Gear Ratio Q's

[imported_TheMan]

I'm hoping someone can help me understand this a little better.

The 3.0L Alpha setup has a 1.98:1 gear ratio.
The 4.3L Alpha setup has a 1.84/1.81:1 gear ratio.

The difference is 0.15 rotations of the engine per 1 rotation of the outdrive. Seems relatively minor at a glance.. Overall their is ~7% difference in shaft RPM as a result of this change when both engines are running WOT.

I've heard people say that you must change the gear ratio if you go from one engine to another, but I'm trying to understand why instead of just being told that. What will happen to the performance if it isn't changed, and why can't it be overcome by dropping the pitch an inch or two? I can understand performance at the very top end, just having trouble with other areas. I also realize you would need to be careful in order to make sure the engine doesn't over/under rev too much..

P.S. I don't doubt that changing the gears is the RIGHT thing to do, I'm just looking for an explanation of what happens if you don't.

 

[dcg9381]

Re: Gear Ratio Q's

Theoretical impact:
4000rpm, 1.84:1, 19 pitch, 10% slip, 35.2 mph

Same deal on a 1.95:1 out drive would be:
4250rpm


Generally, changing propeller pitch 1" will adjust your WOT RPM by about 400... Note, I've never done this - but to me it seems reasonable that you could adjust for 1.84 vs 1.95 gear ratio with propeller pitch.

 

[Bondo]

Re: Gear Ratio Q's

I don't doubt that changing the gears is the RIGHT thing to do, I'm just looking for an explanation of what happens if you don't.

Ayuh,........

Basically,.... it's about putting the available Horsepower into the water,... Efficiently,.......

Normally, Props come within the pitch range of 17" to 21",....At best from 15" to 25",.......
When going to a Smaller motor, usually it runs out of available HP before you can find a prop Small enough......
And of course,....
Going to a Larger motor,..... You won't find a prop Big enough......
Even if you should find a prop Big, or Small enough,.....
You'll end up running at an Inefficient rpm range........