Best Rpm

By looking at an rpm and horsepower graph, how would you determine where the most effiecent RPM would be?

Thank You

By looking at an rpm and horsepower graph, how would you determine where the most effiecent RPM would be?

Thank You

You cannot. The horsepower curve is only going to tell you how the engine power output changes with engine speed.

You could try to find a brake specific fuel consumption (BSFC) versus engine speed curve which will show the change in work obtained per unit mass of fuel with change in engine speed.

However, the BSFC curve is not going to be directly related to your fuel consumption or mileage (that is distance per unit of fuel or vice versa). Although, BSFC is going to effect mileage so are the vehicles characteristics (aerodynamics, friction and losses, speed, gearing, etc), road and environmental conditions (flat, uphill, downhill, windy, temperature, etc), and other changing quantities.

If you are looking only at the engine in terms of power out over power in the BSFC should be sufficient. But if you are looking for something more complicated than that you may have a hard time obtaining the information.

On most well-matched street engines the BSFC is vaguely (and I mean very vaguely) associated with peak VE since that is where dynamic compression is at its highest. The peak VE is loosely associated with the torque peak, so if you're looking for peak BSFC on a street engine, look to within 1500 rpms on either side of the torque peak. If you're looking to find peak FUEL efficiency, that's a whole different animal. That dyno chart exists only at WOT. Peak MPG will occur as a function of foot position, fuel quality, condition of the O2 sensors (if used) speed and amount of accelerator change (since both acceleration and deceleration can cause enrichment)

At a single throttle opening where friction, altitude, temperature, and every other parameter remains constant, BSFC will more accurately correspond to fuel economy, but never confuse the two. Thermal efficiency expressed as BSFC refers to how many BTUs of the fuel make it to the crankshaft as power... or as Alastor put it perfectly... power in vs. power out. But BSFC has nothing to do with how much mass of fuel gets consumed per mile.