Gear ratio of outdrive

[High Bidder]

I picked up a spare OMC Cobra outdrive that was off a 4.3 V6. It has a gear ratio of 21:19. My boat has 5.0 Ford engine and has a ratio of 21:18. Can I just use a prop with a greater pitch to give me the same max RPMs and about the same top end? I am not really concerned with top speed but doo want to maintain the correct WOT RPMs.

High Bidder

[BullGator]

I am a MerCruiser guy, but it looks to me like you would be ok. Your ratio from the V6 drive is 1.10 to 1. The V8 drive is 1.16 to 1. They are very close. Using the V6 drive, your rpms at max wot would be slightly less, if at all. I doubt the difference would be very noticeable.

For comparison, a Merc (Alpha 1, gen 1) 6cyl drive is typically 1.65 to 1, while a V8 drive is 1.5 to 1. As you can see, there is quite a bit more difference between the 6 and 8.

Hopefully an OMC lover will chime in to verify....

But to answer your question, a prop change will put the rpms exactly where you want them.

[MCF]

Going back to the 1970's Trailer Boats did several motor swaps on i/o's usually moving up from a four cylinder to an eight cylinder and keeping the lower unit from the four. Switching out the prop was the solution to the gearing or rpm issues.

[ReelPower]

Your lower unit gear ratio may also be different, creating a larger total gear ratio difference which may need compensation through increasing the prop pitch.

I just went from 5.0 to 5.7 motor w/the 5.0 omc cobra. Best advice from bayliner gurus was that the difference was minimal and the WOT range was the most important issue. Run it, find your WOT rpm speed and then compensate if necessary with the goal being to achieve the higher end of the recommended WOT based upon the ratio difference and to avoid bogging the engine.

Check out baylinerownersclub.com, you must be signed up to get in the "technical" forum. Wealth of info and I just started a thread on this topic.

[tangler]

The difference between 21.18 and 21.19 is hardly gonna be noticed in your final drive ratio. You can fine tune it with your prop if nec.

[High Bidder]

Thanks for the replies. I was sure hoping I could just use a higher pitch prop. Last year I changed from a 19 pitch prop to a 18 pitch because I was 400 RPMs below the recommended WOT. Hopefully the 19 pitch prop will work on this other outdrive and give me close to the right RPMs and still a good trolling speed.

Thanks again

High Bidder