Brake Mean Effective Pressure. Huh?

When it comes to engineering sleight of hand, two routines predominate for extracting more power on next season’s model. The first is simple: spin the old engine faster. If it can be forced to produce the same torque at higher revs, more power will result. (Power is a function only of torque produced and the revs at which it’s produced.) Unfortunately, this approach often compromises low-end driveability from greater overlap, etc.

Second trick is to add cubic capacity. Jumping from 2.0 to 2.5 litres means cramming 25 per cent more fuel/air mix into the process. If you’re lucky you’ll get 25 per cent more torque at the same revs for 25 per cent more power. Unfortunately, this option also adds weight.

In other words, a big engine spinning fast can be designed by dummies and yet produce copious power, whereas building a truly efficient, powerful engine (literally more bang for your buck-per-litre) needs a big R&D budget and smarter boffs.

If you want to see past the smoke and mirrors of engine speed and capacity hikes, you have to use the yardstick engineers call BMEP, or ‘brake mean effective pressure’. This pearler is simply power produced, but corrected for cubic capacity and revs. (Recreational thermodynamicists will note that proper BMEP is strictly a little different from this; indistinguishably so for most of us.)

What you do is: Divide the peak power of an engine in kilowatts by its cubic capacity in litres, then divide that number by the number of thousand rpms at peak power. Then compare similar engines. The bigger the number, the brainier the R&D.

Example: The HSV GTS makes 300kW at 6000rpm with 5.7 litres. BMEP = 300 divided by 5.7 divided by 6 (for 6000rpm) = 8.8. The competing DJR HP320 makes 320kW at 5300rpm via 5.4 litres. Its BMEP is a lot better, at 11.2 (320 divided by 5.4 divided by 5.3).

Remember, the result is an indicator of engineering fundamentals. Another way to look at it is the HSV seems to have more unexplored development potential.

Don’t compare atmo and turbo engines using this process; the results don’t add up since turbo engines have an ‘artificial’ cubic capacity boost. Engines with grossly different cylinder sizes aren’t really comparable either. (But most V8s, sixes and fours can be compared directly, as their cylinders are similar.)

techJohn CadoganComment