when they say a car has 250BHP (brake horse power) how do they measure it? like how did they get 250 from the brakes? thanks

 

They're not talking about the vehicle brakes.

An engine dynamometer lets the engine run, but instead of being hooked up to a transmission, the crank/flywheel is hooked up to a brake. That way the throttle can be wide open, & the brake controls the speed. Then it measures the torque and rpm & calculates HP from that. "Back in the day" (huh?) it was a friction brake, held in saddle bearings. Had a lever so you could pile weights on like a beam balance. Called a "Prony brake", I think. Now they probably use viscous or eddy-current brakes with load cells to measure torque.