Brake horsepower, what it really means

Brake horsepower (bhp) is different than horsepower.  Bhp is the output at the flywheel (a heavy, round metal plate on the end of the crankshaft that is a vibration damper, and balance member.  It also smoothes out the power pulses as each cylinder fires), as measured on a dyno-test (where the maximum amount of horsepower and torque are measured).  Back in the old days, there was a device called a “prony brake,” which was used to determine the maximum amount of torque.  Horsepower was then calculated from torque and rpm.  Horsepower is the measurement of the maximum amount of the work an engine can perform.  It can be described as a way to move a weight a given distance- to apply leverage in a given period of time. 

Here is an example:  Let’s say a 5.4 liter V8 makes 320 bhp; the hp would be about 305.  That is because the transmission’s gears have to spin all that horsepower and torque, before it gets to the driving wheels.  If there was no transmission, all 320 horsepower would be going straight to the drive wheels. 

But, since there are transmissions, the automakers often dyno-test the engines to see how powerful they are.  The Smart Car makes 80 bhp, but once the horsepower is put down to the wheels; it totals 70 horsepower.  Back in the early days, 70 horsepower would have been reserved for an airplane!  Now look at cars today, the average economy car makes 115 horsepower!        

For an example of bhp; a horse has 1 bhp, because there is no tranny or gears to turn:  Its “transmission” is simply muscles!  Now, just imagine that horse pulling a wagon.  That would bring the maximum amount of horsepower to about 0.89 horsepower.  That really isn’t much… 

Knowing how much your bhp is comes in handy if you like to tow/haul.  If you are towing and don’t know how much bhp you have, it can be kind of scary!  So be careful!  And, no, I’m not trying to be your mom!