Help - Need advice Re crossflow compression

It's been a long time since I posted. I am a collector of vintage fiberglass boats - the kind that look like the cars of the 50s and early 60s. I like tinkering around with the old outboards the most though. I am in process of making a cool sleeper engine - it has the hood & outer skin from a 1958 Evinrude Fat 50, but the powerhead is a 1968 100hp. The 68 100horse actually burns less gasoline that the other early V4s did.

I plan to use the heads from a 1970 100 horse motor, as it and my 1968 were both 90 cubes. The 1970 heads have a improved combustion chamber design. Most everyone who has experience with the old crossflows knows that the compression wasnt very high. I know that a motor with cranking pressure from 90psi to 110psi is all they had when new. I packed one of the combustion chambers with plumber's putty and rocked the crank over to have the piston smash the putty into the head. I then removed the head & measured the thickness of the putty left in the dome of the combustion chamber. I discovered that there is no less than .250" or 1/4 of an inch from the piston crown to the cylinder head in any given place that I mesured. I also had the head gasket in place when I did this. I am thinking that milling .125" or 1/8 inch from the heads would give a good increase in compression, while leaving no less than .125" at the minimum between head and piston. I have milled heads on the old V4s up to .060" without having pistons melt.

My question is this - how sensitive are the old cross flows to compression boost, nd how much is too much? I plan to run regular pump gas, and this will be a cruiser, not a racer. I can always run surface gap plugs to keep her cooler. I just don't want to blow holes in pistons. Did OMC keep the crossflow compression low for reliability? Am I blowing it if I mill these heads this much?

Sorry for the long post, I just can't get a real answer anywhere else & these questions are eating at me & stalling my project.

Thanks for any advice.

Bruce G

Redding CA

Bruce, Ive got several 35-25hp late 70s & newer 31.8 ci crossflow OMCs in various stages of rat motor. I run these on a hydro & a runabout. These have a closer tolerance combustion chamber than what you stated & range from 120 to 150 psi cranking compression. Performance wise the higher comp. ones accelerate better but are only a little faster. The lower comp ones will run fine with a prop that doesnt strain it too much where more comp tends to go the extra with more pitch. Even tho these motors designed range is from 4500rpm to 5500 [depending on model/year] they will run fine long term 62-6400 Rs. 7000s in the danger zone.
Consiering the twins fire 180 degrees apart while the V4s fire 90 degrees [I think] they probably dont need that much compression. Hopping one up might be neat. Good Luck. :cool: Mike

What is important is the volume before and after.

OMC did increase the compression and porting on the 89ci motors to make the 115 hp version. They generally went to 41.5cc's minimum clearance. I do not know what the clearance was on the std motors.

Yes, compression is generally kept low for reliability. Gas always varied quiet a bit from region to region. Major fudge factors for reliability are always used on outboards to compensate for morons who put them on real barges with the wrong prop etc, that is why the power per cubic inch is so much less than on a motorcycle or jetski where the manufacturer sells the who vehicle with very little chance of overloading it.

Thanks for the info both of you.

Regarding the volume - I figure this is is determined by CC-ing the head? I guess all I need to do is get a bit of clear plexi - grease the head and slap it on there. Drip oil through a hole in the plexi till it is full - drain it & meausre it. (I don't have any fancy CC measuring equipment - so I'd have to improvise) I could then see what the volume of the stock head is currently at.

I am very interested in the tech info about the GT 115 / X 115 that you mention - where would I dig up this info??

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Originally Posted by Bruce G

Thanks for the info both of you.

Regarding the volume - I figure this is is determined by CC-ing the head? I guess all I need to do is get a bit of clear plexi - grease the head and slap it on there. Drip oil through a hole in the plexi till it is full - drain it & meausre it. (I don't have any fancy CC measuring equipment - so I'd have to improvise) I could then see what the volume of the stock head is currently at.

I am very interested in the tech info about the GT 115 / X 115 that you mention - where would I dig up this info??

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Tech 411 I dont know but for parts #s you can go to BRP parts catalogues. It will take a little time to go thru & gives exploded views. Part#s only, no specs.
If any 2nd Effort parts can be found for your motor would be good, may not be available any more.
I do know that after some initial gain on my 1st 25 hop up to gain 2 mph at any point after was grounds for a party :rolleyes: