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I/O Diplomat with 300hp gearing questions

J

jayz8886

New member
I been looking around here for awhile and im new to i/o boats. Its a great place also. Love the Checkmates.

Ok on to my problem. The boats a 82 Diplomat.

I picked up this boat with a cracked 4.3 omc I removed it and intalled a 300hp 350 in the boat. The first test run killed the water pump shaft so I got no real testing done. The problem is when I tore it down for repair it has 20/21 gears in the upper and a 14.5x19 raker.

My question is there anyway to re prop the boat to work with the 21/20 upper?

Or do I just bite the bullet and get some 21/16 gears?

If I do get the 21/16 gears is my 14.5x19 raker gonna work or will I still need a new prop?

Sorry for all the questions just kinda lost on what direction I need to go.

Also the boat is just a family boat not looking for all out speed just good all around use.
 
not sure what props you can put on your drive, nor what the gearset ou refer to puts your overall gear ratio at... never had an omc, but I've been able to find props that work well behind the 383 I put in my 88 diplomat with a 1.81 gear ratio. I've also tried it with a 1.47 gear ratio and it just performs better with the higher gear and higher pitch props - ie. 27-29".

you just need to run what you've got and see where it puts you. see if you can borrow a 23 or 25 that's between 14 and 15" in diameter and see what it does. A word of advice from my own experience, it'll be a LOT more stable and les prone to ventilate with a 4 bladed prop than a 3 and will like stern lifting characteristics for the best overall performance. at least that's been my experience...
 
Sorry about the confusing first post. I did some more researching and it looks like that with my upper gears im going to have a 1.77 final drive. So as soon as my gaskets kits get here I will get it back in the water and test the 19 raker and see how it does.

Thanks for the help
 
Well I got it in the water and with the 14.5x19 an 1.77 final gears its screaming. It will jump out of the water but shows around 40mph on the guage at 4500 rpms. So a local shop has a 3 blade viper that a 14.25x25 that I going to try I think and will see if that will help.
 
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