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Old 06-13-2020, 10:22 AM
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Default Prop Guidance

Ok guys time to turn to the boat gods and get some opinions. I've finished the upgrades on my 1989 Formula 272 SR1. Motors are running drives are installed going to be time to hit the water soon. boat was pretty stock a few years back when I bought it. Was a stock 454 330hp twin screw. The bravo drives had standard mirage 25p 3 blade props. Now I know these boats are heavy and built to handle the rough stuff. I know I'm going to need to go up in prop size. Ive installed stroked 489's at 525hp per side. So a bit of a upgrade. Im trying to decide prop size on this old heavy bird. Im also looking for suggestions on the prop to go with. I was thinking about going with a set of 29p hydro-motive four blade cleaver style, but some said they might not work well on a heavier boat like mine. some have suggested going with a Bravo or a mirage plus. I think id rather go with a Bravo prop between those two but I want to know your thoughts on this. any idea where I should also start with size? I was thinking around a 28 or 30 pitch would be a place to start, but the prop game is something I will admit that I know nothing about.
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Old 06-13-2020, 12:17 PM
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I am assuming you will be running the new engines at a higher RPM then the original. Run the old 25 inch mirage props and see what happens. Record the data and post it. You really need a base line to get an idea what is needed.

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Old 06-13-2020, 12:54 PM
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No way you will be able spin 29 Hydro's with enough rpms. Like was said, try your 25's and then go from there. You'll prob want to spin the 489's to about 5400 rpms.
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Old 06-13-2020, 01:02 PM
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Originally Posted by Griff View Post
No way you will be able spin 29 Hydro's with enough rpms. Like was said, try your 25's and then go from there. You'll prob want to spin the 489's to about 5400 rpms.
That was my next question. What is a safe rpm to shoot for?
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Old 06-13-2020, 03:08 PM
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I will echo what the others have said. Too many people upgrade HP and immediately jump into unknown prop land. Use the original props, get the motors sorted out in the boat, log some good prop data (rpm, GPS speed in increments of 250 rpm from idle to WOT). THEN post the data and we can see where you might want to go.

If you go from a 25" 3 blade to a 29" 4 blade you will bring the motor to it's knees.....The rpm you need to target should be based on the internal parts, and cam/head selection where peak HP is attained. 5400 rpm as Griff suggested would be where I would start.
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Old 06-13-2020, 05:39 PM
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As others have suggested, what is the max RPM/redline on those motors and what RPM's produce the max torque and or horsepower? I would also do as others of suggested and get a baseline with your current props... GPS mph vs. RPMs.
I am still just learning but I would target a prop that let your motor spin up to about 95% of its redline.
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Old 06-14-2020, 01:18 AM
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Originally Posted by Smoke eater View Post
That was my next question. What is a safe rpm to shoot for?
Do you have dyno sheets??? WOT rpm really depends on internals. If your build is pretty typical with decent parts and it makes a true 525hp, its probably making that HP in the 5400-5600rpm range.
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Old 06-14-2020, 07:41 AM
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Originally Posted by Griff View Post
Do you have dyno sheets??? WOT rpm really depends on internals. If your build is pretty typical with decent parts and it makes a true 525hp, its probably making that HP in the 5400-5600rpm range.
I don’t have any dyno time but here’s the list of parts on motors.

Mark 4 489
eagle rods
eagle cranks
srp Pistons
088 heads
inconel valves
Insky springs
crane gold rockers
Comp roller cams
mellings pumps
9.7/1 Comp
4/7 firing order
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Old 06-14-2020, 07:51 AM
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To echo what Griff said...the dyno sheets for my built 496's have been lost but I have been told 5,600 max rpm.
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Old 06-14-2020, 07:59 AM
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Are they carbed or EFI and whats the specs, whats the spec's on the cam,

Last edited by F-2 Speedy; 06-14-2020 at 09:24 AM.
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