Jet Boats: Horsepower vs. Torque
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Jet Boats: Horsepower vs. Torque

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    Senior Member N2GLOCK's Avatar
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    Default Jet Boats: Horsepower vs. Torque

    So I recently aquired a nice and unmolested BBC Gen IV block. I'm thinking about building this over time and eventually installing it in my jet boat or keeping it as a spare until my current motor finally dies. Luckily, I'm in a position that my current motor is running and I won't find myself with a motorless boat in my driveway for an extended period of time. I'm pretty sure that I'd like to do the 468 thing and being in the neighborhood of like 500HP. What kind of number should I be looking at as far as torque goes? How important is torque in a jet boat? I'm currently running an "A" impeller and I will probably be sticking with that so the motor won't be seeing any high RPM's. Mainly, as my son gets older, the boat will probably be doing alot of the ski, tube and wake board pulling thing. However, when it's just me and some friends I'll probably get on it a bit harder.

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    steelcomp was here
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    Quote Originally Posted by N2GLOCK View Post
    So I recently aquired a nice and unmolested BBC Gen IV block. I'm thinking about building this over time and eventually installing it in my jet boat or keeping it as a spare until my current motor finally dies. Luckily, I'm in a position that my current motor is running and I won't find myself with a motorless boat in my driveway for an extended period of time. I'm pretty sure that I'd like to do the 468 thing and being in the neighborhood of like 500HP. What kind of number should I be looking at as far as torque goes? How important is torque in a jet boat? I'm currently running an "A" impeller and I will probably be sticking with that so the motor won't be seeing any high RPM's. Mainly, as my son gets older, the boat will probably be doing alot of the ski, tube and wake board pulling thing. However, when it's just me and some friends I'll probably get on it a bit harder.
    HP is only a calculation of tq over time. The more tq you can make, the more hp you will make at the same given rpm. For instance, if you want 500hp @ 5500rpm, you'll need to make 477 lbs/ft of tq. If you want to make 500 hp @ 5000rpm you need to make 525 lbs/ft of tq. If you were to make that same 525 lbs/ft @ 5500 rpm, you'd be making 550 hp. 467 ci is relatively small and takes some thought to make enough tq at lower rpm's to make decent power. Also, try not to get too wrapped up in the HP number game; usable power is what makes things move. It's a combination between hp and tq, neither being more important than the other. Remember...without tq, there is NO HP. If you increase tq, you will always increase hp. You can increase hp with more multiplication (through rpm) but then you're not increasing tq and this is usually not a reasonable trade off.
    For a 467 in a jet, I'd be looking at trying to make peak power in the 5500 rpm range and make as much peak tq as you can in the 4000+rpm range. An A impeller needs about 525 hp @ 5500rpm. That means actual, observed hp to the impeller in what ever weather you're seeing that day on the river. That means you probably need to make almost 600 hp corrected on the dyno. Still not a big challenge for a 467. You're talking less than 1.3hp/ci. At 4000 rpm you need about 200hp and 262 lbs/ft for your A impeller which should be a no brainer. As long as your engine's power curve stays ahead of the impeller's power curve, you'll continue to accelerate the impeller. The nice thing about the jet is that at lower impeller speeds the relative loading is light but here's a little curve ball for you to think about; A jet pump is one drive system that is 100% loaded even at idle.
    Last edited by scott foxwell; 12-18-2013 at 01:59 PM.
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    Senior Member N2GLOCK's Avatar
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    Quote Originally Posted by steelcomp View Post
    HP is only a calculation of tq over time. The more tq you can make, the more hp you will make at the same given rpm. For instance, if you want 500hp @ 5500rpm, you'll need to make 477 lbs/ft of tq. If you want to make 500 hp @ 5000rpm you need to make 525 lbs/ft of tq. If you were to make that same 525 lbs/ft @ 5500 rpm, you'd be making 550 hp. 467 ci is relatively small and takes some thought to make enough tq at lower rpm's to make decent power. Also, try not to get too wrapped up in the HP number game; usable power is what makes things move. It's a combination between hp and tq, neither being more important than the other. Remember...without tq, there is NO HP. If you increase tq, you will always increase hp. You can increase hp with more multiplication (through rpm) but then you're not increasing tq and this is usually not a reasonable trade off.
    For a 467 in a jet, I'd be looking at trying to make peak power in the 5500 rpm range and make as much peak tq as you can in the 4000+rpm range. An A impeller needs about 525 hp @ 5500rpm. That means actual, observed hp to the impeller in what ever weather you're seeing that day on the river. That means you probably need to make almost 600 hp corrected on the dyno. Still not a big challenge for a 467. You're talking less than 1.3hp/ci. At 4000 rpm you need about 200hp and 262 lbs/ft for your A impeller which should be a no brainer. As long as your engine's power curve stays ahead of the impeller's power curve, you'll continue to accelerate the impeller. The nice thing about the jet is that at lower impeller speeds the relative loading is light but here's a little curve ball for you to think about; A jet pump is one drive system that is 100% loaded even at idle.
    Wow! Thanks alot for that explanation. Pretty detailed and in plain English. I had no idea that torque played such a big role in making HP. On the cubic inches that I am looking at running, what area of the motor would I need to pay special attention to in order to make good torque #'s?

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    Quote Originally Posted by N2GLOCK View Post
    Wow! Thanks alot for that explanation. Pretty detailed and in plain English. I had no idea that torque played such a big role in making HP. On the cubic inches that I am looking at running, what area of the motor would I need to pay special attention to in order to make good torque #'s?
    Heads, intake and cam will be the most important. Keep the heads small; oval port or small rectangle port. For the intake manifold and carb, I'd go with the Victor Jr single plane/850 Holley and I'd recommend a hyd. roller camshaft (or solid if you like that sort of thing). I would not recommend a flat tappet cam of any sort. If aftermarket aluminum heads aren't in the budget, you can easily make your power goal with a set of 049 or 781 GM iron oval port heads with a valve upgrade and a little clean up in the ports and chambers. The only real disadvantage there would be weight. They're more than capable of making the power. I would not recommend a GM rectangle port over the ovals.
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    BTC cardcarrying member sunkisst's Avatar
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    Default Scott Foxwell...

    Quote Originally Posted by steelcomp View Post
    HP is only a calculation of tq over time. The more tq you can make, the more hp you will make at the same given rpm. For instance, if you want 500hp @ 5500rpm, you'll need to make 477 lbs/ft of tq. If you want to make 500 hp @ 5000rpm you need to make 525 lbs/ft of tq. If you were to make that same 525 lbs/ft @ 5500 rpm, you'd be making 550 hp. 467 ci is relatively small and takes some thought to make enough tq at lower rpm's to make decent power. Also, try not to get too wrapped up in the HP number game; usable power is what makes things move. It's a combination between hp and tq, neither being more important than the other. Remember...without tq, there is NO HP. If you increase tq, you will always increase hp. You can increase hp with more multiplication (through rpm) but then you're not increasing tq and this is usually not a reasonable trade off.
    For a 467 in a jet, I'd be looking at trying to make peak power in the 5500 rpm range and make as much peak tq as you can in the 4000+rpm range. An A impeller needs about 525 hp @ 5500rpm. That means actual, observed hp to the impeller in what ever weather you're seeing that day on the river. That means you probably need to make almost 600 hp corrected on the dyno. Still not a big challenge for a 467. You're talking less than 1.3hp/ci. At 4000 rpm you need about 200hp and 262 lbs/ft for your A impeller which should be a no brainer. As long as your engine's power curve stays ahead of the impeller's power curve, you'll continue to accelerate the impeller. The nice thing about the jet is that at lower impeller speeds the relative loading is light but here's a little curve ball for you to think about; A jet pump is one drive system that is 100% loaded even at idle.
    This guy Foxwell is one sharp individual. I'm sure glad I went with him to do work for me. Top end, 540 BBC. I have been reading his wisdom here for a while. He came highly recommended from Tn. Mike also. His website is very convincing too. www.foxwellmotorsports.com steelcomp and Foxwell are the same person.
    Last edited by sunkisst; 12-18-2013 at 04:00 PM.

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    Quote Originally Posted by sunkisst View Post
    This guy Foxwell is one sharp individual. I'm sure glad I went with him to do work for me. Top end, 540 BBC. I have been reading his wisdom here for a while. He came highly recommended from Tn. Mike also. His website is very convincing too. www.foxwellmotorsports.com steelcomp and Foxwell are the same person.
    Thanks for the kind words Jack...and you just reminded me I need to do a little "maintenance" on my website, especially since I don't live in CA any more.

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    gn7
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    Well said Scott.
    Let me add, torque tends to be a product of CID. The HP the engine is designed and built to make will determine where in the RPM the torque curve falls. As you build for more power, the torque band will shift up the RPM scale. But the max torque remains relatively the same, the curve just shifts.
    I think it was Ken Duttweiler that once said, build it for the desired power and RPM, and the torque will take care of itself.
    Translation: like Steel said,, you can't make power without making the required torque. Because torque is IN the equation, HP is simply the product of that equation.



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    I might have missed it but if he only has a block so far I would just put a 4.25 crank in it and start there. Would take very little to get there with a 489 or 496.
    the price difference on cranks would be minimal.

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    Quote Originally Posted by Red Rocket View Post
    I might have missed it but if he only has a block so far I would just put a 4.25 crank in it and start there. Would take very little to get there with a 489 or 496.
    the price difference on cranks would be minimal.
    I agree and was going to mention that in my post. There is no real reason to build a 468 if you do not have a crank. Racing rules is the only reason that would keep me from building a 496 if I was building from a bare block.

    its NOT a matter of price at all. In some cases, the 4.00 crank is actually more money than the 4.25. Very seldom is it more.
    I am not a fan of stock length rods in a 496 stroker either, specially if you don't have rods. But that is arguable with some. Has a little to with things like intake choice and heads selection. I still prefer longer 6.385 rods regardless of the top end. But that just me. I think the stock rod is too short even for a 4.00 crank.



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    Quote Originally Posted by Red Rocket View Post
    I might have missed it but if he only has a block so far I would just put a 4.25 crank in it and start there. Would take very little to get there with a 489 or 496.
    the price difference on cranks would be minimal.
    I don't disagree...he just said he would like to do the 468 thing so I responded to that. There are some benefits a 4" crank has over a stroker. No windage to speak of and I'd stick a 6.535 rod in it. Thing could make 600hp without trying and do it sipping gas.
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    gn7
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    Quote Originally Posted by steelcomp View Post
    I don't disagree...he just said he would like to do the 468 thing so I responded to that. There are some benefits a 4" crank has over a stroker. No windage to speak of and I'd stick a 6.535 rod in it. Thing could make 600hp without trying and do it sipping gas.
    Windage is over rated Stick a Stefs pan on it and take care of most of it. I wonder what the HP lose difference is between a Stefs kickout on a 4.25 and a box pan on a 4.00?

    he did ask about a 468. Just saying he may want to reconsider that if he doesn't have a crank. I don't get too excited about windage in a recreational jet at the RPM they tend to spin. JMO.



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    Thanks to everyone who has responded. The info has been informative and everyones opinion counts. I do not have a crank for this block. That was okay though as I was planning on buying a nice crank for it anyways. When I first started thinking about this build I decided to make it a full roller. Unfortunately, the additional cost of that will prevent me from running aluminum heads. I was planning on either using my existing 049 heads or getting another pair. My current motor has a Victor Jr. already so I'll use that again. My current carb is a custom built 750 w/ an HP main body. I'd like to try and use that if possible. If not, then I'll have to spring on an 850. As far as the 468 thing goes, I had decided on doing that because I'm a little afraid of building a stroker. I guess mainly because I don't know enough about strokers. Maybe I'm afraid for no good reason. Since time is on my side, I have the luxury of being able to do more research on the subject. In the end, what I wanna build is a well planned motor keeping in mind my hull and jet setup. I just don't wanna find myself going thru Summit and Jegs catalogs and just buying what looks cool. Then again, I don't wanna find out that I should of done things differently once the motor is finshed either. Steelcomp posted some pretty large HP and torque numbers. Quite a bit larger than what I was expecting. Looks like I need to get into the 600 HP neighborhood. That has me a little concerned because I'm of the thinking that I may have to sacrifice reliability in order to get to that number.

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    Quote Originally Posted by N2GLOCK View Post
    Quite a bit larger than what I was expecting. Looks like I need to get into the 600 HP neighborhood. That has me a little concerned because I'm of the thinking that I may have to sacrifice reliability in order to get to that number.
    friend of mine built a 489 single 4bbl engine 12 years ago. in a 19' semi-v, won races and championships with it. beat the crap out of it on the river, hundreds of hours of run time over 10 years. even put a small n2 system on it just to play a little harder. engine made just a hair over 700hp on pump gas. super reliable.

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    Quote Originally Posted by steelcomp View Post
    Quote Originally Posted by sunkisst View Post
    This guy Foxwell is one sharp individual. I'm sure glad I went with him to do work for me. Top end, 540 BBC. I have been reading his wisdom here for a while. He came highly recommended from Tn. Mike also. His website is very convincing too. www.foxwellmotorsports.com steelcomp and Foxwell are the same person.
    Thanks for the kind words Jack...and you just reminded me I need to do a little "maintenance" on my website, especially since I don't live in CA any more.


    Ya your a TN boy now, get that shit fixed

    To the Op, I done a 477 ( 4.00 stroke, 6.535 rod ) with 12:1 compression, Foxwell mildly worked over heads, Straub cam and making 750 HP on a tight budget . I agree with the guys on the 496 if starting from scratch. But you could take sum compression out to make it pump gas friendly and a single carb and do what your lookin for. Good luck
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