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Old 07-30-2010, 11:32 AM   #1 (permalink)
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Pulse and DFCO with AT?

I have read a few threads in here about this, but none (that I have read, at least) have given me any good answers.

I can conclude from the other threads that pulsing and DFCO'ing with a manual is a bad idea, but with my AT, I can pulse in TC lockup, and when DFCO'ing, the TC releases and the RPM drops.

Unfortunately I am (at least for the time being) unable to do some scientific testing on this, but I'm trying to do some logical reasoning on this. I'm sorry if this becomes too messy, but here we go:

Say, I'm pulsing in TC LU in 4th at ~95% MAP, and ~2000 RPM. If I let go of the throttle, the TC releases and the RPM drops to ~1600. The alternative is driving in TC LU all the time, at 2000 RPM and maybe 55-65 % MAP on flat road.

I reckon there will be some losses in the TC when DFCO'ing, but have no idea how much. It may be totally wrong, but I will estimate it to be no torque converting when it is transferring torque backwards to the engine, and just consider it as a slipping clutch with equal torque in and out, and only a RPM drop. Then the tranny turns the input axle at 2000 RPM, with the torque needed to turn the engine at 1600 RPM with closed throttle and no fuel.

Then I guesstimate this torque to be equal to the torque needed to turn the engine at 2000 RPM with wide open throttle and no fuel. Ergo, the tranny "puts out" on the input axle the power needed to turn the engine at 2000 RPM with open throttle (and no fuel).

This leads me to the conclusion that Pulse and DFCO keeps the engine/tranny friction losses equal to that of driving at TC LU and roughly full MAP all the time, which should be better than driving at TC LU and a lower MAP. Of course, this temporary conclusion is extremely weak due to my guesses.

And I understand that the gains will be better (or the losses be smaller) at lower speeds or small declines that will give a lower MAP if not pulse and DFCO'ing.

Anyone have any thoughts on this? Any facts, experience or other input? Maybe my logic are flawed?

Last edited by Pappnese; 07-30-2010 at 11:41 AM.. Reason: Typos
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Old 07-30-2010, 12:18 PM   #2 (permalink)
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Yes, it doesn't work, an AT has enough friction just coasting, DFCO is FOR BRAKING not for coasting, it adds even more friction from engine braking losses and excessive transmission losses since you are also going to be spinning the transmission.

Your best bet with an AT is to surge to speed, let off the gas to coax a shift and press the pedal in just enough to almost maintain speed, allowing it to slowly decay over a period of minutes, then slowly/gently accelerate every time you have a downward section of road to regain the small amount of speed you lost.

DFCO is what you use if you have a lot of stops and are not able to coast to the stop.


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