The Great Equalizer: Understanding Brake Mean Effective Pressure

A while back, we posted an article explaining some of the different numbers, other than horsepower and torque, you will find on an engine dyno report. Jeff Smith did a great job of explaining several different calculated values on that sheet (including BMEP), what they mean, and how to use them in your daily life.

A few short months later, there was a very public dyno test, of a very popular new car, that produced some very impressive numbers, to the point of being unbelievable. While right off the bat we made the call to not touch that situation with a 10-foot pole, some of our friends had no such reservations and jumped into the fray with both feet.

Between their opinions and several others that popped up around the internet, there was one figure being used — almost exclusively — to prove the dyno numbers were highly improbably and, very likely, incorrect. Of course, the internet masses screamed that the people relying on numbers and science were simply “haters,” only to be proven wrong a short while later. The dyno numbers in question were indeed incorrect.

So how did people know the numbers didn’t make sense? Quite simply, through Brake Mean Effective Pressure (BMEP). Simply put, Mean Effective Pressure is a calculation of the average engine cylinder pressure throughout the combustion cycle. Adding “brake” in front of it simply means the number was calculated from a torque value measured on an engine dyno.

Here you can see that BMEP is calculated by the dyno software (in PSI) at every datapoint it creates on a dyno sheet. You don’t need fancy software to calculate BMEP though. Really, all you need is the engine’s displacement and torque value.

Breaking Down BMEP

While Jeff gave us the equation (in lb-ft, cubic inches, and PSI) the dyno software uses to calculate BMEP in the other article and some alternative calculations. However, you know we don’t like to just be told to do something, we want to know the “why” and “how” behind it. Luckily for us, we have Jason Fenske of Engineering Explained who not only loves to dive into the minutiae of the how and why but is very good at explaining it as well.

We’ll warn you now, if you don’t want the advanced breakdown of how BMEP is calculated, Completely skip the section of video from 1:19 to 6:25. However, it’s an incredibly interesting lesson, as long as all the numbers don’t melt your brain. Fenske breaks down each of the factors that go into the equation and explains why they are there.

“Is all of this just a glorified way of talking about torque-per-liter?” laughs Fenske. “Well, yes. That’s basically what the equation boils down to. But think about how much more interesting people will find you at parties when you say words like ‘brake mean effective pressure’ instead of torque.”

To show how the BMEP number is useful in comparing different engines, Fenske uses the published peak torque values from a number of popular cars, ranging from low-power sports cars to high-powered exotics, and can compare their efficiency on an even playing field. Granted, Fenske is using metric values, which isn’t what most of you reading this are used to, I know, but it makes for a very simple scale on which to judge a wide variety of engines.

On the left is the equation given to us by Jeff Smith, and is how most dyno software calculates BMEP in imperial units, with "150.8" being a constant. On the right is how to calculate BMEP using metric units, and converting to bar of pressure, which allows for Fenske's easy-to-digest 10 to 15 bar comparison scale.

On Fenske’s scale of 10 to 15 bar BMEP, he says most of the production engines he calculated fall between 12 and 13 bar. “I looked at and calculated about 100 different BMEPs for a bunch of different cars out there. Most are between 12 and 13, but a good number also fell between 11 and 14,” Fenske explains of his selected range.

For example, a Ferrari 812 Superfast engine has a BMEP of 13.9 bar. The LS2 in a C8 Corvette comes in a 13.0, while a 2016 Mazda Miata and Lexus LF-A (which are on complete opposite sides of both the affordability, and ostensibly performance, spectrum) both share a 12.6 BMEP. Surprisingly the Gen-3 5.0L Coyote engine in the 2018+ Mustang has a BMEP of 14.2, putting it into rarified territory for its price point.

However, Fenske also stresses, that if you are going to use BMEP as a yardstick, you need to make sure you are comparing similar engines. Comparing a naturally aspirated gasoline engine to a turbocharged engine on methanol will deliver skewed results. He illustrates this point, by calculating the BMEP of a new Chevy Malibu at 22.2 bar — with no consideration of the forced induction included from the factory.

However, he does point out that you can account for forced induction by dividing the BMEP by the pressure ratio of the boosted engine. Using the new Koenigsegg 2.0L engine making 600 Newton-meters of torque, you see an insane 37.9 bar BMEP, but once the boost is factored in, the BMEP drops to 12.7, which puts that insane engine into Mazda Miata territory.

Then, illustrating not only how boost can skew data, but fuel as well, Fenske breaks down the dyno test of the 2015 Top Fuel engine dyno. It’s BMEP, all in, comes out to 166 bar. Once boost from the blower is factored in, the Top Fuel engine still has a BMEP of about 17.0 bar, which Fenske attributes to the benefits of nitromethane as a fuel.

But, you can compare the absolute numbers to garner general ideas about different engines. To make that point, he calculates the BMEP of the 6.6-liter twin-turbo V12 Rolls-Royce Ghost engine to be 14.9 — WITH the boost, showing just how impressive some higher-end N/A engines are by comparison.

After watching this well-executed breakdown of BMEP, it should be clear why BMEP was the choice of so many when expressing their lack of confidence in the dyno test mentioned earlier. It’s also nice to know how to be able to calculate BMEP from scratch and compare engines in a much more precise way, in both imperial and metric figures, to boot.

Here you can see how different powerplants line up with one another when compared with BMEP. At the bottom, you can see the differences forced-induction makes (or doesn’t make) on overall engine efficiencies.

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About the author

Greg Acosta

Greg has spent fifteen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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