From time to time, someone will ask us “Why does my car put out 290 horsepower on the dyno if it was rated at 300 horsepower stock?” Dyno results can be confusing for anyone who hasn’t read up on the topic. How can the same car put down different numbers just because the dyno manufacturer isn’t the same? These answers are simple much easier to understand if you know how a dyno calculates or measures horsepower and torque.
How Manufacturers Measure HP & TQ
So if Ford says my car makes 300 horsepower, how come the dyno results are lower? It’s actually really simple. Vehicle manufacturers will use a base horsepower (bhp) figure, also known as flywheel horsepower, in advertisements. These are the horsepower and torque numbers before the power leaves the engine and travels through the transmission and the rest of the drivetrain. As this energy is transferred, it will decrease as much as 10-20%, or even more depending on the vehicle. A typical Mustang will lose anywhere between 12-20% of the base horsepower number when tested on a chassis dynamometer. So while the car is advertised at 300bhp from the factory, it may put down as little as 265rwhp on a chassis dynamometer.
How Chassis Dynos Measure HP & TQ
So what is a chassis dynamometer and how does it measure horsepower and torque? Most chassis dynamometers (known as “dynos”) are large single or dual rollers that can weigh as much as 2500lbs. These rollers can either be free-rolling (inertia-type dyno) or equipped with a large drum brake (known as an eddy-current brake) to simulate load and put resistance against the vehicle.
Eddy current dynos use a load sensor in order to measure the amount of resistance being applied with the large magnetic brake. Load force, along with vehicle weight and aerodynamics factored in will allow the dyno computer to properly measure torque. It will then take a derivative of that number in order to calculate an approximate horsepower figure. Inertia dynos know the weight of the drum, and measure the acceleration of the drum as well as the rate of acceleration of the vehicle in RPMs. It arrives at the measured horsepower number and then calculates an approximate torque figure from there.
As always, the real testing is done on the street and at the track! Since there are so many different dyno makes with different ways to calculate horsepower and torque, these numbers can only be taken for what their worth. A dyno can be a very important tool in properly calibrating your Mustang to ensure that every aspect of the tune has been optimized. Just like anything else, your dyno results will largely rely on proper mechanical operation, temperatures, altitude, etc.
Tell us your thoughts, or even post up a link in the comments below to your dyno results and show us what your Mustang put down!