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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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    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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    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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    Quote Originally Posted by bikerfish1100 View Post
    typical status for all manufacturers/all models. the numbers are bigger at the crank, and therefore they sound more impressive.

    Numbers sound bigger at the crank because they ARE bigger. Horsepower measured at the rear wheel has losses through the driveline.
    Tom

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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 36654 View Post
    I think you are incorrect. Power varies with drive train losses. Torque is a function of gear ratio.


    Chassis dynos measure torque and calculate horsepower. (engine dynos may do the same, but I have only done tuning on chassis dynos) The calculation translates out to torque x rpm divided by 5252.

    I can change my torque and horsepower numbers significantly by doing my measurements in different gears. The torque multiplication of the transmission gearing is what causes this.

    There are divetrain losses on the chassis dyno, but the frictional losses that occur tend to be consistent no matter what gear you are in.
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    Quote Originally Posted by 36654 View Post
    I think you are incorrect. .
    I second that remark. The torque reads different in different gears, yes, but so does the speed. As horsepower is a function of torque and speed, the result remains the same.

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    Quote Originally Posted by EMSimon View Post
    I second that remark. The torque reads different in different gears, yes, but so does the speed. As horsepower is a function of torque and speed, the result remains the same.
    The speed of the vehicle is different in each gear. The speed of the engine (rpm) is the same. A chassis dyno is not using vehicle speed when it is measuring torque. It is measuring the "roller" speed of the dyno against a known resistance value and engine RPM. The speed of the vehicle can be calculated from these parameters, but the speed of the vehicle is not used to calculate horsepower.

    Horsepower = Torque X RPM / 5252. This is the formula to calculate horsepower. (RPM is the engine RPM, not wheel RPM) Vehicle speed is not a factor in this calculation.

    This calculation also holds true for engine dynos, in which an engine is not installed in a vehicle. (the HP ratings manufacturers publish)
    Last edited by twheats1; 04-24-2012 at 11:08 AM. Reason: clarity
    Tom

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    Quote Originally Posted by twheats1 View Post
    Chassis dynos measure torque and calculate horsepower. (engine dynos may do the same, but I have only done tuning on chassis dynos) The calculation translates out to torque x rpm divided by 5252.

    I can change my torque and horsepower numbers significantly by doing my measurements in different gears. The torque multiplication of the transmission gearing is what causes this.

    There are divetrain losses on the chassis dyno, but the frictional losses that occur tend to be consistent no matter what gear you are in.
    That would mean you are producing power in the transmission which is not possible. Torque multiplication is simply,

    5252*HP = (Torque * RPM)_1 = (Torque * RPM)_2 = (Torque * RPM)_3

    or, with proper gear selection (RPM) you can achieve a desired torque.

    On an internal combustion engine, you have to recall that the shaft torque vs RPM characteristic is not a flat line, so you have to base your comparisons on the input (i.e., shaft) values at each RPM.
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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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