Gear ratio change
 

Changing from a 1.5 to a 1.35 ratio running a 23 pitch prop at 4900 how Manny rpm will I loose? Also going up 2 inches a 30 ft w a single hupp with a pad and notch

Are you saying current WOT RPM is 4900 and your going from 1.5 and 23 pitch to 1.35 and 25 pitch?

Don't know what speed your getting now but using constant speed your going to loose about 900 RPM

Not an expert, but gear ratio change alone will reduce 450rpm. Rising propshaft height, if that is what you mean will give some rpm back, maybe all if you are lucky. Do you gain any speed, that you have to test.

My 21ft donzi behaved/handled like CRAP with 1.35, switched to 1.50 and it was night and Day..

So, prop rpm matters, alot..

Why would you swap to 1.35 from 1.50 when you are only turning a 23" prop to 4900rpms??

You will loose about 600rpms and being on lower end of prop pitch, you will limit your prop selection.

Gears
 

Rasing with a imco lower I picked up has a 1.35 ratio ,putting on a procharger on 502,and later a 540 boat weighs 7 k almost 60 at 4900

What would be the reason to switch to a different ratio? Better/more prop selection?

Lower
 

Because I have the lower unit gotta check ratio to be sure