What RPM range do YOU get the best mileage at?

[Frank Lee]

Right! It has me going too fast in the F150; there I pick a speed just above where the torque converter would unlock frequently due to slight grades and whatnot.

[California98Civic]

Originally Posted by Frank Lee

Get the stroke then online calculators can tell you the rpm range that 1000-1200 ft/mn piston speed is. ...

With my Honda Civic's 3.54 inch stroke, one of the online calculators suggests that my engine is in the ft/min range you talk about when the engine is turning 1700-2000 RPMs. Interestingly, as I pointed out in my earlier response in this thread, that's one of the ideal rev ranges that I use MUCH the time. Now I might begin to use it nearly ALL the time as an experiment.

[Frank Lee]

All signs point to that piston speed range as a good proxy for when you don't have a BSFC chart.

Years ago I inquired here about optimal piston speed and why wouldn't slower be better (because the calcs showed my F150 so different from the cars and bikes)? There is a sweet spot between too slow (excessive heat loss through head, piston, cylinder walls) and too fast (higher internal friction).

https://ecomodder.com/forum/showthre...peed-1477.html

[2016 Versa]

My Versa with CVT seems to get it's best mileage between 1400-1600 RPM and depending on terrain is usually 40-50 MPH.

[Ecky]

Using Frank's figure of 1100ft/min +/- 100, my engine would be most efficient at 1700rpm +/- 150, or 55mph +/- 5mph.


Something I wonder about, is how ignition timing plays into this.

Here is one of my ignition timing tables, which I built based on maintaining a roughly fixed peak cylinder pressure:




When spark ignited, a flame front spreads from the spark plug through the combustion chamber, and it's ideal to have peak cylinder pressure occur when the piston rod has the best mechanical angle to spin the crankshaft. Flame speed is pretty constant for a given fuel and air mixture, so as the piston moves faster (higher RPM), you need to start combustion a few degrees of rotation earlier to keep peak pressure at the same crank angle.

At higher engine speeds, there's a larger percent of of high pressure before the piston even reaches the top (trying to spin the engine backward) and also too late in combustion, where the angle isn't very good. At least, to a certain point. My understanding is that, although the piston moves faster as RPM increases, after a certain point one does not need to ignite it any earlier, and indeed my ignition timing tables suggest this is true. I don't have a firm grasp yet on why this is so.

[Piotrsko]

I recall 1100 fpm as a magic number, but it was related more to a structural limit, either you couldn't aspirate faster, or perhaps fatigue on the rod ends. Been 40 years too long since I was a hot rodder. You may be correct on the ignition limit as I recall there was no point going beyond a certain amount of advance.

[MeteorGray]

I have always figured that after achieving high gear in any car, the lowest RPM value will yield the highest fuel economy.

[Ecky]

Originally Posted by MeteorGray

I have always figured that after achieving high gear in any car, the lowest RPM value will yield the highest fuel economy.

That's probably true with fixed gearing, because load is *probably* more important than piston speed in the vast majority of cases.

But, let's say you have a hybrid vehicle, which can spin the engine at whatever RPM, and toggle it on and off whenever. Is it going to be more efficient to spin the engine at, say, 3000rpm 90% load for 30 seconds out of every minute, or to spin it at 1500rpm 90% load and leave it running all the time?

[Hersbird]

Originally Posted by MeteorGray

I have always figured that after achieving high gear in any car, the lowest RPM value will yield the highest fuel economy.

I used to think that as well but with direct injection, a turbos, and an 8 to 10 speed transmissions I'm not so sure about that anymore. They are building in lower, and lower highway cruise RPMs all the time looking for better economy at 65 mph. With the turbo and direct injection they can get a good BSFC over a big range, but with 2 or more overdrive ratios the spped can get pretty high before you reach "top gear". Then maybe one of the lower gears would have less aero drag while still being at peak efficiency.

[serialk11r]

Newer engines with high EGR dilution can run better at slow speeds since the diluted mix burns cooler and loses less heat to coolant.