Originally Posted by Pete
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
For the 3.8 GM V6 this is between 1740 to 2100 RPM.
Cheers , Pete.
I've had Pete's post about piston speed in my sig for a long time but never actually did the math. Here's the math for my 2008 VW Rabbit:
2 * 3.6 inch stroke * rpm / 720 = piston speed (optimally 16.4 to 19.8 ft/s)
simplifies to this very convenient number:
7.2 * rpm / 720 = piston speed
which results in numbers like this:
7.2 * 1640 / 720 = 16.4
So, 1640 to 1980 RPM would be the most efficient piston speed. That's a measly 38 to 46 mph in my "high" gear (not really very high at all, but the highest I've got).
I guess that means I should experiment with normal cruising instead of engine-on P&G at that speed, though I usually EOC at that speed (which would have to be better than even the most efficient piston speed).
Also, I wonder if it means I should downshift to keep in that range when cruising at slower than that. There's a heavily enforced 30mph road on my way to work where I have been cruising at 30mph in 5th (~1300rpm), but maybe I should try 4th and see how that affects me. I could actually watch the fuel rate meter for this purpose. I think I'll try that, I may have never tried it at 30mph.
Okay, so my 2002 GMC 5.3 V8's stroke is 3.62204724 inches...almost exactly the same. Now if I could only convince the transmission to shift earlier under heavier throttle...