Normally aspirated 2.4L doing 300 whp...

Recently there have been rumors of a normally-aspirated 2.4L Honda engine doing "300 whp" prowling the streets of Singapore. Let's evaluate the claim.

Based on a popularly optimistic dyno, 300 whp would translate to about 360 brake horsepower, assuming losses of 16.5%. By way of comparison, the S54 engine (3.2L inline 6) found in the BMW M3 CSL needs all of 7900 RPM to produce similar power. So a top of the line engine from BMW M division, with greater displacement and more revs, thereby flowing much more air, only produces the same power as an engine with 2 fewer cylinders, 25% less displacement and less revs. Right.

Let's dig deeper.

Brake Mean Effective Pressure (BMEP) is a synthetic engine performance index; it does not give actual cylinder pressure but affords us a good idea of what is possible for a particular engine in discussion. It can be derived by taking the torque in lb-ft, multiplying it by 150.8 and dividing the result by the engine capacity in cubic inches. A 2 liter engine would be about 122 cubic inches.

A 2006 Nextel Cup Car with a stroke of 3.25" produces 840 HP @ 9000 RPM. Torque would thus be 490 lb-ft. For such an engine, BMEP would be 206 psi. Peak torque is said to be about 550 lb-ft, or BMEP of 231 psi.

Remember, we are dealing with normally aspirated engines, so this is about as good as it gets.

Formula 1 engines: 2.4L V8, approximately 750 bhp @ 18500 RPM. BMEP of 219 psi at peak power.

2007 Porsche 911 GT3 RSR. This is a naturally aspirated monster. With just 3.8 liters of displacement, and FIA restrictors in place it produces 485 bhp @ 8500 and 320.8 lb-ft @ 7250. Running with a compression ratio of 14.5:1, it requires 100 octane fuel to avoid detonation. BMEP on this car at peak torque is "just" 209 psi.

For this mythical K24, stroke would be 3.9". Due to piston speed limitations, let's just say it produces its peak power of 360 bhp @ 8000 RPM. Thus torque would be 236 lb-ft. BMEP at peak power is 243 psi!

Perhaps next year's Nextel Cup and FIA GT cars should run on this K24 instead!

Bonus credit: With a 3.9" stroke, this 2.4L engine will be pushing mean piston speeds of 5200 fpm @ 8000 RPM, or 26 m/s. That's like revving the famous F20C engine in the Honda S2000 to 9455 RPM, when factory rev limit is 8600...

In view of such extreme real-life examples, we are forced to conclude that it is highly improbable that a 2.4L engine of such dimensions, revving at 8000 RPM, naturally-aspirated, is capable of producing 360 bhp.