I am locking to convert my std Bravo 1 lower on my 29 Scarab single engine to a -2 imco sc lower . What rpm i will gain with a -2 imco sc if i use the same prop?
There is exist a rule by inch shorter vs rpm?

Thanks for the help!!

No one can help me about runing a imco -2 sc lower vs std bravo 1 lower, How much rpm gain in general we will see ?

Kinda of a vague question to a specific boat. I'll try though. I have a 28' Nordic, I had a standard lower with 1.5" spacer. I removed the spacer, saw no significant speed difference but better handling. I then went to a -2 also with no spacer (so a 3.5" reduction in prop depth) and again, not a huge speed gain but better handling. Can't really say I saw an RPM change, my slip is higher now at mid range RPM's, but right on the money at WOT.

Remember you will lose drag by going shorter, but you will have less prop in the water too. Talk to B Blades, bet he will have advice-

launchpad475,,, so you kept the same prop ,,,, pitch, rpm,1.5 drive, mph ?

oh, looking to do the same, my prop shaft centerline 7 3/8 deep,, (twin SBC) ,,,, what are your centerline guys?

Well, I answered somewhat generally. I've tried numerous props with all the changes along the way, and what I found (on my boat) was bringing the prop up improved the handling a bunch and the speed was negligible. Mine would chine walk real bad with the deep prop, it also LEANED real hard to one side from prop torque. That all went away with the changes. The down fall is a little more midrange prop slip, but I run moderate power to an XR so I'm ok with some extra slip.

the -3.5" was a 30P with 1.5 gears. Taking out the 1.5 gears same set up did nothing for speed, handling improved.

When I went to the -2, I went down to 1.36 gears and a 28P. I netted some speed here, because I could get the RPM's where I wanted. (I was running out of blade with the 30 and 32P and couldn't spin the motor fast enough to make em work.) Plus I feel that on my boat, turning those bigger pitches made the boat lean over. Doesn't do that anymore.

Since then I have made engine changes so, the comparison isn't really linear at this point. I've seen 92 with the vortech, I see 90 with the Whipple now. Totally stable, no chine walk. 1.36 with 28P and 56-5700 rpm. I also run a stand off box, so my combo is somewhat unique.

01' Nordic Heat, 540 with Whipple.

At wide open throttle your RPM won't go up just because you raised the prop height. Unless your slip goes way up. Your RPM goes up with the higher prop shaft height because of loss of drag that then allows the boat to go faster and the faster speed raises the RPM.
I took one of the engines and -2 shorties out of my SCARAB and installed it in my Daytona. Same engine same drive. I had 17% slip in the SCARAB but only 11% in the Daytona. In the SCARAB this combo ran 5400 RPM in the Daytona it ran 6000 RPM. The prop shaft on the SCARAB was 5 inches below the bottom - in the Daytona it is 1/8 th inch below the hull. The lower slip in the Daytona makes the RPM lower than it would have been if it were 17% instead of 11%.

I am now running a -3 inch shorty that has my drive at 7/8 th inch above the lowest point of the hull.

I went from std Bravo to -2 Imco lowers on the Black Thunder. I gained about 2 mph and rpm stayed the same or went down a hundred or so. I think it was because the imco lower smoothed the waterflow to the prop and resulted in less slip. Midrange slip increased and I lost 2-3 mph at cruise speed.

Tinkerer ,,,,, what are your final results , details if don't mind