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Old 06-20-2008, 01:06 PM   #21
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Quote:
Originally Posted by almightybmw View Post
input shaft spins, fluids move, primary and secondary shafts move. When loaded (ie on level ground) the car doesn't move, but in the air, the friction is close to nothing, therefore shafts spin. Does this mean there is no neutral? no, it just means the tranny is built solid and with tight tolerances. Neutral still exists. The engine has no direct mechanical connection with the tires.
In an automatic the engine never has a direct connection with the wheels unless the TC is locked. The only exception that I know of is the old "Variomatic" transmissions from the late 50's. I forget who made them, but they utilized belts, not a fluid connection to the rear wheels.

-Jay
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Old 06-20-2008, 01:18 PM   #22
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OKay, sounds like we all know the same stuff about how it works, and pretty much come to a conclusion that internal fluid friction is a pretty good deduction as to why--regardless of how you start or whether in D or N along the way--the rpms climb at speed if in Neutral. Can we prove it via testing?

Comp, you said you've seen it in your 77 Chevy? I guess I want to try it in my 02 GP then to see the results as well.
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Old 06-20-2008, 02:16 PM   #23
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I'd like to try it in my 86 Chevy and my 81 Regal, but neither of those have a tach.

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Old 06-20-2008, 02:44 PM   #24
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Ok, so the question is...is that fluid connection enough to increase engine RPM by 10% when the driveshaft is spinning at highway speed?
looks like it ,,,this is a good reason sticks get better mikeage,,,,less HP loss
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Old 06-20-2008, 06:20 PM   #25
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In most engine there is an idle control valve, which has a air by pass pipe in front of the butter fly plate, its a solenoid device and it allows a small amount of air to go around the plate to increase idle speed, its also connected to a speed sensor from the gearbox, the same one that gives the speedo reading on the dash, so it knows when the car is moving and will keep the idle above 1200rpms

It also helps keep idle higher when there is a heavy current draw from the alternator, modern cars have so many sensors and control systems in place, its amazing they don't break down more often.
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Old 06-21-2008, 07:33 PM   #26
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I was cruisin' round the yard sales this morning, doing P+G, and I was noticing that at low speeds sub 30mph I do seem to get lower RPM coasting in gear than in neutral. Coasting down from 50ish, the engine only seems to drag more than Neutral down to about 40ish, after that I get no apparent huge difference in glide distance. Around 20mph-ish the motor seems to push just enough at idle speeds of around 850 to keep moving indefinitely, whereas if I shift to N, the rpm jumps to 1100ish... I am curious now as to whether it's using more fuel or not at that point. I'm figuring that it is probably using about the same in gear or in N below 50. I should be getting DFCO above 50 as well... so all in all it seems like there's a very narrow band, between 40 and 50 where N coast is of any real benefit. Although N coast at highway speed down hills and behind semis seems probably worthwhile. At a very rough figuring, knowing gas gauges are in no way accurate, and thinking my first quarter tank is above the full mark, and the rest of the scale is about 3/4 of the 15G tank, then I used around an 8th doing about 40 city miles this morning, which seems to indicate 28mpg-ish, probably too optimistic, but still around 22mpg if the gauge is "accurate" around the middle.

Edit: in case you're wondering about the economy of yard saling with gas so high, I did score a $200+ mechanics vice for $25 and a $100 carpet "runner" for $1, both things I needed, for an apparent cost of about $7 in gas. I'd have used the same amount of gas probably driving round stores "shopping" for them if I'd had the cash to buy new.
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Old 06-21-2008, 08:41 PM   #27
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coasting is always better

coast as much as you can !!!!!!!! stop watching the mpg gauge, stop counting rpm's !!! coasting in gear = braking. You are always better off coasting.
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Old 06-22-2008, 05:53 AM   #28
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Quote:
Originally Posted by RoadWarrior View Post
Around 20mph-ish the motor seems to push just enough at idle speeds of around 850 to keep moving indefinitely, whereas if I shift to N, the rpm jumps to 1100ish... I am curious now as to whether it's using more fuel or not at that point.
Almost definitely yes. You can certainly find out easily with a certain DIY fuel rate monitor that you know I really enjoy.

Quote:
I'm figuring that it is probably using about the same in gear or in N below 50. I should be getting DFCO above 50 as well... so all in all it seems like there's a very narrow band, between 40 and 50 where N coast is of any real benefit.
Which of your vehicles is that for?

It's definitely not universally applicable. In my VW, coasting in neutral is far more efficient than DFCO, as determined by running whole tanks of gas one way or the other. If you think about it, using DFCO as a replacement for coasting can't possibly be more efficient. Consider the following:
- In neutral, you have aerodynamic drag, rolling resistance, and 1100rpm of engine friction and pumping.
- In DFCO, you have the same aerodynamic drag and RR, but now you have 1500 or 2000 (or whatever) rpm of engine friction and pumping, plus some additional drivetrain friction.
Which one uses more energy?

Quote:
Edit: in case you're wondering about the economy of yard saling with gas so high
Even if it wasn't the most efficient option, it's probably something you enjoy. That's reason enough.
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