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JNorthcut
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I realize that all engines are different, but does anyonen know about what the best engine speed is to shift at for optimal efficiency, or know where I can find some efficiency curves (power output vs. fuel consumption rate) for various engines?
I had always thought that the lower your rpms when you shift the better mileage you will get, but after thinking about it I am not so sure.
Any insight anyone can give me would be helpful.
Thanks,
-J
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RyanDL
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There was a recent thread on this in (I believe) the General forum.
Ryan
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floundericiousMI
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Honda used to have a graph like that for engines like the H22 in my prelude. Fuel burned per horsepower per rpm.
The bottom line is this:
A reciprocating engine operates with maximum efficiency around its torque peak and burns less fuel on the slower side of that peak. On the prelude, the low cam would peak its torque at about 3000-4000 rpms, so the most efficient CRUISING speed was 2500-3000 rpm, which correlated with 50-60mph.
For accelerating, you want to be sure you don't let the engine get slowed down to the point that you go into anti-stall. So you should shift high enough that your engine drops to about 2000 rpms once the next gear is engaged. Again, this is different car to car, but in the prelude that was approx @ 3500 for 1-2, and about 3000 for the others.
If you're really serious about your fuel efficiency, you should consider the following: Changing your shift points doesn't do so much to affect fuel economy.
The big things that affect fuel economy are:
1) Smoothness of throttle position change..the engine computer has a complex map that does measure the rate at which throttle demand is increased. If you increase too sharply, it engages one of the "max acceleration" programs as it believes you need full power NOW. If you go more smoothly, you can keep it in the cruising mode and avoid getting a sudden dump of fuel into the injectors
2) Smoothness of shifting and clutchwork... I noticed, with one of my manual cars a while back, that I was lifting off the throttle pedal too late in the shift process...as a result, the engine was revving up a little bit during a shift. This gave me rough/slow shifts but it also made my mileage HORRIBLE (lost 10-25% of my MPG). When I finally started paying more attention to that shift and lifting off earlier, my mileage went (under the same driving cycle and highway/city road speeds...not babying it by any means) from ~25mpg to 32-35!!!!
3) Smoothness of driving... This one's a head scratcher, as well. Consider this: Your speed through a corner has a large bearing on how much fuel you have to burn to get back up to speed. If you take a corner smoothly and carry much of your speed through it, you won't have to recover so much momentum.
4) Staying "on center" while cruising... Also, while driving up the road, try as hard as you can to find "center" on the steering rack and keep it there without turning the wheel back and forth. The PS system is driven off the engine either by electricity or by mechanical/hydraulics linkage. If you're constantly correcting back and forth in your lane, you're not only scrubbing speed off as cornering resistance (very small amounts, but still noticable), you're also constantly drawing boost from the PS pump. That can be a constant drain on the engine as well that drags down fuel economy.
That's most of what I can think up right now...
FL
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gila
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Good points. How about weight and tyre pressure, wheel alignment?
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floundericiousMI
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True enough....those all wind up tying in to point number 4 (in that they involve varying the amount of loading put into the contact patch) and have to do with the tyre scrubbing and directional control system losses.
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