I just did a compression check on my rebuilt 502 Gen 5 w/ a very mild Cam (crane 13721, I think), changed all valve train to roller. And a new stock crank.

Other then changing to roller, everything else is stock set up. My question is is'nt 185 lbs of compression high? I even used another compression gage just to make sure I was getting correct readings. 185 lbs. on all cylinders

I wouldn't want it any lower.

The reason I ask is because the Merc service manual reads 150. Does this change my ratio from 8:1 or something close to that? I would still like to use regular unleaded.

Changing a cam on a motor can have a big affect on cranking compression that you are describing here. I would say that your static compression would have to be about 9.5 to 10.0 to 1 to get a 185lb number. Valve opening and closings can raise the compression somewhat so if no other real changes were made, ie. pistons, block decking or head milling then I would guess the cam has raised your cranking compression. It may not be a problem with timing set conservatively but be careful to watch & listen for the sounds of knocking. If it does occur to much you could replace the head gaskets with about a .020 to .030 thicker head gasket and probably reduce this compression ratio to about 9.0 to 1 to 9.5 to one which would be a good range for an iron head motor.

Ray @ Raylar

Thanks for the info.....learn something new again

Did you go to higher compression pistons with your rebuild?

My new 502'S have 185 as well all cylinders. I am running stock comperssin pistons and head have only been milled about .002 to .003 for clean up. Cam has a big play in cranking compressin. But this number although effected by static CR does not indicate the static CR. If you have not changed any thing to increase CR IE: Domed Pistons, Milled heads, smaller cc heads, IMO you will be fine on regular fuel.

Depending on cam this number could swing as much as, from 150 to 200 or possiably more.

Just like the original J & E pistons, Crane #139721 Cam, Retrofit Rollers to fit it a Gen 5 block, Crane Gold Roller Rockers 1:7, Push rods and springs. Everything else is stock 502 , except the exhaust (Revolution Marine).

Can any one tell me what my horse power might be?