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Old 01-06-2013, 03:45 AM   #11 (permalink)
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But it's more than just the efficiency of the engine at play. If the engine were "too powerful" - and you had the gears for it - you may have to drive 100 mph+ to be at the peak efficiency RPM. But the aerodynamic drag in that situation would wipe out the small efficiency gain of the engine. And running in a lower gear just to keep the revs up to the "most efficient RPM" would cause worse MPG, too.

Also, if that engine is "too powerful", pulse & glide would become less effective too, because it would accelerate you too fast, making that most efficient RPM range just a momentarty "blip" on your trip profile.

I came to this conclusion when trying to decide what speed to climb hills at in my Jimmy. According to the BSFC charts for my engine, it would be 2800 RPM at around 75% to 80% engine load. Well, when I tried to go up the hill under those conditions (via my ScanGauge), it downshifted like mad (a disadvantage of an automatic), and then accelerated right past the speed limit in a lower gear. - and my MPG sucked big-time. Now, if it wasn't an automatic, maybe I could have found a speed in top gear that would allow me to work the engine that hard at that RPM . . . . . the penalty of the automatic, I guess.

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Old 01-06-2013, 05:42 AM   #12 (permalink)
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Quote:
Originally Posted by PaleMelanesian View Post
True. Even in my Odyssey with an autotragic transmission. I just have to be more careful to avoid it downshifting.

One of my common drives is a two hour stretch of highway, half flat and half rolling hills. Consistently, I get 30+ mpg in the rolling hills half, and struggle to reach 28 in the flat part. Hills provide natural p&g even if you're not trying.

With a manual transmission it's not even close. Hills all the way, every time.
Today I drove with my family across some mountainous roads in our automatic SUV and I felt a little piece of me die when the transmission was kicking down a gear up the hills and braking down the hills My mother hates it when I shift to neutral and all, and besides the automatic just idles high when you put it in neutral while moving.

Manual transmission anyday, ftw.
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Old 01-06-2013, 03:05 PM   #13 (permalink)
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Quote:
Originally Posted by wmjinman View Post
But it's more than just the efficiency of the engine at play. If the engine were "too powerful" - and you had the gears for it - you may have to drive 100 mph+ to be at the peak efficiency RPM. But the aerodynamic drag in that situation would wipe out the small efficiency gain of the engine. And running in a lower gear just to keep the revs up to the "most efficient RPM" would cause worse MPG, too.

Also, if that engine is "too powerful", pulse & glide would become less effective too, because it would accelerate you too fast, making that most efficient RPM range just a momentarty "blip" on your trip profile.

I came to this conclusion when trying to decide what speed to climb hills at in my Jimmy. According to the BSFC charts for my engine, it would be 2800 RPM at around 75% to 80% engine load. Well, when I tried to go up the hill under those conditions (via my ScanGauge), it downshifted like mad (a disadvantage of an automatic), and then accelerated right past the speed limit in a lower gear. - and my MPG sucked big-time. Now, if it wasn't an automatic, maybe I could have found a speed in top gear that would allow me to work the engine that hard at that RPM . . . . . the penalty of the automatic, I guess.
Well no matter how much power you have, you can control that power with the throttle. The amount of power it takes to rev the engine in nuetral to 7000rpm compared to maxing out 3rd gear at 7000rpm is completely different.
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Old 02-15-2013, 01:07 PM   #14 (permalink)
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I would say that running in hills (involuntary pulse&glide) is better than pulse & glide on flat roads at the same average speed. The reason is that you lose less energy to aerodynamic drag.
On flat roads, with p&g, you have to exceed average speed half the distance, which hurts drag exponentially. In hills, you can keep a more constant speed (if the decline is right). If the decline is to steep and the speed gets too high, the advantage is lost (you won't get as far horisontally for your built up potential energy).
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Old 02-15-2013, 03:49 PM   #15 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
"COZX2" claimed to top 100 mpg out in the mountains...
His driving around in the thin air at 10,000 ft elevation in the Colorado mountains also helped. The local air temperature here at 500ft elevation would have to reach 170F for the air here to get that thin.

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