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Originally Posted by Suberman
F = ma. Power is not relevant. F means torque in the case of an engine.
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But what is important is the force F that is accelerating the car, NOT what torque the engine is making.
The force F that is accelerating the car is *directly* related to engine POWER.
F = 550*power/speed in ft/s
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Indeed, power is just torque x time
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No it is not. And it is not torque divided by time or "torque over time" either.
It is torque multiplied by rotational speed.
It is also force multiplied by linear speed (thrust * road speed).
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and bhp is a derived figure. The horsepower curve is not measured but calculated. The torque curve is measured and converted into bhp (that's the origin of the b for brake)
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Actually, disconnect the plug clip that senses engine rpm at the dyno, and you will get POWER only, and not engine torque. It is the engine torque that is derived from power, not the other way around. You know engine POWER because you know the dyno drum speed and torque, hence you know dyno drum power. You don't need to know engine rpm to know engine power because it is the *same* as dyno drum power. You *do* need to know engine rpm in order to derive engine torque from power.
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It is a common misconception that power to weight ratio is a helpful number and comes from a time when horsepower was the only figure the engine manufacturer published.
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NOt a misconception at all. Power is the relevant quantity.
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Power is indeed the rate at which Work is done and Work is just not related to acceleration.
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The RATE of doing work, which is what power *is*, is *absolutely* related to acceleration.
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Power is a constant speed factor and is a good indicator of top speed, but not acceleration.
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Of course it is the *only* way to estimate acceleration. Engine torque is not by itself useful.
Power is not a "constant speed factor", it determines acceleration potential.
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With two vehicles compared and each has the correct gearing power to weight can be considered to be useful for comparing predictions about acceleration intervals but only as a stand in for torque. Look at the torque curves and the weights and you'll literally see the acceleration curve differences. This is especially true when comparing supercharged engines or diesel to gasoline for example.
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If torque/weight is more important, why is the S2000 so much quicker than the FR-S/BRZ? Power/weight is what is important. Engine torque doesn't tell you enough.