How Efficient are 911 Engines in Converting Air to Driving Force

[jluetjen]

I've been plugging some engine numbers lately to see how a 911 will stand up in SCCA E-Prod competition since it was recently classified in that class. The competition is 240/260Z's, 240SX's, Mazda Miata's, Honda Preludes and assorted English Sports Cars. Anyhow, one way that an engine is rated is by how well it coverts air and gas to energy. The usual measure is Brake Mean Effective Pressure and can be thought of as a measure of how much torque can be created by a given capacity. Some engines (think F1) spin a zillion rev's to pull the air needed to create HP. Some engines are limited in the rev department (think NASCAR) and so they have to try do create more force lower in the rev. range. But which works better??? In general the highest BMEP's are generated in air restricted classes (such as IMSA, GrandAm and FIA Sportscars or F3), but I haven't included any of those in this poll.


So; which of the above engines do you think creates the highest BMEP's?

Hint: It's not as obvious as you might think... I'll post the answers in the next day or so. Until then, here are some numbers for other engines:

Nissan Super Touring Motor: 248 PSI (The highest rating that I've found yet!)
Olds IRL Motor: 228
Honda F3 Motor: 204
PL Newman's C-Prod Datsun 280Z: 184
Group C Jag V12: 173
Porsche 924 D-Prod (Anyone remember this Factory racer?): 164
Ferrari 308: 149

[masraum]

Ok, I'm ready for the answer. I haven't a clue except for weirdness sake I'm going to say the Ford Cosworth or the RSR2.8. WAG

[jluetjen]

Good Guess (or rather guesses) Steve!

Keith Duckworth really had his sums together when he designed that DFV which eventually became the DFR. Apparently one of his goals was to have have good swirl in his heads to provide good charge mixing and combustion. But the 4 valve heads apparently flow well and the combustion chamber also has good "squash". I'm sure that the rest of the package was just as good.

In general Porsche's engines show up real well compared to contemporary designs, such as Ferrari's, Jag's, Datsuns. I've also added the recent 911GT3 and you can see that it's generating a BMEP comparable to a 908 which was no sloutch! The winner from Porsche's designs is the 917-4.5. The 5.2 was actually a little lower, but all versions of the 917 were better then other Porsche designs. It just goes to show how well they refined an engine who's basic design started with the early 911.

Here's the complete list:

Nissan Super Touring Motor: 248 PSI (The highest rating that I've found yet!)
Olds IRL Motor: 228
Honda F3 Motor: 204
Ford Cosworth DFR: 202
Porsche 917-4.5: 197
Honda V12 F1 Motor: 197
PL Newman's C-Prod Datsun 280Z: 184
Ferrari 312P F1 Motor: 182
Porsche 908: 180
Porsche 911GT3: 180
Porsche 906: 179
VW Super-Vee Motor: 178
Group C Jag V12: 173
Porsche 911 Carrera 2.8: 173
Porsche 924 D-Prod (Anyone remember this Factory racer?): 164
Porsche 911S 2.2: 164
Ferrari 308: 149
Ferrari 275 GTB-4: 149
Ferrari Dino 246: 138

BTW: The formulae for calcuating BMEP is (HP * 792,000)/(displacement in inches * RPM) or (Torque * 150.8)/Displacement in inches. In this case I used the first formula since HP numbers were more readily available then Torque numbers for many of these engines. Another footnote is that the BMEP at the torque peak will be higher then what I showed for each engine. Engine's generate their highest BMEP's when generating their peak torque. I guess that makes sense.

These calculations can be helpful in generating a "Target HP" for an engine build. If you know how many rev's you're going to turn, you can plug in a ballpark BMEP and out comes your target.

Alternatively; If you know how much HP you want, you can use this formula to back into how many rev's you need to turn based on an estimated BMEP. From there you can pick the required cam to turn the rev's and then build the rest of the engine to support those rev's.

Finally; it can also be used as a sanity check for dyno numbers. If you plug the dyno results into formula and come out with 190+PSI from a 911 motor - question the dyno data.

Have Fun!