Disengaging auto trans for coast.

[apgrok1]

Originally Posted by 88CRX

Your saying the torque converter is still engaged, only the lock is disengaged when in neutral. If so, then i should still feel the car engine brake, or accelerate when i push the gas. It doesn't engine brake or accelerate when in neutral.

Ok, here is how an auto trans works
A. The torque converter has an outer shell which;
1. is connected to the flywheel
2. is also connected to the system pressure pump, so that any time the engine is running, the trans will always have system pressure. This is what also contributes to the inefficiencies of an auto trans. You are always using some power to turn that pump.
3. has fins on the inside of the shell to push oil against the stator
4. has the lock up clutch in it to provide 1:1 ratio between the engine and the rest of the trans, otherwise there is always slippage between the shell and the stator. It was designed this way.

B. The stator is a multibladed fan-looking part that is internal to the TC shell. It is connected to the input shaft of the rest of the trans. It is also connected to the lock-up clutch. When the lock up clutch is disengaged, the stator has no connection to the shell other than the oil being thrown against it by the fins. This is what turns the stator. When the clutch is engaged, the stator is locked to the TC shell, providing the 1:1 ratio.

C. The rest of the trans is a series of planetary gear sets. A planetary gear set consists of a central cog (called the sun gear), three or more cogs that are engaged to the sun gear (called planet gears) and are all mounted to their own carrier plate, and finally a ring with teeth on the inside (called a ring gear) that is meshed with the planet gears. Usually, one would lock one of the three sets of gears stationary to get an input and an output from the other two. For example, if the ring gear was held stationary, the sun gear would be the input and the carrier plate fro the planet gears would be the output as the planets would travel around at a different speed. Internal clutches hold these gears stationary. There maybe three or four of these sets of gears.

So, the internal clutches of the transmission are what allow your trans to connect the stator to the wheels, AND are what are disengaged to provide neutral. When there is no pressure to any of the internal clutches, the planetary gears are allowed to turn in any direction at any speed to which ever gear has the least resistance. Even though the TC shell may still be pushing the stator to turn, the input power has nowhere to go as all the internal clutches are disengaged.

[msirach]

You need to "blip" the throttle to match your engine rpm to your mph that you are traveling. It should make it a seamless (no noise or feel) transition.

[Formula413]

I was just about to make a new thread to ask pretty much this same question. I have started using this technique recently (that is, shifting my auto trans into neutral when coasting) and I haven't noticed any strange noises or harshness when shifting back into drive. Sometimes I try to match revs, sometimes not. I would love to have a definitive answer as to whether this could hurt anything, since it does make a significant difference in FE and I would like to keep doing it. I'm inclined to think it is fairly safe since the torque converter is a fluid coupling device which is designed to slip.

[apgrok1]

The items that usually wear out are the oring seals inside the clutch packs. They get abused from high trans temps (when towing) and contaminated oil, but that's it. Keep you're oil changed and your loads light and it shouldn't matter how many times you put it in neutral.
However, racing the engine and throwing it in drive may cause damage to the clutches themselves. I would think a minor blip on the peddle would be ok, but I have never had to do that in my car.

[Krieg]

What do you guys think about downshifting (for those of us with tiptronic trannys).

Does that do damage to the auto tranny? Clutch packs? o-ring seals?

I've been downshifting to a stop to keep the revs over the magic 1500 rpm number (where the injectors shut off).

[apgrok1]

You should be OK, but I would think that down-shifting would waste fuel.

[88CRX]

Originally Posted by apgrok1

Ok, here is how an auto trans works
A. The torque converter has an outer shell which;
1. is connected to the flywheel
2. is also connected to the system pressure pump, so that any time the engine is running, the trans will always have system pressure. This is what also contributes to the inefficiencies of an auto trans. You are always using some power to turn that pump.
3. has fins on the inside of the shell to push oil against the stator
4. has the lock up clutch in it to provide 1:1 ratio between the engine and the rest of the trans, otherwise there is always slippage between the shell and the stator. It was designed this way.

B. The stator is a multibladed fan-looking part that is internal to the TC shell. It is connected to the input shaft of the rest of the trans. It is also connected to the lock-up clutch. When the lock up clutch is disengaged, the stator has no connection to the shell other than the oil being thrown against it by the fins. This is what turns the stator. When the clutch is engaged, the stator is locked to the TC shell, providing the 1:1 ratio.

C. The rest of the trans is a series of planetary gear sets. A planetary gear set consists of a central cog (called the sun gear), three or more cogs that are engaged to the sun gear (called planet gears) and are all mounted to their own carrier plate, and finally a ring with teeth on the inside (called a ring gear) that is meshed with the planet gears. Usually, one would lock one of the three sets of gears stationary to get an input and an output from the other two. For example, if the ring gear was held stationary, the sun gear would be the input and the carrier plate fro the planet gears would be the output as the planets would travel around at a different speed. Internal clutches hold these gears stationary. There maybe three or four of these sets of gears.

So, the internal clutches of the transmission are what allow your trans to connect the stator to the wheels, AND are what are disengaged to provide neutral. When there is no pressure to any of the internal clutches, the planetary gears are allowed to turn in any direction at any speed to which ever gear has the least resistance. Even though the TC shell may still be pushing the stator to turn, the input power has nowhere to go as all the internal clutches are disengaged.

Oh i see, so the lock you were talking about is not the torque converter's

The trans still spins, but no load

[Krieg]

Originally Posted by apgrok1

You should be OK, but I would think that down-shifting would waste fuel.

I guess that depends on if there is a fuel cutoff over a certain RPM or not.

If so, downshifting gets you to an RPM above the cutoff. No fuel is being injected, so mileage is infinite.

The alternative is shifting into neutral. In that case, the injectors are firing to maintain idle. Mileage is good, but not infinite.

If there is no fuel cutoff, I agree that downshifting would waste fuel. Don't do this on an old carbureted engine, for example.

[Formula413]

Little off topic here, but how do you know whether or not your injectors shut off and if so when? I really have no idea how to calibrate the cutoff feature on my Scangauge so I just left it at 0.