Question: I was sitting around one of the local automotive shops last week, doing some bench racing, when I heard someone mention BMEP. When I asked what the term meant, he gave me a long-winded answer that left me more confused than ever. So what is BMEP, and why not just use "torque" or "horsepower" instead?
Fast Eddie
Via the Internet
Answer: The acronym BMEP is an engineering term that means "Brake Mean Effective Pressure." "Mean" is another word for "average," which in this case means the average effective pressure of all engine-stroke cycles. BMEP is a function of the temperature of the gases in the cylinder. To increase the temperature, you need to either burn more fuel or increase the volumetric efficiency of the engine.
Facts: Torque is a function of BMEP and displacement, while horsepower is a function of torque and rpm.
Getting back to basics, remember that a typical automotive engine converts pressure into torque. The more pressure, the more torque. It's that simple. The hard part has always been trying to make more pressure.
Take a look at the chart and table on this page, which were taken from a training session at Katech. The chart shows the relationship between the piston speeds (PS) of various engines and the BMEP readings those engines generate. Note that the average pressure is highest at peak power and falls off from that point on. The actual BMEP numbers vary between engines, but the overall curve is very predictable.
Two things immediately jump out. First, all of the engines-with the exception of the LS7-are at approximately 200 psi BMEP at peak power. Second, the piston speed is approximately 4,500 fpm (feet per minute) on average. This is roughly the maximum speed a durable street piston can attain, given current technology. This isn't surprising, considering that all of these powerplants were based on solid engineering data.
All of the above engines were designed for maximum power and basically unrestricted in design, with high compression ratios, low friction, and unrestricted inlet and exhaust systems. The exceptions are the Winston Cup V-8, which was required to use a single four-barrel carburetor, and the LS7, which is installed in a mass-produced street car.
Listed below are a few equations you can use for quick power analysis. A few items will be needed for the calculations. They are cid (cubic-inch displacement), the stroke of the engine, and claimed power at a particular rpm. From these numbers you can quickly tell the following:
Piston speed: PS = Stroke x rpm/6 (Anything much higher than 4,500 fpm is suspect.)
Torque required for hp: TQ = hp x 5,252/rpm
BMEP from TQ: BMEP = TQ x 150.8/cid (Anything much higher than 200 psi is suspect.)
BMEP from hp: BMEP = (hp x 5,252/rpm) x 150.8/cid
So the next time someone tells you how much power he made, get out your calculator and keep him honest.
| | BMEP
@HP | | PS
@HP |
| ENGINE | CID | HP | RPM | PEAK | STROKE | PEAK(FPM) |
| Judd CV020 | 213 | 577 | 10,800 | 199 | 2.487 | 4,477 |
| '91 IMSA GTP | |
| 6.0L | 364 | 738 | 8,000 | 201 | 3.400 | 4,533 |
| '92 WC V-8 | 357 | 683 | 8,000 | 189 | 3.350 | 4,467 |
| Gen IV LS7 | 427 | 505 | 6,200 | 151 | 4.000 | 4,133 |
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