torque vs HP in jet applications
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torque vs HP in jet applications

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    Senior Member $uperJoe's Avatar
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    Default torque vs HP in jet applications

    from what i read here it seems the best way to go faster is more HP so in turn you can turn your imp faster. seems logical. does engine torque play into any part of the effect?

    im asking these as extreme examples for comparison. i know most people have big block engines and some small block engines as well. small block guys are happy and also told the best way to go fast is to build a big block. is it because it is easier to build HP with the cubes?

    example: lets say i have a 12V cummings diesel producing 344 HP and 795Ft/lbs. it will only turn 3500 rpm safely. could i put a very aggressive imp with a small nozzle to increase thrust and compensate rpm?

    what about a 3.5L nissan motor that with turbo makes 450hp @6.5psi and 6200 rpm and only makes 287ft/lbs. will the small torque numbers kill my hole shot or can it be compensated with a smaller imp?

    ive read all the charts i can find and saved most of them for my own resources. lets here some real motor guys chime in on this one

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    I'm subscribing to this one. Sounds like I might learn something as long as you can keep the bickering to a minimum.
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    example: lets say i have a 12V cummings diesel producing 344 HP and 795Ft/lbs. it will only turn 3500 rpm safely. could i put a very aggressive imp with a small nozzle to increase thrust and compensate rpm?
    there's a thread or 2 or 3 or 4 that have been down this road. I always learn something new.

    In your example above the way to run that engine would be an over-drive between the engine and the pump. If the over-drive were 2:1 were you could spin an impeller to 7,000 would be the way to go ( more than likely) You'd have to figure out how much trq the overdrive will "eat" and then size the impeller after that.

    If the OD ate 125 lbs foot at 7,000 then your engine would net 670lbs ft at 7,000 at the output shaft ( 670 x 7000 / 5252 = 892 hp)
    You'd run an impeller that absorb that power at 7,000. ( now that I wrote that I think either GN7 or BP told me it wouldn't quite work out that way.... ) well they can correct me if I'm wrong.

    The other way to do what you want to do would be to run a large Hamilton jet. maybe even a two-stage. I think the Hamilton would work better if you have an extra $20k laying around.

    Note: someone correct where needed........

    Hope that helps......

    S CP
    Last edited by Sleeper CP; 09-27-2012 at 02:52 PM.

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    Or Seth, either one Budweiser's Avatar
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    Quote Originally Posted by Sleeper CP View Post
    there's a thread or 2 or 3 or 4 that have been down this road. I always learn something new.

    In your example above the way to run that engine would be an over-drive between the engine and the pump. If the over-drive were 2:1 were you could spin an impeller to 7,000 would be the way to go ( more than likely) You'd have to figure out how much trq the overdrive will "eat" and then size the impeller after that.

    If the OD ate 125 lbs foot at 7,000 then your engine would net 670lbs ft at 7,000 at the output shaft ( 670 x 7000 / 5252 = 892 hp)
    You'd run an impeller that absorb that power at 7,000. ( now that I wrote that I think either GN7 or BP told me it wouldn't quite work out that way.... ) well they can correct me if I'm wrong.

    The other way to do what you want to do would be to run a large Hamilton jet. maybe even a two-stage. I think the Hamilton would work better if you have an extra $20k laying around.

    Note: someone correct where needed........

    Hope that helps......

    S CP
    A 2:1 overdrive will cut torque output in half and double output speed. Horsepower is a wash if you overlook any losses in the OD.

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    Or Seth, either one Budweiser's Avatar
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    Quote Originally Posted by $uperJoe View Post
    from what i read here it seems the best way to go faster is more HP so in turn you can turn your imp faster. seems logical. does engine torque play into any part of the effect?

    im asking these as extreme examples for comparison. i know most people have big block engines and some small block engines as well. small block guys are happy and also told the best way to go fast is to build a big block. is it because it is easier to build HP with the cubes?

    example: lets say i have a 12V cummings diesel producing 344 HP and 795Ft/lbs. it will only turn 3500 rpm safely. could i put a very aggressive imp with a small nozzle to increase thrust and compensate rpm?

    what about a 3.5L nissan motor that with turbo makes 450hp @6.5psi and 6200 rpm and only makes 287ft/lbs. will the small torque numbers kill my hole shot or can it be compensated with a smaller imp?

    ive read all the charts i can find and saved most of them for my own resources. lets here some real motor guys chime in on this one
    Look at horsepower as an indicator of how much fuel you can burn. More fuel more power. More power more top speed.

    Look at torque as the twisting force.

    In technical speak, horsepower is torque and rpm. The twisting force and how fast it's twisting.

    A jet typically flashes to it's highest rpm, low rpm torque isn't really a factor.

    I'd take the Nissan... hands down.

    Love diesels though... gotta!
    Last edited by Budweiser; 09-27-2012 at 03:56 PM.

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    Quote Originally Posted by $uperJoe View Post
    from what i read here it seems the best way to go faster is more HP so in turn you can turn your imp faster. seems logical. does engine torque play into any part of the effect?

    im asking these as extreme examples for comparison. i know most people have big block engines and some small block engines as well. small block guys are happy and also told the best way to go fast is to build a big block. is it because it is easier to build HP with the cubes?

    example: lets say i have a 12V cummings diesel producing 344 HP and 795Ft/lbs. it will only turn 3500 rpm safely. could i put a very aggressive imp with a small nozzle to increase thrust and compensate rpm?

    what about a 3.5L nissan motor that with turbo makes 450hp @6.5psi and 6200 rpm and only makes 287ft/lbs. will the small torque numbers kill my hole shot or can it be compensated with a smaller imp?

    ive read all the charts i can find and saved most of them for my own resources. lets here some real motor guys chime in on this one
    i am not a real motor guy, just ask GN7, BUT


    look at an impeller curve...no torque on it. It aint that there is no torque, or that it dont increase as impeller rpm increase. It is easier to deal in HP. More HP, more thrust (mostly). Look at engine combinations (used on jets) HP TQ curves. Mostly the max torque happens below where the pump's rpm is limited by HP absorbtion. 450hp wins, torque looses. Torque is only half of the power equasion...like saying a 10 amp bulb is brighter than a 5 amp bulb...depends on the volts...in this case amps is torque, volts is rpm and watts is HP. 10watts is generally brighter than 5watts. Impeller/nozzle combo is (to me) more like style of bulb (incandescent, led, MV, etc.)

    I have never gotten a good answer as to how impellers' efficiency changes with size. I have experimented with nozzles... and have some data on impeller changes.

    Still not enough to make up for the 1.3 hp increase between the two engines. Notwistanding the 12v diesel probably weighs more than the Nissan 3.5.
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    Quote Originally Posted by Budweiser View Post
    A 2:1 overdrive will cut torque output in half and double output speed. Horsepower is a wash if you overlook any losses in the OD.
    yes, you reminded me thanks

    I think GN 7 and I had a conversation about a lever and torque.

    So the trq output at 7,000 would be 1/2 ( less the amount used by the OD)


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    Or Seth, either one Budweiser's Avatar
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    Quote Originally Posted by $uperJoe View Post
    example: lets say i have a 12V cummings diesel producing 344 HP and 795Ft/lbs. it will only turn 3500 rpm safely. could i put a very aggressive imp with a small nozzle to increase thrust and compensate rpm?
    To clarify a bit... this example produces maximum torque of 795, however it occurs at a lower rpm than it's stated maximum rpm.
    The above example, [email protected], is actually making 516 foot pounds at 3500. If it were able to maintain it's peak tq all the way through it's max rpm it would be 530hp.

    How much can you lift (torque)? How many times in a minute can you do it?(horsepower)

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    Default Theory vs. Reality

    I wont bother getting into the theory conversation. I have had several combinations from 402ci bbc to 632 ci bbc. If you want to have fun and win drag races build a torque monster. Now for you theory guys Im not saying put a 3000rpm red line diesel engine. My favorite engine to date is the 555 BBC Blown Alky motor with a PSI blower, it doesnt turn any more rpms than the 632 Big Chief motor with 2 Foggers, but pulls so much harder that it makes the Nitrous motor look Silly. Just a little input based on the Truck loads of $$$$$$$$$$ spent over the years.
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    Senior Member $uperJoe's Avatar
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    in torque as a lever it makes perfect sense. basic physics. i at about 7k rpm it may not be worth it to only turn 300ish hp bieng that it weighs as much as a blown big block

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    Quote Originally Posted by $uperJoe View Post
    in torque as a lever it makes perfect sense. basic physics. i at about 7k rpm it may not be worth it to only turn 300ish hp bieng that it weighs as much as a blown big block
    I'm glad Budwiser chimed in here maybe this time I'll remember

    a 300 hp engine that weighs that much you'd be going backwards.


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    Quote Originally Posted by Sleeper CP View Post
    I'm glad Budwiser chimed in here maybe this time I'll remember

    a 300 hp engine that weighs that much you'd be going backwards.


    S CP
    Now you know why NOBODY tries to run a Peterbuilt thru a gear box.

    Anybody here ever try to build a HP motor WITHOUT BUILDING TORQUE? The whole torque VS HP thing is a joke.
    It REALLY comes down to RPM ACCERATION, how fast the thing can get thru the RPM power band. Engine accleration under load. Again, why we don't race with Peterbuilts.
    You CANNOT build 1000HP without building a shit load of torque. Even a 1066HP @ 7000 engine makes 800 lbs ft.

    The ONLY time an engine, any engine can be considered a TORQUE engine, is if it redlines, or its power curve drop off before 5252. If it spins 5300, its making more HP than torque. Period. And you and NOBODY else can make it do otherwise. If it makes 500HP and 490 Lbs, is it a HP engine, or a torque engine?



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    Senior Member $uperJoe's Avatar
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    so if that pushes the diesel out then the next contender would be the nissan motor. its about 330 lbs complete plus turbo and intercooler at 450HP @6.5 lbs and running 93 octane fuel. i would loose a bit here in az as we only have 91 on the pump unless i convert to e85. dropping 500lbs out the back of the boat should make it jump out the water harder. or at least with all the work to potentially get it in there i would hope

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    It's not about torque, and it's not about horsepower, it's about power, more specifically, usable power. Any engine that accelerates a given load fastest from point A to B, wins, regardless of horsepower. In the case of a jet, the engine is accelerating the impeller, not the boat. The impeller has a power absorption curve. As long as the engine's power output exceeds the impeller's power absorption, the impeller will continue to accelerate. Once the two lines on the curve intersect, the impeller will stop accelerating.
    Last edited by scott foxwell; 09-27-2012 at 10:43 PM.
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