Re: Sloth Kills Fuel Economy!

Garth Almgren wrote:
Around 3/29/2008 12:22 PM, Scott in SoCal wrote:

On Sat, 29 Mar 2008 12:05:22 -0800, "Daniel W. Rouse Jr."
<dwrousejr@xxxxxxxxxxxxxxxxx> wrote:

So does driving over 3250 RPM in any gear when driving a fuel efficient
four-cylinder car, based on my observations of driving several four cylinder
cars including rental cars.


Wow, that's quite a conclusion there!

What, do you suppose, accounts for this 3250 RPM barrier, and why is
it the same for every fuel-efficient 4-cylinder engine? What RPM is
best for a non-fuel-efficient 4-cylinder engine (like the one in the
1973 Chevy Vega I used to have)? What about 6-, 8-, 10-, and
12-cylinder engines?


I can tell you right now, 3250 RPMs would put my V6 Mustang at almost 88 MPH, not its most efficient speed. The Mustang gets its best mileage at around 65 MPH and 2300 RPMs, and that's with a stone-age carb.

3250 RPMs in the Jeep which is a somewhat modern MPI 4 banger would put it at about 73 in 4th and 92 in overdrive, also not realistic speeds for fuel efficiency. 60 MPH (which is the slowest I can use overdrive) returns the highest MPG, and that's roughly 2100 RPMs.

So if you're just going to base it all on RPMs, my observations of my vehicles place the best point at an average 2200 RPMs or so.

BSFC is a much more important number than peak torque, IMO.


Sometimes - not always, but often enough that there seems to be some correlation - the lowest BSFC is at or near the torque peak.

nate

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