BMEP—that’s short for Brake Mean Effective Pressure—is an important yardstick in engine development. Here’s Jason Fenske of Engineering Explained to sort it out for us.

There is more than one way, naturally, to consider the potential work we can perform when we combust fuel and air in the cylinders of a piston engine. One familiar method includes measuring the torque produced at the crankshaft, as we do on the dynamometer, which we can then state as pound-feet (English) or newton-meters (SI) units of rotational force.

But if we choose, we can also back up a few steps and regard the engine’s output not as rotating force aka brake torque at the crankshaft, but more directly, as the explosive or expansive force produced within the cylinders. Calculated as an average, we call this value Brake Mean Effective Pressure, or BMEP for short. This gives us a tool to study the engine’s effectiveness independently of displacement, rpm range, bore/stroke ratio, and a host of other factors. With this yardstick, we can study wildly different engines designed for entirely different purposes—say, from a Toyota Prius and a Formula 1 car—and compare them on a fair and equal footing. (Along with Brake MEP, engine developers also consider Indicated MEP, Friction MEP, Pumping MEP, etc.) 

Of course, there’s a lot more to it, and fortunately, Jason Fenske of Engineering Explained has put together this concise and entertaining explanation. BMEP is a powerful metric among engine developers, but there’s no reason it can’t be equally useful among car enthusiasts. Video below.