Omc 25h.p.

**JohnsonM50** · 06-25-2006

The Evinrude/Johnson stock lower units mostly used 12 to 21 gears on mid sized motors however my old book lists 18 & 33 hp motors = .571 to 1, the ha ha newer 22s & 31.8s have the same except for the thru hub types.

**RichardKCMo** · 06-25-2006

Originally Posted by Roy Hodges

check with any old timer . the stock outboards had a 36 class for 25-30 hp johnrudes ,at the last it was (i think ) won by 35 hp chryslers . somebody jump in and set us straight , please

That i am, i don't race but spectate a bit.
I've been trying to find info on old omc motors, it seems some techs. can only relate to cc,s not cu. in. and other info without the book.
Skoontz , i like reading about things to make omcs fast, as theres not much out there except fo mercs.
RichardKCMo

**JohnsonM50** · 06-25-2006

Thats the way I find it too, dealers outlooks toward performance enhancements seem taboo, -you dont do that- or its already Hi Perf. Taking into account that a boat motor needs to rely more on tork than screemin H P cause a boats always goin up or maybe close to level but not exactly. All their accel. and speed are eek-ed out of 1 gear [forward] and they dont coast.
In comparison with motorcycle motors of similar ci. bike motors ROCK and have those 5-6 gears [hey why dont we got gears]
So its challenge to make em fly and a pleasure to hear from another who knows about it. Any one whos seen my posts knows Im a 31.8ci hack but I know a couple guys who have 22ci that smoke me! The secret... adapted AB quicky. I also saw a B mod with a 22ci OMC at Millville dominating the straightaways. Maybe some day 31.8s will run C mod, I wonder what lower unit could be used.

**RichardKCMo** · 06-25-2006

Omc 35s have already been tryed in c and d , too fast for 1 too slow for the other. Outa be somewhere to fit though.
RichardKCMo

**JohnsonM50** · 06-26-2006

Originally Posted by RichardKCMo

Omc 35s have already been tryed in c and d , too fast for 1 too slow for the other. Outa be somewhere to fit though.
RichardKCMo

I Can guess you mean fast in C, slow in D cause of cid. What did they use for a lower unit? Were they using pipes or stacks? The 70s 50hp OMCs are I believe 43 +or- ci. and not very slow. A Bass D lower unit would be good for either.

**Skoontz** · 06-26-2006

Your best money is going to be spent below the cavitation plate reducing drag/creating transom lift first. Then after you get that dialed in, work the engine mods. The only way to make a screw together OMC gearcase work well is by removing it and adapting and Quicksilver unit, or some other reasonable unit. And when you run a stock OMC gearcase, the way to make it work is with a Merc propeller. At least that's what we did. There are also certain year OMC thru hubs hat blew water out at too slow of a speed. Post
'78 was when they worked a little harder on hydrodynamics.

**JohnsonM50** · 06-26-2006

Im gettin more than 50 from a screw together lower, ground down alot, coned, above and below the propshaft clearanced for better props, the fin thinned, the screw holes filled and I havnt removed the R teeth yet.
Bein on a Bezoats hydro and spinnin a 16P prop helps too but a set of gears would be cool. I think a 31.8s too much for an AB merc and they are too valuable to find out the hard way.

**JohnsonM50** · 07-01-2006

Now this LU is full forward, teeth removed from R gear so no more opposite spin friction or cradle- dog friction and no spacer friction because the dog is pinned forward thru the shaft, not spaced between gears + no more double gear lash. Ive been tryin a prop that would probably be pretty fast but it seems to slip excessively, maybe its because the torpedos kinda fat despite what Ive done to it. I tried some tweekin on the prop hopin to get better traction. That and going back to a non cut flywheel as advised, I want to test it this weekend. Im tryin to get an omc to be as fast as a C moda.. ha ha.. got a lonnng way to go!