance?

What prompts this post is I took a compression on an OMC and had 80psi. A leak down test showed a 40psi drop above the ports and a quick seal before the ports opened. Mesuring the no 2 cylinder showed .003 out of round, .003 taper and .009 clearance. WOW. No wonder I was 1.st, 2nd or 3rd across the line then still ended up last in every race.


There is now a new block and pistons on top of the lower unit.

Bob Smiley