Brake Mean Effective Pressure

Laying in my rack while stationed on the USS Kitty-Hawk (Aircraft Carrier CV-63), is when I first read about Brake Mean Effective Pressure (BMEP). From Kevin Cameron’s “Sportbike Performance Hankbook”, 1st Edition. If you have been a sportbike guy since the 1990’s, it’s the book with a black ZX-11 doing a wheelie on the cover. Whether you can get your hands on the first or second edition, reading chapter three will blow your mind. It’s titled, “How Engines Work”. You may say, “I know how engines work, Scott!” If you thought, “Revolutions Per Second” blew your mind by reducing the denominator, Kevin’s chapter three, plus all the other chapters, is filled with fascinating information. Did you know that combustion temperature at full throttle is 2,600 degrees Celsius? That temperature is not something someone would normally remember. So how is that mind blowing? It’s higher than the melting point of any metal in the engine. Any easy response is, “Yeah, that makes sense; that’s why there is coolant and/or cooling fins.” But he also points out that the combustion process is less than 10% of the time. The other 90% is the rest of the crankshafts revolutions to complete the cycle. Fascinating, you may think, but did you know there is also a boundary layer of stagnant air also insulating the metal parts? He goes on to explain these things you normally don’t think about. Pick yourself up a copy.

Ok, so I got on a bit of a tangent. The second time I read about BMEP, I was in my wife’s childhood home. In the mountains of the Philippines. Reading Kevin Cameron’s, “Classic Motorcycle Race Engines”. In the book, he lists the BMEP for most classic race engines and throughout most of the engine’s development years.

What is BMEP? It’s the power measured at the crankshaft (Brake horsepower), averaged (Mean), along the power stroke (Effective Pressure). Mr. Cameron explains it by saying, “BMEP is the effect of many things acting together: cylinder filling, combustion efficiency, heat loss, friction and combustion ratio. BMEP is the profit left after all the engine’s internal bills have been paid.” The same thing we get from an engine dynamometer. Or a chassis dyno, with the addition of drivetrain losses. Although not as accurate as measured from the crankshaft. However, if you keep your drivetrain variable to remain constant. Then, you have a reference point for torque and horsepower. If you modify your engine, you will see an increase or decrease. I see no need for BMEP. It seems like an elaborate formula to basically describe horsepower or torque per liter (displacement). Mathematics loves to reduce and simplify, but there must be some reason for the BMEP formula. If you’d like to see how the formula is derived and some modern cars are ranked; watch this video:

All this beating around the bush to say I don’t know what it is. If I knew why it was needed, then I would be able to say what it is used for.

So why would I write a newsletter about something I don’t understand? It was suggested from a reader and voted for (tied) in a poll; two newsletters back. So I took the topic and ran with it.