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Lets say you were building a 45 ci OMC twin for top end lake fun, would you benefit from changing the gears to nitro gear ratio? The way I see it you'd spin a smaller wheel and it would make it more efficient.

Not sure what boat this will go on, but going to the 1.86 gears will require way too low a pitch propeller. That will limit prop selection drasticly. This wopuld be a mistake.

This would be for something like a T boat or a tunnel light. I'm just playing with the idea.

Mike
When the OMC 2 cylinder 60hp first came on the market in AU in the 80's the factory illegally changed the gears to the nitro 15:28 and ran the Mercury 50/650XS props that were 11 X 18, 11 X 20s and the motors went like a rocket and were a lot quicker than the standard 12:29 gears. At that time no other motors such as the 60HP Suzuki could get anywhere near them. At that time the APBA rules in AU was that the motors had to be homologated and OMC could not produce the papers to allow the 15:28 gear ratio. It took some time and many arguments and finally OMC were forced to run the 12:29 gears. I ended up buying one with the 12:29 gears and compared to the 2 cylinder Suzuki was a dog of an engine. Had to run a 12 X 27 OMC Cleaver. It would be easy to build a 15:28 gear ratio box to suit the 45ci OMC to see what advantage you will get. If the class rules allow for a change in gear ratios I would suggest that you do it as you will be a lot quicker. I guy in AU use to run both the 15:28 2 cylinder OMC and the 15:28 3 cylinder OMC on the same Tunnel Hull boat with a lot of success.

Stephen Medaris

[quote="steves jem"]Mike
When the OMC 2 cylinder 60hp first came on the market in AU in the 80's the factory illegally changed the gears to the nitro 15:28 and ran the Mercury 50/650XS props that were 11 X 18, 11 X 20s and the motors went like a rocket and were a lot quicker

The only change I would make is the word...quicker to faster. The quote even has the correct props. The tunnel application is best.

Good stuff guys. I just thought that if the SST 45 uses a 1 to 1 gear ratio the 1.87 to 1 would be better suited for something in those boat classes than the stock gear ratio, you would be spinning a smaller wheel but probably would cavitate a bit more on take off. Thanks, I figured someone must of played with this.

[quote="steves jem"]I ended up buying one with the 12:29 gears and compared to the 2 cylinder Suzuki was a dog of an engine.

Here is a mention of the 2 cylinder Suzuki motor being very strong in that class. They were rare in North America in a short shaft version. There were a lot racing overseas. Suzuki was not interested in boat racing.

Yeah, Suzuki never comes to mind when thinking racing

As Houndog said, you will require a 17-18" pitch small diameter prop to make the 1.86 work. Also, you will have to replace the vertical drive shaft, since the 1.86 had a 4 tooth spline and the 2.42:1 does not. That will require re shimming the vertical shaft. The end result will be a slight gain in top speed, with a small loss of acceleration. That would be a mistake on a race boat, in that acceleration wins races, not top end.

If you are rigging a tunnel boat and not a "T" boat, the prop pitch you can run will be higher; perhaps a 20-21", with a tad more diameter.

Changing gear ratios is a very interesting topic. Changing them changes the way the motor performs. A high ratio say 2 to 1 gives more punch [acceleration], a low ratio say 1.5 to 1 gives less punch and more top end. The lower ratio 1.50 normally goes faster, but takes more time to get there. In race classes like SO where all the motors are the same, it is not noticed because the performance is the same for every boat.

It Canadian OPC races in the 1990's it was a common for Mercury 650XS racers to take advantage of the 3 gear ratios that fit that gear case 2 to 1, 1.84 to 1 and 1.64 to 1. The last two ratios were the best. One got you off the dock faster and one gave you a couple of miles per hour more top end. To-day the props are much improved so the switching would not be necessary.
I think with the 2 cylinder OMC the new props can give both the best acceleration and equal top end.

The F4 class [60 hp 4 stroke tunnel] is a later example of switching gear ratios. The class uses the same motor on every boat. The first motors ran a 1.64 ratio. The boats looked dead coming off the dock start and lazy accelerating out of the turns. Added to that they had no top end. Mercury changed to a 1.84 ratio which has helped to a small degree.