joerambo

200ft, south Texas

tkperf

South Texas has great "air" so elevation is not an issue. I calculate your prop slippage at 18% and blueamerica's numbers at 8% so you may want to look at your prop situation. Also check your nose cone for damage and look at your X demension.

It takes a pretty "big" camshaft to make power at 5500 rpm. It sounds like you are turning to small of a prop for your set-up. You may be better off turning your engine at 5000 rpm, with a bigger prop.

What are your cam spec's? New cam vs old cam and what was the differance in rpm if any between the two.

CA_Southpaw

How do you calculate prop slippage?

tkperf

RPM x Gear Ratio x Prop Pitch x Constant = Theoredical Speed

For Gear Ratio, take 1 / Your Drive Ratio. So then 1.5 = .667 1.36 = .735

Constant = .00947

Example: Formula for joerambo 5500rpm x .735Gear Ratio x 24Prop Pitch x .000947 = 91.87MPH

76/91.87 = .827 or 82.7% of 100% so 17.3% slippage

Hope this helps!

quest

For bravo 1 4 blade props take 1 from the pitch ie for a 24 pitch prop use 23 in your calculation. for a 26 bravo use 25 in the calculation and for a 28 bravo use 27. the Bravos are pitched 1 less than their posted number.

yes 76 is slow for 550 hp you should be in the low 80's

tkperf

I think Mercury spends alot of money on research and development and if they say 28 pitch, Then I would think it would be 28 pitch. If you have proof otherwise I would be interested in hearing it.

Also if you are correct in 1 pitch less than blueamerica is running a 4% slippage. That would be very very very very good.

tkperf

Sorry, I ment blueamerican

CA_Southpaw

"So then 1.5 = .667 1.36 = .735"

What would be the factor for 1.46 gears?

cjsdad62

.685 CA Southpaw.