Brake Horsepower vs Flywheel Horsepower

     Horsepower is measurable, meaning that there are numbers involved and can therefore be compared.  The problem with measuring horsepower is it isn’t always measured in the same way.  When we add two with seven, we know there is only one definition for “two” and “seven”.  But when talking about cars, there are infinite unknowns which, as variables, we can’t always eliminate.  While 500 ponies is greater than 499 ponies, the problem is how both horsepowers are measured.  Differentiating between flywheel horsepower (engine horsepower) and brake horsepower (power to the wheels) definately helps, but it’s still not perfect, but I’ll leave that to another post.  There is also the problem of not being able to accurately convert between the two (flywheel and brake horsepower).  Flywheel horsepower is usually between 10% to 20% greater than brake horsepower due to drivetrain losses, but what does that really mean?

     When we  change parts to make our car faster, we usually use horsepower to determine whether it was successful/useful/worth the money or not.  We say that we’ve “gained” or “added” 10 horsepower to our engine/car, but that’s not always correct.  What we measure when we dyno our cars to find the horsepower is the power that’s conveyed to the wheels.  So from releasing the energy from fuel via combustion to sending them to the wheels to increase our velocity, there’s a bunch of stuff in between.  What the dyno measures is just the end product (the energy sent to the wheels).  We know there are various ways to make our car “faster” or “quicker” (there is a difference), but sometimes the only way we measure that is through horsepower.  Let me explain.

     One of the easiest ways to make our car faster is to decrease the weight of our wheels, since unsprung weight (or mass) has a larger effect on acceleration than sprung weight (or mass).  It takes more energy to rotate a mass than to push it linearly.  So if we’ve installed lighter aftermarket wheels and take it to a dyno to measure the effects, let’s say that we see an increase in 2 horsepower.  Now that’s not alot, but it’s still measurable through the dynometer, and thus we can make comparisons.  Lighter wheels = more power, right?  Not exactly.  Nothing has changed in both the input and output of the engine whatsoever.  Your engine still produces the same amount of power it did before you replaced your wheels. 

     But how does that make sense if our dyno clearly shows an increase in horsepower?  And an exact 2 hp at that?  An increase in brake horsepower doesn’t always equal an increase in engine horsepower.  And the engine is the only place to produce horsepower, not the wheels.  This is a direct example of that.  We can’t always convert our brake horsepower to flywheel horsepower just to fluff our power numbers.  Reducing the weight of our wheels does NOT make our engine produce more power.  The same amount of air is fed into the cylinder and the same amount of fuel is burned through combustion.  There is no change within the engine, nor is there any change from the input and the output of the engine.  The flywheel horsepower stays the same, but what has changed is the rotational losses of when the energy of the fuel is being sent from  the engine to the pavement (the brake horsepower).  And this is what seperates flywheels horsepower from brake horsepower.  Brake horsepower measures the power being put to the ground, while flywheels horsepower measures the power the engine produces.

     So while our dyno says there’s an increase in horsepower the engine doesn’t see any change.  Thus, while we haven’t “gained” any power, we’ve “freed” or “unlocked” some power through weight reduction.  A quick example of rotational losses would be the driveshaft, clutch, flywheel, brakes, tires, wheels, etc, anything that the engine has to turn while transmitting the power to the ground.  Reducing the weight of any of those parts will reduce the amount of power sapped before the engine power reaches the ground, no matter how small or large the weight decrease.

     So the next time you dyno your car, take care when converting your brake horsepower into flywheel horsepower.  The best thing is to just leave your brake horsepower as is, and to not convert it.

     //Drive on.


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November 2009
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© 2009 Rusi Li

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