4.750 stroke too big for performance boating
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4.750 stroke too big for performance boating

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    Default 4.750 stroke too big for performance boating

    A friend of mine spoke to a reputable engine builder who stated that 4.750 stroke crankshafts were too big, and the subsequent large engines they create, won't RPM in a boat, even thought they will on a dyno.
    The load on the dyno is allowing the motor to RPM, where as in the boat the load, continually gets larger as the speed increases. I wouldn't see where anything but TQ and HP would make any difference, the propeller doesn't know what is on the other side. If the motor, irregardless of size, produces more TQ and HP than it takes to drive the boat, the boat will continue to accelerate. Am I flawed in my thinking or is this more of that bogus boat theory, that doesn't wash. Matter of fact I would think the large TQ that a big motor produces would be more beneficial, since TQ ultimately is what drives a boat across the water. Chime in experts.
    Last edited by Jspeeddemon; 03-20-2013 at 03:20 PM.
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    Ain't Right Racin piston in the wind's Avatar
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    what drive ? how much power ? What deck height on the block? bore? compression? power adder? Heads? cam? Need more input...
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    Senior Member wagspe208's Avatar
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    Hmmm.... the boat guys around here build em from time to time$$$$$.
    A tractor stroke isn't going to turn 9500 all day. That is what gears and prop are for.
    If you put peanut port heads on them they don't run well either.
    Wags
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    Quote Originally Posted by Jspeeddemon View Post
    A friend of mine spoke to a reputable engine builder who stated that 4.750 stroke crankshafts were too big, and the subsequent large engines they create, won't RPM in a boat, even thought they will on a dyno.
    The load on the dyno is allowing the motor to RPM, where as in the boat the load, continually gets larger as the speed increases. I wouldn't see where anything but TQ and HP would make any difference, the propeller doesn't know what is on the other side..
    it's more than just the prop it's the drive system. A lot depends on the out-drive and gears. building an engine that can spin 7,400 but having a drive system that's going to hold it back to 6,200 doesn't do anyone any good.

    I know a couple of 632's that spin to 7,400 in a jet.

    Need more information on the application.

    S CP

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    It was a blanket statement about crank size, really irregardless of motor specs. He called asking about some carb information and got in a lot more stuff. The motor in question made 866HP @ 6300 and 815TQ @5400. It was mentioned that it wouldn't be able to spin up in the boat due to the 4.75 stroke, like it did on the dyno, because the loading was different.
    True but if the power exceeds, the force it requires to continue accelerating the boat will continue to go faster LOL, doesn't matter what the cubic inch or stroke is.
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    [QUOTE=Jspeeddemon;2018962]It was a blanket statement about crank size, really irregardless of motor specs. He called asking about some carb information and got in a lot more stuff. The motor in question made 866HP @ 6300 and 815TQ @5400. It was mentioned that it wouldn't be able to spin up in the boat due to the 4.75 stroke, like it did on the dyno, because the loading was different.////////// It would work in a boat ( I have a pair close to them in a 30' cat)
    but in a boat oil windage can be a problem with a 4.750 stroke for any length of time. real Deep pan or Dry sump is a must.
    Last edited by jimclauss; 03-20-2013 at 06:38 PM.
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    Jet boats are water brake dynos.
    Chris Straub
    Straub Technologies

    3HP is an A$$ Whooping!!! JW
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    Really not a great combo unless the deck height of the block is in the 10.700 range and you plan on running a dry sump
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    Titan pumps, Williams pans with 2 inch kickout, over 11" deep, with windage tray of course. Ran them in 30' Cat last year, before upgrading with Profiler Intakes, pans. The Profilers real woke the motors up TQ and HP, 158HP and 125 ft/lbs TQ @peak.
    Just amazing to me, that if it is outside of what people do, it won't work, because it's different.
    My point, with the whole thread, is that pretend it was a 496 with a 4.25 crank and 16:1 that made 866 @6300 and [email protected], would it matter to the boat what size motor it was. A motor
    makes a power curve in a certain graph and if that graph line stays above the power, required to shove the boat thru the water, it doesn't matter what is on the forward part of the flywheel, even if it was an electric motor. When these motors are installed in the 30' Daytona they bleong to, the video will prove the Initial statement untrue. By the way the motors still made 840 plus HP @ 6500, so they don't fall off quickly. Dyno pulls were stopped at 6500, because at 6500, the Daytona runs 140MPH.

    Are there inherent problems with a Big Shaft spinning up in a boat for long periods of time, Sure. But it has nothing to do with the power, as the boat uses it. The horsepowercurve and TQ is what matters.
    People get paid well in motorsports markets, to build stuff, with archaic viewpoints, of what will work and what doesn't, but down the street a guy slams a fuel injected, Turbo Toyota motor in a boat and it runs well. Check youtube.

    The world ain't flat, but some people thought it was, and if sailors went too far they would fall off a cliff. Big LOL.
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    Either there's more to what the "reputable" engine builder said, or he doesn't understand boats. 4.75" stroke motor haves some compromises in them, but they can work just fine in a boat. Windage from the crank can be a problem and getting enough air to them has some challenges, but they can be made to work just fine. We did a 4.5" stroke deal and I was surprised how much different the prop/gear/hull set up was from a similar HP 4" stroke engine. Once we got it sorted out, the boat runs pretty good. Wouldnt be my first choice for a max effort deal, but a fast lake hot rod or bracket boat, no problem.
    Quote Originally Posted by gn7 View Post
    ....... David 519 is 100% correct........

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    Quote Originally Posted by David 519 View Post
    Either there's more to what the "reputable" engine builder said, or he doesn't understand boats. 4.75" stroke motor haves some compromises in them, but they can work just fine in a boat. Windage from the crank can be a problem and getting enough air to them has some challenges, but they can be made to work just fine. We did a 4.5" stroke deal and I was surprised how much different the prop/gear/hull set up was from a similar HP 4" stroke engine. Once we got it sorted out, the boat runs pretty good. Wouldnt be my first choice for a max effort deal, but a fast lake hot rod or bracket boat, no problem.
    Yep. Its all in the geaing and prop, and possibly even the setup of the boat itself. But there is no reason a 4.75 stroke won't work. Like Cstraub posted, a jet is nothing more than a water break, you can dial in the pump for what ever RPM you desire at what ever torque input. If you can dial in a dyno to ALLOW a given RPM, you can do the same with a jet. V-drives aren't much different. Between gears and prop you can get it to do what ever you wish.

    If it didn't work, Sonny Leonard would't sell as many offshore engines as he does. And Rudy Ramos would have never won so many races in the Allison powered boat that they outlawed it.

    Very seldom does the term "no replacement for dispalcement" not apply, specially to boats. Not always the most feasable, and certainly not the most economical means of making power. But cubic inches and brute torque can be made to run right along side smaller, faster spinning engines.



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    Quote Originally Posted by gn7 View Post
    Yep. Its all in the geaing and prop, and possibly even the setup of the boat itself. But there is no reason a 4.75 stroke won't work. Like Cstraub posted, a jet is nothing more than a water break, you can dial in the pump for what ever RPM you desire at what ever torque input. If you can dial in a dyno to ALLOW a given RPM, you can do the same with a jet. V-drives aren't much different. Between gears and prop you can get it to do what ever you wish.

    If it didn't work, Sonny Leonard would't sell as many offshore engines as he does. And Rudy Ramos would have never won so many races in the Allison powered boat that they outlawed it.

    Very seldom does the term "no replacement for dispalcement" not apply, specially to boats. Not always the most feasable, and certainly not the most economical means of making power. But cubic inches and brute torque can be made to run right along side smaller, faster spinning engines.
    Thank You Bob, I knew your common sense approach would get what I'm talking about. HP and TQ curves are just that, whatever the vehicle is will accelerate or stop doing so, when the power it takes to continue making progess become equal. You'd be shocked at who said this, but I'm going to leave it at that.
    There may be more to the story, he was trying to convince his audience, that his program is better, sales and marketing.
    Last edited by Jspeeddemon; 03-20-2013 at 09:16 PM.
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    Somebody give me the 110,000.00 for one of these and I'll drop it in my GN in a heartbeat, and love every minute of it. With a 5.875 stroke, and 5.220 bore, and still spins 8000, I would say that built correctly, and 4.75 stroke is not a problem.
    Even if built to be a RPM torque monster, you can make it work.

    LINK: 1005 cid & 2150 HP






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    Quote Originally Posted by Jspeeddemon View Post
    , but down the street a guy slams a fuel injected, Turbo Toyota motor in a boat and it runs well. Check youtube. .
    Now you've gone and used the "T" word.
    David 519 is probably done with this thread now.

    Twin Turbo 1800 HP V-Drive lake boat

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