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Thread: A horsepower question.

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    It's a way of life! oldnslow's Avatar
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    Question A horsepower question.

    If a bike is rated at, lets say 90 hp at 6000 rpm, is it producing 90 hp in any gear as long as it is turning at 6000 rpm.? Please lets not get crazy with hp loss due to drive train frictions and such. Lets agree that the bike is rated at 90 hp at the rear wheel at the ground.
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    No. The horsepower figures are the maximum the engine produces under loaded conditions. If you were to put the amount of air and fuel necessary to produce 90 hp into an unloaded engine, it would over rev and destroy itself.

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    Left Coast Rider
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    Quote Originally Posted by oldnslow View Post
    If a bike is rated at, lets say 90 hp at 6000 rpm, is it producing 90 hp in any gear as long as it is turning at 6000 rpm.?
    If its at full throttle, yes.

    Thrust, of course, is something entirely different.

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    Norm Norms 427's Avatar
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    Quote Originally Posted by BC1100S View Post
    If its at full throttle, yes.
    +1

    If you ride down a street holding a steady 6000 RPM in 1st gear the throttle would not be wide open or even close to wide open. In this case it would be making only a fraction of the 90 HP, maybe only 10 HP or so. Taking it a step further, if you're stopped at a stop sign and rev the engine to 6000 RPM in neutral it would only be making maybe 5 HP or so.

    But under full engine load, wide open throttle, yes it would be producing the 90 RWHP regardless of which gear.

    In other words, at a drag strip when you're WOT (wide open throttle) going through the gears, then every time the engine swung past 6000 RPM it'd be making 90 RWHP.

    Another scenario, if you're bike goes 130 mph in 6th gear at 6000 RPM then it'd be making 90 RWHP in 6th @ 130 mph. But going 6000 RPM in 4th gear you'd be going maybe 95 mph but if you're on a level road under normal conditions your throttle would only be open maybe 75% and so the engine would be making less than 90 RWHP. Engine load is the key.
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    Horsepower is a function of torque and speed. HP= M x 2 ? n. Thus it will be the same at the rear wheel in different gears when the engine is at its max output.
    In higher gears and higher rpms, the torque is less and in lower gears at lower rpms the torque is higher. The resulting HP remains the same. Theoretically.

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    Registered User easy's Avatar
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    We probably need someone to weigh in on this, but I understand that the factory listed horsepower is usually measured by the factory at the crank at the rpm stated.

    E.
    Last edited by Easy; 04-25-2012 at 08:22 AM.

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    Quote Originally Posted by Easy View Post
    We probably need Paul to weigh in on this, but I understand that the factory listed horsepower is usually measured by the factory at the crank at the rpm stated.

    E.
    typical status for all manufacturers/all models. the numbers are bigger at the crank, and therefore they sound more impressive.
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    Quote Originally Posted by EMSimon View Post
    Horsepower is a function of torque and speed. HP= M x 2 ? n. Thus it will be the same at the rear wheel in different gears when the engine is at its max output.
    In higher gears and higher rpms, the torque is less and in lower gears at lower rpms the torque is higher. The resulting HP remains the same. Theoretically.
    Actually the horsepower reading will vary in the different gears, due to the torque multiplication that occurs in the transmission.

    To be accurate on a chassis dyno, one needs to pick the gear ratio closest to 1:1.
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    Registered User 36654's Avatar
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    Quote Originally Posted by twheats1 View Post
    Actually the horsepower reading will vary in the different gears, due to the torque multiplication that occurs in the transmission.

    To be accurate on a chassis dyno, one needs to pick the gear ratio closest to 1:1.
    I think you are incorrect. Power varies with drive train losses. Torque is a function of gear ratio.
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    Quote Originally Posted by twheats1 View Post
    Actually the horsepower reading will vary in the different gears, due to the torque multiplication that occurs in the transmission.

    To be accurate on a chassis dyno, one needs to pick the gear ratio closest to 1:1.
    This is why I said in my original post "thrust...is something entirely different."

    The engine produces 90hp. It will not produce more, or less, horsepower in a different gear.

    To quote a highly respected member here, "I have no idea, how discussions like this get started and how they morph into something totally different."
    Last edited by BC1100S; 04-26-2012 at 04:50 PM. Reason: typo

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    I have no idea, how discussions like this get started and how they morph into something totally different.

    This was the original question:

    Quote Originally Posted by oldnslow View Post
    If a bike is rated at, lets say 90 hp at 6000 rpm, is it producing 90 hp in any gear as long as it is turning at 6000 rpm.? Please lets not get crazy with hp loss due to drive train frictions and such. Lets agree that the bike is rated at 90 hp at the rear wheel at the ground.
    And the answer is: Yes! As long as the engine revs at 6,000 rpm, it turns out the rated horsepower of 90HP. No matter what gear you are in, you would measure 90 hp ( times efficiency, of course) at the rear wheel. The torque is different in different gears, yes, but so is the speed of the rear wheel in different gears, means the horsepower is the same - again under the assumption that the losses would be the same in different gears.

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    Registered User Rod Sheridan's Avatar
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    Quote Originally Posted by EMSimon View Post
    I have no idea, how discussions like this get started and how they morph into something totally different.

    This was the original question:



    And the answer is: Yes! As long as the engine revs at 6,000 rpm, it turns out the rated horsepower of 90HP. No matter what gear you are in, you would measure 90 hp ( times efficiency, of course) at the rear wheel. The torque is different in different gears, yes, but so is the speed of the rear wheel in different gears, means the horsepower is the same - again under the assumption that the losses would be the same in different gears.
    No it doesn't.

    The engine only produces enough power to match the load, plus drivetrain losses.

    If load is low (flat ground, full speed in low gear), the engine power output is low.

    If the load is high, (High speed vehicle, high load due to aerodynamic and frictional load) then engine power output is high.

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    And the descent into madness continues....

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    Registered User rxcrider's Avatar
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    Quote Originally Posted by oldnslow View Post
    If a bike is rated at, lets say 90 hp at 6000 rpm, is it producing 90 hp in any gear as long as it is turning at 6000 rpm.? Please lets not get crazy with hp loss due to drive train frictions and such. Lets agree that the bike is rated at 90 hp at the rear wheel at the ground.
    As you run past 6000 RPM at WOT, you should come close. If you don't have the throttle held all the way open, no.

    Other factors that will change your actual output relative to the rated value include:
    fuel charecteristics
    fuel temperature
    air pressure
    air temperature
    oil temperature
    head temperature
    humidity

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    Quote Originally Posted by rxcrider View Post
    As you run past 6000 RPM at WOT, you should come close. If you don't have the throttle held all the way open, no.

    Other factors that will change your actual output relative to the rated value include:
    fuel charecteristics
    fuel temperature
    air pressure
    air temperature
    oil temperature
    head temperature
    humidity
    Question: Have you ever had your bike on a dyno??? How are you getting to the rpm that is supposed to give you max hp (here: 6,000 rpm) without the throttle fully open?????
    In case you didn't know: On a dyno, you are running your engine against a load.
    You will have a hard time getting your engine up there without the thottle wide open. And of course, your SPECIFIED horsepower is at SPECIFIED ambient conditions.
    That's why a lot of dyno runs provide adjustements for the ACTUAL ambient conditions.

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