Help Modifying OMC 31.8 cid

[Emustard]

Do you guys have a relief cut in the side of your cylinder wall or is that just something they did on the 20hp or is it on all the 31.8cid 20,25,30,35hp.

[JohnsonM50]

I've read of that but never saw one, most of my stuff is early 80's or older. Best reason I think noted was easier starting & that it had no effect on performance. If anything raising the exhaust port time slightly would be typically for more top end. The rings & compression stroke 'cc's are the key to compression. It can vari from motor to motor about 120-135 w/o any significance in speed. 150 from what Ive seen takes milling. 2 of mine are to the point where the pistons will hit the head w/o the head gasket. I don't know how much can be milled but be advised they are already close.

[Smokin' Joe]

Do you guys have a relief cut in the side of your cylinder wall or is that just something they did on the 20hp or is it on all the 31.8cid 20,25,30,35hp.

Where, for what?

[Emustard]

On the exhaust side on the middle port there is a cut about a 1/4 inch above the exhaust port and 1/8 inch deep. They call it a relief cut and I think JohnsonM50 is right it's for easy starting.

[JohnsonM50]

On the exhaust side on the middle port there is a cut about a 1/4 inch above the exhaust port and 1/8 inch deep. They call it a relief cut and I think JohnsonM50 is right it's for easy starting.

Something I read here on BRF.

[JohnsonM50]

To adapt a gear case & save the pump. [I think this answers & asks a question] Luck has it the the OMC pump plenum happens to be right where the Yamato & I beleive Konig too water ports come up.



So the Q being since there is a bearing below the pump then the top driveshaft bearing of the gearcase within a couple inches, robing potential speed. I decided to err on the side if security there & so far they hold up. To lube the isolated pump bearing I put an upper & lower hole to squeeze lube in.

[JohnsonM50]

As far as gearfeet go you can get the most out of a motor with one that is a good match. I'd be afraid to adapt an Merc AB or C to a 31.8.. could be too much hammering at 3" bore A 'D' maybe but other units can be had cheeper usually.
V reeds are something I went as far as to buy from a bigger motor, stare at for awhile & decide they were 1. Too big, 2.would be too small & still not fit well if one petal station were cut off. so I traded them off to someone building a pulse jet.
I'm convinced that 'ReaL' power gain can be found in some reed improvement but that leaves to question why? Its obvious why the reed wall doesn't quite do it but is the answer more volume, more velocity or just better open/shut capacity? On this particular motor you would almost be better off re-designing the crankcase front.

[Eric, the pic above is one of the lower crank case opening clearanced to be like the upper].
The sort of funnel area behind the reeds is alot of C.C. volume for one thing & since its roundish theres no good way I see to get V reeds on/in w/o increasing C.C. volume as much as to possibly counter any advantage from using an add on reed compartment.
Ive done way more work on the 31.8's than I have results to show for it. One guy who I admire here is Tim K. He's done the same kind of V reed adaption to a Merc. & its awesome. If he were to take on a 31.8 I'd bet there'd be a work of art that works... meanwhile this idea has me thinking. As you see in the pic the rods swing pretty close to the inner surface of the case so shaving it back is limited & presents a mount stud issue. Filling it in some comes to mind but getting it right so it doesn't become a blockage is one concern & I'd hate to think of what is 'breaking loose' would look like.

[JohnsonM50]

Raising the motor, With the club foot application I think in a way I was lucky. The skeg is small for the 50mph neigborhood & the prop pretty big compared to the usual race boat situation. That could result in poor control & blow out. The cone is all epoxy just stuck on the assembled foot & shaped. When it was time for sevice I carefully hacksawed it at the seam line & re-assembled it with silicone. Well over 5 y.o. & its still there??, yes somehow. I'm thinking it didn't do much for recordable speed but may be why it works given the 9 or so inch prop thats on it. It is run submerged as it won't grab much higher but is right at the surface, [probably explains how the skeg works]
When you do go up, whether the prop will work is one thing but cooling w/o a low water pick up is another. Ive heard of plugging the top hole on a thru hub but have not done it. I've come up with an auxillery pick up:


This is a Yamato 102 adapt to an OMC clubfoot type case with a revese gear water inlet so it was easy to make a threaded copy to induce more water. It was a nightmare plumbing the 102 pickup to the OMC & it sorta worked but I didn't trust it. This works as just an add on w/o changing prop thrust pickup, just puts more water into a questionable situation.
On a thru hub you would have to drill & thread a hole at a good location to do this & then decide whether or not it works along with the side inlets. I'm thinking leaving some of it [bottom hole?] open but don't know. Would guess it to depend on how high your prop can go as well as water level at speed. Keep in mind that when the water goes by the rear bottom edge of the boat it begins immediatly to seek its own level so back at the foot it will be higher than the boats bottom.