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Old 02-18-2011, 06:40 PM   #1 (permalink)
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Freewheel coasting with an automatic transmission.

Is there a way to free coast with an automatic tranny ?

We've had great articles on taking control Manumatic - EcoModder and on freezing the slushbox Torque Converter Lockup - EcoModder

Shifting into N and back to D makes noises that don't sound healthy to me.

Is there a way to "disengage the clutch" as it were ?

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Old 02-18-2011, 08:27 PM   #2 (permalink)
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If there's a way other than shifting to "N" I'm not aware of it.
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Old 02-18-2011, 09:22 PM   #3 (permalink)
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Jason,
Good to see people are enjoying my write ups on the wiki.

To my knowledge shifting into and out of N is the only true way to disengage the "clutch" Note: there is no actual clutch on a auto.

a half way of doing it is to disengage the torque converter (if you have manual control)

I routinely go in and out of neutral on my automatic without any issue. Before i had control of the torque converter I did have to rev match otherwise there would be a good crunch as the wheels span up the engine.
so for me if i was above 60 kph while in neutral i would rev up to 1.5k then shift into drive. or if i was at 100 kph i would rev up to 2.2k then shift into drive.
This stopped any crunching or noises.

after i got control of the torque converter i stopped rev matching as the unlocked torque converter will soak up any rev mismatch as its not a direct mechanical connection.
so now at 100kph i have the torque converter unlocked then i shift direct into drive, the engine comes up to speed based on the wheels backpowering through the torque converter (the car slows a little bit) then i switch the torque converter lock on and away i go.

Hope that helps, good luck
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Old 02-19-2011, 08:44 AM   #4 (permalink)
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So with rev matching, it shouldn't be any different than a manual.

Is there a way to depressurize the TQ so it free spins?

That would probably take too long to refill.
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Old 02-19-2011, 06:24 PM   #5 (permalink)
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correct rev matching is the same in auto as it is for a manual

regarding the TQ (torque converter) free spinning by depressurizing. This is a bit above my head, it may be possible to depressurize the TQ but i would expect it would be increadibly hard and possibly dangerous for the TQ.

Having said that i have noticed once or twice that if i am in drive and say traveling at 60 kph then turn off the TQ lock up the engine will only take as much petrol as if i was in neutral. This indicates that if the wheel side of the TQ is spinning faster than the engine side then it will not back power the engine when the TQ is unlocked.
Note: I have not checked this so it may not be too accurate you may need to check with your own car.
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Old 02-19-2011, 08:16 PM   #6 (permalink)
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I coast with the engine on and off in my car but it doesn't make any unsettling noises, if it did make noises I didn't like I would not continue to do it.
I've finally learned to listen to my intuition, when something seems like it might not be a good idea there's probably a reason.
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Old 02-20-2011, 08:12 AM   #7 (permalink)
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There should be no problem with a slush box being put in Neutral and then back into Drive again. They are designed to be able to do this as a safety feature when driving on ice and emergency stops to disconnect the driven wheels from the drivetrain. But rev matching will help the process when returning to drive.
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Old 02-21-2011, 09:16 PM   #8 (permalink)
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I go into neutral at 55mph and coast down to 35mph twice each way to and from work. I do not do a rev match, I just pop it back into D. Been doing this for about 6 months now, and haven't had a problem yet.
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Old 02-21-2011, 09:37 PM   #9 (permalink)
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The issue is the torque converter, small engines have relatively high stall speeds (lots of slippage at low engine RPMs, to compensate for a relative lack of low RPM torque) while V-8s generally have much lower stall speeds (to better utilize the low RPM torque they produce). So with the higher stall speed converters popping it back into gear with the engine idling doesn't load the drive-train due to the high slippage, the low stall speed converter imposes a significant load on the drive-train even at idle hence the fun noises.
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Old 02-22-2011, 01:15 PM   #10 (permalink)
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I've been shifting into neutral more and more with my VW TDI. The only time it protests is shifting back into D at speeds lower than 10mph. Otherwise it's completely seamless, as long as I keep my foot off the go-pedal for 1 or 2 seconds as it re-engages the wheels.

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