Quote:
Originally Posted by Peter7307
As a rule of thumb, most engines achieve their best fuel economy at an RPM corresponding to a piston speed of 5 to 6 m/s (16.4 to 19.8 ft/s). Piston speed (ft/s)= 2*stroke(inches)*rpm/720.
Piston speed = 2 x Stroke in inches x rpm / 720
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Where did you hear this rule of thumb? I had heard 20-25 ft/s but had heard other sources open it a bit more on the lower end.
You should be specific that the formula as entered gives piston speed in ft/s when you noted the rule of thumb values for ft/s as a side note. Nobody would be able to drive if they tried to work this formula to achieve an output of 5-6.
I find it easier to calculate piston speed in terms of feet per minute where the target range corresponding to 20-25 ft/s is 1200-1500 ft/min and the formula is
Piston speed= Stroke in inches x RPM /6
I personally find my best mileage when I center the engine's operating range around about 20 ft/s, shifting at 25 and letting it come down to about 16 in the next gear. Steady-state highway is shifted because my aerodynamics are pisspoor but I still find the best balance of fuel economy and time economy at 20 ft/s (75 mph). I can achieve better fuel economy at slower steady speeds, but I have to drop at least 10 mph to see any significant increases at which point driveability makes it harder to maintain FE due to constant downshifting (5spd trans).