I currently have an Ilmor 625 that should have a WOT range of 5800-6000rpm. I've been running Bravo 1 props but I've been having way to much slip so I tried running a 5 blade. My last numbers were 25p 1.34gr 5000rpm and 73mph. My max speed to far was with a Bravo 1 26p at 81mph at 6000rpm. Obviously I'm way off on my numbers so the first thing I thought I'd do is change my gears to a 1.50. Anyone have any idea how much RPM I should gain by changing gears?

In theory about 600, but you have to take into consideration that you will now be closer to the "powerband" of your engine which should get you some more rpm and speed.

So, short answer is damn close to where you need to be.

But, based on the numbers given if your slip stays the same you and you run at 5800 rpms the Bravo with the 1.34 will be faster.

You don't think my slip might come down as well or does that have nothing to do with it?

The gear ratio change should only bring 400-500 additional R's.
Changing gears will not affect the slip %. You need to change X dimention, propeller configuration or CG to help the slip numbers. (other things can help too)

Brett

Thanks Brett,

That run was with a -2 shortie, maybe I should try it with a -1.

By the way Brett, I talked to Brad yesterday and gave him a update. As soon as I try the new gears I'll give you a call.

Technically it doesn't. However, if you are over-trimming a bit now in attempt to get higher rpm, you may see lower slip numbers after the gear change if you are now using less trim.