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Thread: 31.8 omg 25hp vs 35hp gearing quandary.

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    Default 31.8 omc 25hp vs 35hp gearing quandary.

    I have two motors Iím tinkering with and I pulled down both lower units and found the 35 has much lower gearing than the 25.

    The 35 is a 76 model and the 25 is an 84.

    I want to use the 35 power head and what Iím wondering is which lower units gearing to use for the best top end speed.

    Would the lower gearing of the 35 lower unit coupled with a larger more efficient prop be the way to go, or would the higher geared 25hp lower units gears with the smaller higher pitched prop be best.

    This motor will be used on a 14í aluminum semi vee.

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    i am very intrested i wondered this myself.

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    Yeah. I know from racing RC boats that the larger prop with less pitch will be more efficient and on a 16’ or larger boat glass boat would be the way to go due to the weight, but I feel the 14, tin boat might be right on the edge of going one way or the other.

    And to clarify, I’m talking about standard props but discussion of choppers or other options is welcome also.

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    light and fast its always the tallest ratio.
    that means maybe less acceleration and less load carrying ability but will always mean best top speed.

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    Quote Originally Posted by Powerabout View Post
    light and fast its always the tallest ratio.
    that means maybe less acceleration and less load carrying ability but will always mean best top speed.
    Yes, I can understand that as a general rule to go by.

    The issue with these motors is that they are of the same displacement with only minor internal changes between the 35 and 25/20.

    The carb and intake are the known differences, with port timing and one having points (35hp) and the other electronic ignition (25hp) being the other variables .

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    Hello,

    Just as an suggestion I would just do some testing. I would call Ron Hill at Hill Marine... He is the owner of this web site and Hill Marine is on the top for their contact information. In the racing community the answer is always... Testing, Testing and then Test some more!!!

    Good luck on your project.
    Dean Hobart

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    Thanks for the input so far guys.

    Being this is a points motor, is there any guideline as to the upper limit on RPMs are?


    Being its not governed it will most likely rev higher, but what is safe?

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    If your rig is light and can get aired out to reduce the water drag as much as practical the smaller dia higher pitch prop will give the best top end. If you go for fully ventilated with part of the prop out of the water you can go for more pitch with the water air mix. There is a limit here if weight is too much there will be planning off difficulty and loss of acceleration. It is a compromise with props depending on what you want from the rig.

    As Dean stated you need to TTTTTTTTTTTeeeeeesttttt a lot.

    Ex on my hydros with the Merc H tower and quicki gear case with 1:1 gears with a small dia lower pitch gives the best top end but lacks acceleration and getting the right prop for 1:1 is not easy. With the 16:21 gears in the quicki a larger dia and more pitch is needed and works best for accel and reasonable top and props for this gear ratio are easier to work out but still a lot of Testing.

    I prefer the CD ignition, points do not give a good steady signal because of a lot of wear and rpm variables and setting the timing to be right on with piston position, CD is right on with a clean signal to the plugs. Play with both powerheads

    On point ignition I use a dial gage and a accurate buzz box (from Aircraft Spruce) to get accurate on the timing at the desired piston position to be the same for all pistons. I play with 2 cylinder jobs.
    " Three may keep a secret if two of them are dead" Ben Franklin

    Location: SW Orlando, Fl
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    Team Member JohnsonM50's Avatar
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    The lower units: Non thru hub, it's significantly lighter, has a taller gear ratio [.571:1] and is narrower. It is not by any means aqua-dynamic tho but a fabricated nose helps a little. The biggest downside is prop availability unless you can rig up a better prop on a stock hub. Ron Hill makes a signature prop that will fit the larger thru hub unit. From talking to people and reading about the GTpro class I'd say generally you're likely to get better performance from the thru hub unless you do alot to the non thru. Interchange? The power heads and the tuner it came with are interchangeable, the lower units to towers are not. I've taken a non thru hub to reach 52mph on a pals hydro. Those have a fairly weak clutch dog system unless you have the heavy duty 3 dog gears and clutch. To solve that and make this a direct drive I made a pin to fit in the detent hole to permanently lock the slide clutch forward. Then I took R gear and ground the teeth off by spin grinding it till the teeth cleared the drive gear by about an 8th" It now doesn't counter rotate or mesh anymore acting only as a spacer relieving some drag. This unit then was thinned as much as possible and an epoxy cone was added. I also found a real nice right rotation 13.5 P 2 blade speed prop from Prop19 on ebay and inserted a stock hub. [this being a 31.8 unit I put a prop shaft from a 22ci in, it fits and is pin drive, not splined]. Before doing all this, Low 40's was the best I could get from this till it [again lol] popped out of gear. Note, spring loading the shifter didn't help much.

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