Effect of Change in Outdrive Gear Ratio

[Peaceful Pirate]

Not being a mechanical engineer, I need to seek additional wisdom. What effect on WOT and prop pitch should be expected if you marry a late ?60?s (1968) 120 Mercruiser power plant with a mid ?70?s 140 outdrive? I know that the original 120 stern drive (sr#2366557) had a ratio of 2.01:1, and the 70?s 140 stern drive (sr#4670850) has a ratio of 1.98:1. The two will match up.

The original 120 drive (which died a sudden death!) ran a Michigan Wheel 033008, 15 1/2 X 19 prop and would top out at 4,000-4,100, which is in the WOT range for the 120 engine.

Many thanks in advance!

 

[QC]

Re: Effect of Change in Outdrive Gear Ratio

The ratio change is exactly 1%. I wouldn't expect you to notice anything as far as performance is concerned.

 

[Haut Medoc]

Re: Effect of Change in Outdrive Gear Ratio

I would put it on & see what it does......
The WOT on the 140 should be 4200-4600......
The ratio will have little effect, the extra 20 HP might though.....
I'm thinking that prop might be OK.........​

 

[QC]

Re: Effect of Change in Outdrive Gear Ratio

I think the engine stayed the same . . .

 

[Haut Medoc]

Re: Effect of Change in Outdrive Gear Ratio

I think the engine stayed the same . . .

Misread that.....
I agree, the effect will be negligible....
The WOT should be 3900-4300....