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I have a 2000 nsx with approx 550 hp, 1.36 gears, and a 24 pitch 4 blade bravo. The boat turns 5500rpm at 76 mph on gps. Does this sound reasonable? It seems like I have the slowest nsx in history. I do have a B&M 250 I'll try to get on this weekend. I've had the boat for a couple years and I'm not sure I could live without it but I need to get serious and start pushing 100. What's it going to take?

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An NXS with a 502/502 runs in the neighborhood of 78mph... what exactly is your engine. what rpm are getting and what is the powerrange of the package. With 550 hp you should be running mid 80's. A 525efi runs 84 or a little better.

Don

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secondly ... a 502/502 with 1.36 gear generally uses a 26 prop.. with an additional 50 hp i would think you would be running a 28.

Don

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It was a 502 mag EFI. I put on a carb, intake, new hydraulic roller blower cam,roller rockers, ignition, and did the heads(shaved and stock valves). The motor runs great at 32 degrees timing, planes pretty well and runs up to 5500-5600 rpms. Maybe my prop is junk? Its a bravo 24 and you can tell its been worked at some point. I've tried a 27 3 blade mirage and couldn't even get on a plane. I have a hydromotive quad IV 26 but it runs worse. I don't think the boat likes or needs the stern-lifting prop.

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I also put HP 500 exhaust (gil) on the motor.

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The boats definately like the bravo 4 blade prop... if your prop has been reworked multiple times it could have lost some diameter and have lots of slip.....
Have the motors been dynoed ? where did you get 550 hp ?.... if the cam is for a blower application.. it is possible that you are not making even 450 hp...

Don

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your numbers show about 15% prop slip... thats higher than it should be in my opinion..

Don

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I would dyno the motor for sure.

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Sounds a little slow. I am still dialing in my new 2600 AO that has a 525EFI, 1:36 gears and 15 1/4 26 Bravo 4 blade. I have seen 82 @4900 this past weekend, still have some more work, but boat should be in mid 80's when done. Ed really builds a great cat. PS it is my third 2600 in nine years...

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What elevation do you run this boat

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200ft, south Texas

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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.

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How do you calculate prop slippage?

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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!

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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

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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.

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Sorry, I ment blueamerican

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

What would be the factor for 1.46 gears?

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.685 CA Southpaw.