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I want to know whether decreasing the mass of the frame of a car will increase its horsepower. From what I understand horsepower is a measure of the car's ability to transport load, and decreasing the frame's mass will increase this ability. I tried to search this on internet and all I got was that it depends on the horsepower of the engine not the car itself. It is very confusing. If my assumption that the car's mass has an effect on horsepower is right, then what is the possible way for me to calculate it?

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Doesn't matter the weight of the car. The horse power is the power of the engine. Noting to do with the frame. –  user28936 Sep 20 '13 at 18:54

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Let the engine's power be $P$, the weight of the frame be $M$ and the transport load be $m$.

From the definition of power $P=W/t$ where $W$ is the amount of work and the work in linear case is $W=Fs$, we get the formula $P=Fs/t=Fv$.

In this case, force exerted to the car $F$ is the constant. It's not change because it's the capability of the engine.(for ideal)

From the $F=(M+m)a$, we know the relation $a\propto\frac{1}{M+m}$.

$a$ is the acceleration of the car which can be the transport ability.

If the car has small weight of body frame, then the car can carry more loads than heavy frame with the same acceleration.

Add. Horse power doesn't matter with the weight of the car. It is the power of the engine. Noting to do with the frame.

But if you want to increase the net force exerted to the car in the direction of progress to get better ability in transportation, you have to consider particular factors.

The mass of the frame is the important thing but the other factors are also important.

The force exerted to car from the engine is not the whole force(net force).

Air resistance, frictional energy loss, etc. These factors interrupt the car's direction of progress, which can be interrupt the transportation ability.

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You're thinking of the power-to-weight ratio. The "horsepower" is just a measure of the amount of power the engine is capable of producing, which is independent of the weight. You can increase the power-to-weight ratio by increasing the power of the engine, or decreasing the weight of the car.

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