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Old 06-28-2007, 07:35 AM   #21
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I think a simple explanation would be max torque. Since the amount of work needed to get the car to speed is a constant, running the engine at max torque would be the most efficient.

Not possible of course, but maybe keeping the RPM's in the max torque range yields the best FE on acceleration ????????
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Old 06-28-2007, 07:38 PM   #22
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Probably the main thing about CVT's that lets them get better FE than normal automatics.
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Old 06-28-2007, 08:12 PM   #23
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Originally Posted by MetroMPG View Post
I believe this is true too, but it's counter-intuitive isn't it?

Can you explain why you think this is true? I have some ideas, but I'd like to hear yours.

I may split the topic off into its own thread.
I believe that efficiency of the motor is more or less the same whether you are accelerating quickly or slowly in closed loop mode. What makes the difference in terms of P&G vs. steady state cruising is rotations per mile, or rpM if you will. In other words, by accelerating more quickly to speed and assuming the same shift points, one can reduce the distance used for acceleration and therefore the number of rotations before going into a low rpm glide and lower net rpM.

Conversely, not gliding nets the exact same rpM whether accerating fast or slow - leaving any improvement in efficiency to whatever differences exist in the BSFC between the two.
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Old 06-28-2007, 10:23 PM   #24
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Quote:
Originally Posted by lca13 View Post
I think a simple explanation would be max torque. Since the amount of work needed to get the car to speed is a constant, running the engine at max torque would be the most efficient.

Not possible of course, but maybe keeping the RPM's in the max torque range yields the best FE on acceleration ????????
Most non turbo motors reach peak torque at relatively high rpm, and most cars make an excess of power, so maximum torque isn't needed for normal accelleration. I took a quick look through a few issues of Car and Driver and it looks like many cars/trucks of various displacement make peak torque around 4krpm, even the Smart car makes peak torque at 4500rpm. It seems the Metro XFi makes peak torque at 3200rpm, which is better. www.answers.com/topic/suzuki-g-engine . MetroMPG, what's your favorite shift point?

On a turbo motor max torque is determined by the turbo. Typically most turbo cars reach peak torque at lower rpm, and torque peak is seen a few hundred rpm after the turbo reaches full boost. For example, the non-turbo version of the motor in my car makes peak torque at 5k rpm (135HP@6krpm, 125ft.lbs@5k) The turbocharged version reaches max torque at 3k rpm (195HP@6krpm, 203ft. lbs@3k) It's no coincidence that the stock turbo reaches full boost around 2800rpm. Subie WRX is an exception, reaching peak torque around 4krpm despite a small, quick spooling turbo. VW 1.8 turbo motor has a tiny turbo that spools up quick, so it's peak torque is around 1800rpm.
But since we're not using max torque to accellerate, and not spooling the turbo up to create boost pressure, are we benefiting from the turbo's torque amplifying abilities? Or is it just along for the ride?
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Old 06-29-2007, 04:34 AM   #25
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My car has max boost at about 2000rpm . Although that is with a little T2 turbo. On my car, if I cut the injectors, but leave the boost gauge on, I can get positive boost by going downhill, and flooring the accelerator, at about 3000rpm. This positive boost is simple by the 'air pump' action of the engine.

When driving along normally, (in vacuum just before the enrichment phase), on my car at least, the turbo is spinning quite fast, and I guess it is therefore helping to force the air into the engine, by using 'waste' pressure from the exhaust, and therefore seems to reduce the energy wasted dragging air into the engine. However... I don't know how big an effect this is.
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Old 06-29-2007, 04:57 AM   #26
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its not pressure that pushes the turbo, its the expanding hot gasses trying to escape.
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Old 06-29-2007, 10:04 AM   #27
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The expanding hot gasses create pressure on the turbine, which causes it to spin.
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Old 07-01-2007, 09:26 AM   #28
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Quote:
Originally Posted by lca13 View Post
>>what corollary?

I was just trying to lighten it up a little. The corrolary was, instead of "If you would have read more closely" how about "If I had written it more clearly...."

Just teasing a little.... often times when I am trying to communicate and people don't get it, sometimes I need to look in the mirror. For what it's worth, I thought your post was very clear.
ahhhhh i understand now. hahaha i know exactly what you are saying and i was thinking about editing my post but i didnt know thats what you were talking about. looking back at my post a week later, i think you are right. i shouldve eased up.
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My lowest shift point is around 2000 RPM. At that at full throttle, I'm boosting about 10psi. That means I've almost doubled my "displacement" not good for FE.

2500 is my car's favorite shift point. It operates the smoothest 2000 and up (for accelerating, at least I can cruise at 1500, but it stutters a bit if I try to accelerate out of 1500) and I've got some pretty wide gear ratio gaps.

And from checking my tripminder computer, I DO get better FE cruising at 65 than at 55 Or rather, I get identical. At 65, I'm around 2300rpm and my computer hits the ultra-lean in cruise, and also the EGR kicks in, raising my FE that much more. Plus I've got a very streamlined car as is.
for just cruising, the only way you are going to prove to me that your car gets identical or better mpg at 65 vs 55 is with a scan gauge.

as far as the acceleration argument, i think spoke too soon and did not realize that every car is different. some motors will get best mpg with half throttle and shifting as soon as possible, others will not, and there is no real way to be certain where exactly all motors perform best without a phd.
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Old 07-01-2007, 03:01 PM   #29
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But don't let a PhD fool you either. That just means they know enough to make it look like they really know what they are talking about - whether they do or not!
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Old 07-01-2007, 04:53 PM   #30
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A turbo can be used to make a car more fuel efficient. Check out the B5 1.8t Passat vs the the NA 1.8 Passat only offered in europe. The more powerful 1.8t is more fuel efficient. A turbo allows you to use a smaller engine with taller gearing therefore reducing internal friction. The 1.8t of the late 90s was equivalent in power to a 2.5 liter NA engine of the same era. But again it all depends on how its engineered and how you use it. The original 1.8t was rated at only 150hp by using a very small turbo that was biased for low rpm boost (flat torque curve starting at 1800rpm) instead of peak hp.
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