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Old 01-24-2021, 09:22 PM   #1 (permalink)
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Cruise RPM on the torque curve

Where is the best RPM for fuel economy on the torque curve?
Is it near peak torque? More? Less?
Does it vary by engine design/size?
And why?

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1966 Chevelle w/ 3.6L (217 ci) 4 cam direct injected V6, 6 speed double OD auto trans and 3.07 rear end. 3260 lbs with full tank of gas.
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Old 01-25-2021, 02:47 AM   #2 (permalink)
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I'm not totally with it right now, but...

A BSFC map would be more useful to look at than a torque curve. You can see what kind of fuel consumption you'd have based on the engine load and rpm.

Peak torque would generally be where an engine is most efficient...but only if you need all the power it makes at that RPM. And only if you keep you engine out of "enrichment" mode...usually starting somewhere around ~80% load.

So to answer your original question...you'll get the best fuel economy when you're in as high of gear as you can get without lugging the engine. Why? because otherwise you'll be running at less than optimal(close to maximum) engine load. (Efficiency takes a nose dive the lower the engine load, unless you're running a diesel engine. EGR mitigates this somewhat, but far from completely).

This is where having a smaller engine comes in. If you want maximum efficiency, you want to run it at high load as much as possible; a small engine will have to work harder, more often, to keep the car moving. Whereas if you're cruising in your v8 with the throttle barely open, you'll almost always be in an inefficient area on the BSFC map.

Engine design does matter...but if you had two engines with the exact same design, other than displacement, the smaller one would come out on top every time...unless you're constantly overloading it (too much weight, too many hills, maintaining too high of a speed), which, again, would push it out of it's efficient zone and the larger engine would get pushing in to it.

Uhm, yeah. something like that.
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Old 01-25-2021, 11:07 AM   #3 (permalink)
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Iíve had zero luck finding a BSFC map for my engine. Itís pretty small displacement, for the car anyway, and not particularly torque-y (sic). I find that when cruising on the highway between 55-65 it constantly wants to drop a gear to keep the same speed. Super annoying.
So I thought maybe Iím in a bad spot torque-wise on the curve???
FWIW, I have 26.6Ē tall tires, 3.07 rearend gears and the top two trans gears are 0.85 od and 0.67 od.
Itís pretty dam efficient for what it is when I drive it like there an egg under my foot but if I drive it normal it shifts around a LOT. This canít be good for efficiency.
Iím thinking maybe a lower rearend ratio (higher numerically) might get me closer to a sweet spot. How can I run a test to verify prior to changing?
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Old 01-25-2021, 03:24 PM   #4 (permalink)
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You might want to see if a tuner can change the shift responsiveness...it's all electronically controlled, if you're running what I think you are.

Only other thing to suggest is some kind of fuel consumption meter, so you can see for yourself how much fuel you use at what speed and load.

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