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Thread: What is BHP?

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    tobycruse's Avatar
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    What is BHP?

    Before you all say "what type of stupid question is that!" hear me out!

    We all know and feel what torque is, you know when a car has more torque or less torque than another.

    But what is BHP? How do you describe a car that has more BHP than another?
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    Quote Originally Posted by tobycruse View Post
    How do you describe a car that has more BHP than another?
    You can simply say it is faster than the other car
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    Generally anything that really revs will make good power and not a huge amount of torque, so if you have any experience of bike engines or high revving N/A engines then thats the feeling of power

    A high power / low torque engine on a road application is pretty much pointless unless its in a very light car
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    "Brake horsepower is measured by taking a drum filled with a fluid and attaching that to an engine. The power required to turn the drum is the brake horsepower - as the drum is braked by the fluid."

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    A bit of plagerism

    The term "horsepower" was coined by the engineer James Watt in 1782 while working in the performance of steam engines. This occurred while using a mine pony to lift coal out of a coal mine. He conceived the idea of defining the power exerted by these animals to accomplish this work. He found that, on the average, a mine pony could pull (lift by means of a pulley) 22,000 foot-pounds per minute. Rather than call this "pony" power, he may have increased these test results by 50 percent, and called it horsepower i.e. 33,000 foot-pounds of work per minute.

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    Quote Originally Posted by PW-Motorsport View Post
    A bit of plagerism

    The term "horsepower" was coined by the engineer James Watt in 1782 while working in the performance of steam engines. This occurred while using a mine pony to lift coal out of a coal mine. He conceived the idea of defining the power exerted by these animals to accomplish this work. He found that, on the average, a mine pony could pull (lift by means of a pulley) 22,000 foot-pounds per minute. Rather than call this "pony" power, he may have increased these test results by 50 percent, and called it horsepower i.e. 33,000 foot-pounds of work per minute.
    increased because a horse is fifty percent bigger. Mmmm. Obviously its got something to do with horse abilities.

  8. #7
    Some car manufacturers still quote a power output rating called PS, which stands for Pferdestrke (literally, 'horse strength'). It's alternatively known as DIN (Deutsches Institut fr Normung ) hp as opposed to SAE (Society of Automotive Engineers) bhp.

    PS is a slightly higher figure than the SAE bhp. As a rule of thumb, you can knock off one PS for every 100PS to reach a rough bhp figure. If you want to be completely accurate, multiply the PS figure by 0.9864 to reach the bhp total, or bhp by 1.0139 to get back to PS.

    An EU Directive tried to replace the use of PS with kilowatts in the 1990s. One bhp is equivalent to 745.7 Watts, so multiply bhp by 0.7457 to get a kW figure, or kW by 1.341 to do the sum in reverse.

    This is why Audi (and others) tend to quote power in both PS and kW.
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    Quote Originally Posted by jonnyc View Post
    Generally anything that really revs will make good power and not a huge amount of torque, so if you have any experience of bike engines or high revving N/A engines then thats the feeling of power

    A high power / low torque engine on a road application is pretty much pointless unless its in a very light car
    That makes sense! Thanks Jonnyc!
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    Even when you understand what BHP actually is, it's only part of the story. How the BHP is used is more important, how the car translates it to the road surface, controls the power, the weight of the car vs. BHP etc.
    A Lotus Elise is a good example of a little-ish amount of HP put to good use in a tiny light car. Whereas the new RS6 with 572 BHP weighs about the same as a bungalow ....Pointless in my opinion.
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    Quote Originally Posted by PW-Motorsport View Post
    A bit of plagerism

    The term "horsepower" was coined by the engineer James Watt in 1782 while working in the performance of steam engines. This occurred while using a mine pony to lift coal out of a coal mine. He conceived the idea of defining the power exerted by these animals to accomplish this work. He found that, on the average, a mine pony could pull (lift by means of a pulley) 22,000 foot-pounds per minute. Rather than call this "pony" power, he may have increased these test results by 50 percent, and called it horsepower i.e. 33,000 foot-pounds of work per minute.
    I'm a bit confused by that as it uses 3 variables in foot-pounds per minute. Length, weight and time. Even more confusing as foot-pounds is a measurement of torque. And we know that you can't convert torque to BHP by dividing it by time!

    One way to think about torque rather than power is to think about the actual force required to turn the wheel. The heavier the vehicle the higher the resistance to the wheel turning and you need more torque to overcome it. The same goes for going up hill, it's effectively adding weight to the vehicle so more torque is needed.

    BHP on the other hand still baffles me. I think generally if you have 2 identical vehicles with the same power but different torque they will be about the same performance. But if you add weight or go uphill the one with more torque will go faster. I may be talking complete crap though as I don't know that maths behind it.
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    BHP is a number which is used to start off all sorts of heated arguments and debates after a Rolling Road session.
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  13. #12
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    Quote Originally Posted by Iggu View Post
    BHP is a number which is used to start off all sorts of heated arguments and debates after a Rolling Road session.
    Now that is VERY true lol

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    Torques are the amount of rotational force put to the wheels from what I can understand about it.

    BHP is how quickly and engine can develop its potential. So the difference in the same size engine is how quickly it can reach his peak performance hence the more BHP you have the quicker you accelerate.

    I was always told though torque wins races for pulling you out of the corner
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    Torque at the wheels is different to torque produced by the engine. There is a crossover point of something like 5250rpm engine rpm where anything above that and the gearbox increases the torque at the wheels over what is produced by the engine. So cars like VTEC powered cars which have low engine torque levels actually produce good torque at the wheels but only once the engine is putting in speeds above that into the gearbox.
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    Just found a link to what I was talking about. http://www.revsearch.com/dynamometer...orsepower.html
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    lucky it wasnt a shetland pony mr watt used huh

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    torque is the force to turn the wheels, power is the amount of energy (joules per second) being used when that force is doing work

    an example of high torque but low power would be an electric motor - when stalled it develops maximum torque but isnt expending any energy doing work so is delivering zero power, similarly a diesel has a high torque but at low revs cant deliver as much energy - as the revs go up, the torque drops off rapidly and so the delivered of power is limited.

    as power is proportional to torque and revs, high revs but a low torque can deliver the same energy, but needs a bigger reduction of the revs through gears and the gears multiply the effective torque like a long lever - the overall result is the same work done by either a high torque, low revs high gear or low torque high revs lower gear engine.

    if your torque drops rapidly with revs, you have to change gears to get the engine working at a higher torque output eg like a diesel, but ultimately the continuous acceleration requires energy to be expended and that depends on power and weight ie bhp/tonne (force=mass x acceleration), it doesnt really matter whether its through low revs, high torque or high revs lower torque.

    when torque is in excess of the energy (power) requried to deliver that speed, you get a sensation of greater push, so a diesel or turbo petrol with a big mid-range torque will feel a lot quicker because its mid-range acceleration, but only over a limited range - ultimately the continued acceleration depends on power.
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    I liked this one when someone explained it to me:

    Imagine 2 cyclists starting a race uphill.
    Cyclist 1 has massive thighs like tree trunks - ultra strong and powerful. But he's a chain-smoking fatboy who runs out of puff after a few pushes of those legs.
    Cyclist 2 has skinny little stick legs with not much strength in them - but he's ultra fit and can spin those legs as fast as bees' wings.

    When they both start off Cyclist 1 will push ahead for a while as his massive legs spin the wheels round easily. Cyclist 2 will soon catch up though once he gets his speed up and Cyclist 1 runs out of puff. At that point they are putting out the SAME power (bhp) but how they got there is different.

    Cyclist 1 is a Audi A3 TDi (all torque)
    Cyclist 2 is Honda Civic Type R (all revs)
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    God all we do on here is torque torque torque

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