Hey guys I tried a search and turned up nothing. I was reading my "tuning accel DFI gen 6" book last night and this among many things was brushed upon. But they didn't give enough info to draw any conclusions. Other than it is important and you should be concerned with it.

So can some one explain exacley what it is, how to measure it. What it should be. How to adjust it ect?

 

Simplifed, its the THEORETICAL average pressure in the combustion chamber on the power stroke. The higher the pressure, the more force on the piston, more torque on the crank, etc. more power out of the engine. BMEP is calculated from the torque, as measured on the dyno. Its a measure of the efficiency of the engine. Try Google.... you will find many explanations. Here's one:

http://www.epi-eng.com/ET-BMEP.htm

 

Fred beat me to it. His link is a good one.

The easy explanation of Brake Mean Effective Pressure (BMEP) in psi units, is "torque per cubic inch". It is a good way to compare engines of any cylinder size, number of cylinders, and their usage on the basis of how well they use air and fuel to produce torque. If you look at BMEP at power peak rpm(pprpm) it's a good way to see how any engine is doing it's job.

A fairly close conversion is: 1.0 lb-ft/cubic inch = 150.8 psi. So as an example: a ~7.0 L (427.65 cu. in.) engine with a 150.8 psi BMEP @ 6200 rpm power peak would be making 1.0 x 427 = 427 lb-ft of torque @ 6200 which is, of course 427*6200/5252 = 504.8 hp. Coincidently one OEM engine is right there.

150 psi BMEP (@prpm) isn't all that good. A Nextel Cup 358 making 850 @ 9000 has a BMEP of ~209psi. A 750 hp @ 18750 rpm 2.4L (144.6 cubic inch) F1 engine has a pprpm BMEP of ~219 psi. Remember Brake power (or torque or MEP) is what's left after friction and pumping losses are subtracted out. Friction (FMEP) and Pumping losses (PMEP) never get to the flywheel to do useful work. As rpm increases fricton and pumping losses increase, so making the same or more BMEP at twice the rpm is much more difficult.

A 600 fwhp @ 6500 383 would have a BMEP of ~191 psi. where a H&C 355 making 400rwhp @ 6700, about 455 fwhp, would have a pprpm BMEP of ~152 psi, right in there with that 7 L OEM engine.

One reason to be concerned with BMEP is what to expect from an engine design or modification. You are not going to get 210 psi pprpm BMEP with many 23° SBC heads for something you can street drive. Reason: on a 355 that's (210/150.8) or 1.39 lb-ft / cube or about 494 lb-ft @ pprpm. Let's say a 7500 rpm power peak. That's about 705 fwhp. That would be one strong 23° DD!

So if you look for 185 to 195 psi BMEP and a reasonable pprpm you can make a realistic estimate of your potential power. That puts a 383 peaking @ 6500 @ 582-612 fwhp, or in the 500-530 rwhp range. That's stout for an LT1, but possible for an LS engine with good heads.

If you build a 210 psi BMEP 400-450 cube LSX engine based on OEM or aftermarket components, with pprpm in the low 7s, you'd have one powerful contender for bragging (if not dragging) rights.

Oh, yeah, to calculate BMEP divide the torque (at whatever rpm you choose) by the cubic inches and multiply the result by 150.8. If you have hp and rpm, this will work: BMEP = (HP * 5252/rpm) / displacement * 150.8

Homework questions:

1) Where is any engine's highest BMEP? Why?

2) What's the BMEP @pprpm for an NHRA Pro Stock (500 cube) engine making 1440 @ 9400? Are you impressed?

3) What's the BMEP for a 10 cc (.61 cube) model airplane or model car engine making 2.0 hp @15000 on 5%-10% nitro? Are you impressed?

More than you wanted to know, I suspect.

 

Hey guys..... both posts are outstanding.

 

Outstanding both of you.

OldSS

I going to take a shower and then do my homework.

A little thought just popped into my head though. IS BMEP directley correlated to DCR? As the BMEP goes up is it safe to assume trying to avoid detenation becomes a concern?

 

1. Peak torque or peak VE as this highest torque and thus the highest BMEP is acheived at this point.


2. 242.65 Holy Shet!!

3. 173.11 That seems pretty reasonable, but amazes me those little parts can take pressure that high.