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Thread: HP vs. Torque

  1. #1
    mrossum
    i have been following a thread elsewhere in this furum about how much hp it takes to spin a pump at a given rpm. my question is this: if i have a panther pump that won't spin a whole lot of rpm anyways, why wouldn't the torque number be more important to me than HP?
    at the lower rpm's, the torque is doing the work. at least that is how i was taught when building engines. if you look at the graph of a dyno pull, the torque curve and hp curve intersect at some point. anything to the left of that point represents torque working, and anything to the right represents the hp working. i am by no means an expert, but this is how i was taught.

  2. #2
    steelcomp
    HP and torque are just words to identify a work force. It's the work force that's doing the work, not hp, and not torque. It makes no difference where they cross, because if you call them somehthing else, they may cross in a completely different place...just depends on the converson factor. (look at metric power conversions) Torque is basically a stationary work force...hp is torque which has been converted to relate to time and distance (RPM) using a conversion factor. They're both basically saying the same thing. Because of the conversion factor (and nothinig else) hp and torque cross at 5250rpm. Below that, torque will always be higher than hp, and above, the opposite. That's why it "seems" as if lower rpm is torque related, and high rpm is hp related. It's only a function of math, not the work force being applied.
    Hope that helps a little.

  3. #3
    Duane HTP
    Good answer. You stopped this argument dead in it's tracks.

  4. #4
    cfm
    On the subject but off the subject (subject is drag racing - not boats - but may give us some better thought)) - just a quote we should all read and think about. Kinda put a different twist on things:
    Darin Morgan writes:
    The simple explanation is that raising rpm effectively increases an engine’s displacement. This might seem nonsensical because the volume displaced by the pistons doesn’t change, but consider the effects of filling and emptying the cylinders faster in real time. An internal combustion engine is an air pump, and if we turn that pump faster, we can theoretically burn more fuel in a given amount of time and consequently produce more power. For example, an eight-cylinder engine running at 6,000 rpm fires its cylinders 24,000 times in one minute (assuming perfect combustion). Increase the engine’s speed to 8,000 rpm and it will fire 32,000 times per minute, a 33 percent increase. The volume of air and fuel that moves through the engine is now equivalent to an engine with a much larger displacement. There are also 8,000 additional power pulses per minute transmitted to the crankshaft that can be harnessed to turn the wheels and accelerate the car.
    Raising engine speed is analogous to supercharging or turbocharging a motor; the goal is to increase the volume of air and fuel that moves through the engine. The airflow is increased with a forced induction system by pressurizing the intake system; in a naturally aspirated engine, the airflow is increased by raising rpm. If done correctly, both approaches will increase power.
    A higher revving engine also permits the use of a numerically higher gear ratio to multiply the engine’s torque all the way down the drag strip. Let’s say an engine that produces 1,000 horsepower at 7,000 rpm is paired with a 4.56:1 rearend gear ratio. If this engine is then modified to produce 1,000 horsepower at 8,000 rpm, it can now pull a 4.88:1 or 5:14:1 rearend gear without running out of rpm before reaching the finish line. The numerically higher gear ratio gives the engine a mechanical advantage by multiplying its torque by a greater number to accelerate the car faster – in effect, it has a longer lever to move the mass.

  5. #5
    steelcomp
    Darrin's a sharp cookie, but his first sentence should read:
    The simple explanation is that raising rpm effectively increases an engine’s dynamic displacement. Raising an engine's rpm dosen't change it's static displacement. That's a given. The only time that ever happens is when things start taking on different material properties, and that's never a good thing in an engine. (like when an eight cyl engine becomes a five and a half cyl engine) :cry:
    What was the context of this quote? What is the question that this was the "simple" explaintion of? Just curioius.

  6. #6
    Tom Brown
    It's all about torque. Horsepower is just a number the tight t-shirt wearing dyno monkeys like to cite.

  7. #7
    Jbb
    It's all about torque. Horsepower is just a number the tight t-shirt wearing dyno monkeys like to cite.
    Testing out the flame retardant touque?....

  8. #8
    mrossum
    all answers above were GREAT answers that shed much needed light on this subject.
    back to the original question, though: shoud i be more interested in building a ton of torque at lowqer rpm, or a ton of horsepower at higher rpm???

  9. #9
    steelcomp
    you should be looking to make usable power. Don't focus on irrelevant names. There's a combination of "hp" and "torque" that'll give you what you're looking for. What's the application?

  10. #10
    mrossum
    you should be looking to make usable power. Don't focus on irrelevant names. There's a combination of "hp" and "torque" that'll give you what you're looking for. What's the application?
    18 ft Haskell flat bottom with 355 small block chevy and panther pump

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