Fuelly Forums - Trick for P&G On an automatic.. Hint, you need Cruise Control

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Long post, hope you find it useful reading.

Quote:

Originally Posted by ************* (Post 108056)

That's a dumb way at looking at what you've read. Just because the theory behind the idea is incorrect, it doesn't make the idea itself any less valid. Furthermore how the transmission shifts is completely irrelevant to my discovery because when gliding, the tach maintains a constant speed, as if it was in neutral which goes back to what my "intuition was".

Very well then, that's a fair statement. I will proceed to study what you have said and reply based upon your observations of your particular vehicle and the replies of others. I may truncate posts to minimize the length of this post.

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I think I discovered the answer for those who would like to do pulse and glide on an automatic. While I have not tried this on any other cars thus far, I believe it will certainly work with Toyotas equipped with cruise control. I have an automatic, actually at the moment, all the cars I have are automatics. One problem is, if you wanted to do Pulse and Glide, you have to shift into neutral in order to coast which is fine but to shift BACK into drive while you're still moving WILL damage the transmission, for an automatic at least. This is based upon your vehicle's transmission. This does not apply generally. Now if you tried to do the Pulse and Glide with out turning off the engine and just letting off the gas, you'll end up with engine braking which is BAD because that prevents you from GLIDING.

I noticed something when I have cruise control enabled and I catch a slope. For me, at 55mph the tach reads around 1500rpm, So I have cruise control set at 55mph and I catch a slope, now the car is accelerating to 60mph, but if I watch the tach, something happens. Instead of the RPMS increasing in line with the higher speeds which would lead to engine braking and the quick slow down to 55mph, something else happens. So what I see are the RPMs drop to 1200, then 1000 and I think at one point 800. This means that the transmission goes into neutral when the car accelerates faster than the set speed of the cruise control and when the throttle isn't used, i.e. when coasting down a hill. This is all and well for your vehicle, but my tests have shown my car does nothing similar. If cruise is set, it will maintain rpms to match the speed, assuming I'm above 40mph and the TC lockup ( referred to TCL from here) is engaged. Assume Cruise set at 65mph. Assume I crest the hill at 65mph and begin coasting. As my speed increases, the TCL keeps the engine in perfect snyc with my speed, providing a smooth transition to ON throttle when I can no longer coast without applying throttle. The car most likely (IE I haven't plugged a device in to check) is in DFCO, using little to no fuel while providing small form of engine braking, helping to limit a runaway speed.
It does not do this with Cruise OFF.

This is great news, when I'm going faster than my set cruise controlled speed, the transmission goes into neutral in order to not cause engine braking which means it won't hurt my coasting at the higher speed. Well.... since noticing this while I'm driving on the highway, before I get off I disengage the cruise control and try to coast for as long as I can. Unfortunately if I disengage it too quickly, I'll slow down so quickly and will end up having to speed up temporarily. The reason I'd slow down faster than I'd want to is because of the engine braking that normally occurs when your foot is off the throttle and cruise control isn't enabled. As stated above, it appears your vehicle disengages the tranny with Cruise ON and coasting down a steep enough grade, while keeping the tranny engaged with the Cruise OFF down the same grade. Backwards from what my setup does.

Well cruise control can come in handy for another thing, for when you want to GLIDE. Gliding is a problem on an automatic unless you have no respect for your transmission or all your routes are down hill on at least 5% grade because of engine braking. At any speed with Cruise OFF and coasting--regardless of grade--my car will disengage the tranny from the engine--to a point--and coast very nearly the same as it would in neutral.

So when I discovered that the transmission disengages when the car goes faster than the set speed of cruise control so that you will coast while at the higher speeds, I figured I'd try something else. Instead of canceling the cruise control right before I got off the highway in order to coast, I instead held the cruise control knob downward in order to keep telling the cruise control to "decelerate", meaning to reduce my set cruise control speed. What this did was effectively do the same thing as when I had the cruise control set to 55mph and the car accelerates to 60mph due to other forces besides the throttle such as being on an incline. So by telling the cruise control to decelerate, the transmission basically goes into neutral and I was then able to glide a much farther distance since there was no engine braking. I do the same, but different. I put it in neutral. Up a specific ramp, disengaging the Cruise by switching it off will place me at a certain spot/speed. By placing it in neutral I coast further, IE same spot, but faster. I'd give specifics, but not knowing the length of the ramp or the time it takes me to coast that distance makes the placemark pointless. If I hold the "coast/set" button on the Cruise, it will do the same as manually putting the tranny in Neutral. What's the difference? None. Both tell the tranny CPU to release the TC from the engine. How the CPU logic plays out I don't know, but either way, electronically (coast button) or physically (shifter) it reacts the same.

Summary: Pulse and Glide doesn't work with an automatic unless you have no respect for your drivetrain. You can shift out of gear while moving but to shift back on an automatic while moving WILL destroy your transmission. You can't easily glide on an automatic because you will have some sort of engine braking somewhere except in one case. If you're going 55mph, have cruise control enabled and then hold "decelerate" on the cruise control continuously, the car will not use engine braking. It will not use engine braking because in this specific scenario the transmission will self disengage and effectively be in neutral, therefore permitting an actual glide. True enough. I haven't met a car yet that doesn't release the motor from the tranny when you press the decel/coast/set button.

I have found this only to be true on one car which is a Lexus, but the cruise control functions similarly on all toyotas. I plan to test this on my other cars when given the chance and I believe they will all peform the same due to them acting the same in similar circumstances. I do not know if this will work for you but there is no harm in trying. That's a fair disclaimer I believe a few of us didn't take notice of. I apologize for being brash.

I applaud you for taking the time to study your vehicle and how best to gain mpg with P&G. However, taking it personally when someone has a different situation or has a greater knowledge about the subject seems callow. Why am I writing in red? There, that's better.

I think you have the right attitude about researching your vehicle for better mileage, but how you presented it could have been better. You're right about coasting downhill, cruise off, TC unlocked. The speed causes drag, raising the rpms of the engine. RoadWarrior raised a valid point as the first reply: What's the difference between the Cruise engaging neutral or you? If you're coasting downhill, and Cruise ON, 1000rpms, what happens when you reach the bottom and the Cruise has to raise the rpms to match the vehicle speed? I don't believe it does anything different than what manual shifting to neutral does. Both cases the TC has unlocked, slowing the fluids within, removing the majority of the connection between tranny and engine.

I could theorize and state more scenarios, but honestly I'm getting bored with writing this post (I say that jokingly). There are a lot more variables to test and remove to figure out exactly how the tranny reacts, in all situations. I myself will be on a roadtrip next weekend, and I plan to test some of these things you've brought up; it's made me think about how my tranny reacts to the different situations.

Again, I'm apologize for saying this thread is worthless. After reading all of it several times, I see even I have some things I need to test and prove regarding P&G in an automatic, specifically mine.

This thread is giving me a headache.

*************, I can understand why you feel like you're under attack. However please know that that is not the case; while I can't speak for everyone in this thread, I doubt anyone has any different intentions than I do. I merely want to find out what new information you have and how you got it, and either change my own practices or educate you on how you may be misinformed.

As such, I am concerned about my own practice of P&G in my automatic. You still haven't backed up this statement:

Quote:

Originally Posted by ************* (Post 107936)

to shift BACK into drive while you're still moving WILL damage the transmission, for an automatic at least.

You can shift out of gear while moving but to shift back on an automatic while moving WILL destroy your transmission.

with any theory or anecdotes. Can you say why you believe that? If it's going to destroy my transmission I'd rather stop.

Quote:

Originally Posted by ************* (Post 108019)

But it is my understanding that a torque converter that is slipping the most that it can would still yield higher engine RPMS than having the transmission in neutral.

It sounds like you think the TC can slip in only one direction. Perhaps this is where things are unclear? The TC happily allows slippage in either direction -- the engine could move faster or slower than the transmission.

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If it's not shifting into neutral and simply just telling the torque converter to slip" or even to completely disconnect which I'd say is basically neutral, then what we should be asking ourselves is this:

What would put more stress on an automatic transmission? Shifting it into neutral while moving and then shifting it back into Drive, something the manual tells us NOT to do. Or when attempting to glide, telling the cruise control to "decelerate" which we speculate either shifts the transmission into neutral for us, in a controlled manner and or causes the torque converter to slip and or entirely disengage.

Theoretically there could be more wear from shifting in and out of N/D vs. just unlocking the TC (which in most vehicles can be done with the gas pedal just as easily as with the cruise control). In practice I don't think the wear is significant, especially if you rev-match.

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Modern toyotas seem to have a funny way of defining "idle speed". When I coasting, I shifted the car into neutral and even though it indicated I was in neutral, the "idle speed' was around 1000RPM despite the engine being fully warmed up. Also getting the car to be at its lowest idle speed has been quite a challenge and for the most part I've found it to idle around 1000rpm but sometimes 800rpm even when fully warmed up.

This was discussed pretty thoroughly in another thread just a couple days ago. It was decided that either friction in the transmission (due to the transmission fluid) is turning the engine a little faster (I think this is the case; if you put the car on blocks, start the engine, and put it in N, the wheels will start to turn a little) or the computer is idling it a little faster (as it does under other conditions, such as cold idle, or limp mode, or when the A/C is on).

Quote:

Originally Posted by ************* (Post 108055)

Well considering a lot of vehicles aren't meant to be dinghy towed

Dinghy towing is done with the engine off. This whole time, have you been talking about EOC? Most automatics should not be used for EOC; they depend on the input shaft for their oil to be pumped to keep them cool. I P&G in my automatic but I do not EOC.

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I doubt many people would be perfectly rev matching since that would mean more work and less benefit to mileage.

Nice thing about automatics: They have that torque converter, and bands/clutches in the transmission, that take up small RPM differences without breaking a sweat (and do a damn good job on large RPM differences too).

As for anecdotes related to why you don't shift into D from neutral while you're still moving, well apparently those who were drag racing, after finishing a race would shift into neutral and then later shift into D, only to have their transmission to explode from doing this.

Here are some links supporting this:
https://www.elantraxd.com/forums/arch...p/t-10957.html
https://forums.evolutionm.net/archive...p/t-28943.html
https://www.dieselplace.com/forum/sho....php?p=2696485
Last one being funny
Just search for something like this: drag racing shifting into drive from neutral

Dude, all that stuff is about dropping it in to Neutral after deliberately revving the engine high while the car is STOPPED for the purpose of taking off from a line. Drag racing is completely different from what we are talking about.

Also, the difference in force on the TC and tranny are substantial, especially if you bother rev-matching at all. Difference between stopped car and high revs (links specifically mentioned 4000+ RPM), is the full force 4000 RPM the TC has to absorb. Difference between cruising RPM (2000-2500) and neutral idle RPM (600-1000) is at most 1900 RPM. I'm also fairly certain that inertia and other factors make the difference more than linear (in other words, 4000 RPM difference is going to cause more than twice the wear & tear that a 2000 RPM difference would cause. If that is true, then the few hundred RPM difference that happens when you rev-match is virtually meaningless.

There are further differences that could be elaborated, but I suspect I'm only feeding the troll, so I won't bother...

like theholycow said, this isn't a personal attack and most of us aren't saying you're a bad person. I congratulate your experimentation. I'm just informing you that some of your theories are wrong and some need some expansion based on my not insignificant knowledge of cars and their workings. again, not a personal attack.

OK I didn't feel like taking 10 mins apiece to read your links but quite frankly, ones' a hyundai forum (not known for strength/reliability), one's a lancer forum (some mitsu trannies aren't the strongest either), and ones a diesel forum (bajillion ftlb of torque). You mentioned all of them under the category of "drag raced, shifted to neutral, then back to D and the trans blew up". trust me, an auto shifting while at WOT acceleration is taking a LOT more abuse than low rpm N-D. During regular driving, even at highway speeds, N-D with the vaguest attempt at rev-matching will be perfectly fine for MOST transmissions. certainly toyota transmissions.

I had the line pressure on my old A340 high enough it'd chirp the tires and surge forward shifting into 2nd and 3rd at WOT with under 200 hp in a 3400lb car. I know people that regularly drag race turbo cars with the exact same transmission for years. it wouldn't bat an eyelash at my antics. going from N-D from idle at 80 mph was noticeable but hardly harsh.

Quote:

Originally Posted by ************* (Post 108118)

Here are some links supporting this:

Thanks for the links. I'll have to look at them individually...

https://www.elantraxd.com/forums/arch...p/t-10957.html

Quote:

Is it bad on an automatic...To shift it to Neutral, rev the engine to 4000 rpms, then quickly let off the gas and shift to drive and floor it for a better launch?

This is called a "Neutral drop", and is done to spin the tires like you can do by revving up and popping the clutch in a manual transmission. N->D in P&G is quite the opposite of this.

https://forums.evolutionm.net/archive...p/t-28943.html

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How do u race an automatic lancer?
...maybe just rev it in nuetral in beggining then slam it into drive

Another neutral drop.

This one has a few interesting pieces in it...
https://www.dieselplace.com/forum/sho....php?p=2696485

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2. Will it hurt my Trans. shifting in and out of drive?

Just read through this thread again...does anyone have any insight on this part of his question? Will this hurt these trucks?

no, the allison is very protective of itself and makes it almost impossible for the driver to harm the transmission by doing something stupid.

ie, if you try a wide open throttle neutral drop, the trans computer will take over control of the throttle, bring the engine down to idle, THEN engage drive smoothly, and then return throttle control to the driver.

you can also shift it into reverse while you are driving on the highway, it just kicks itself into neutral until you put it back in drive. (yes, ive done this multiple times just for the amusement factor of seeing people behind me dive for the other lane because your reverse lights do come on)

the only thing it can not protect itself against is if you put it in park while in motion

Those protections do not exist in most vehicles, but are not necessary for N->D under P&G conditions.

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Like most modern GM trans the Allison has a Neutral RPM tracking feature in the computer. It keeps the engine speed at the right speed to re-engage.

Ah, this might explain the question of why the engine revs higher in N at speed than in N at a stop.

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Just search for something like this: drag racing shifting into drive from neutral

Those terms will always return results about neutral dropping.

There's something that can happen with a pressure spike in modified transmissions with the line pressure set wayyyyy high if you shift to neutral above 2500-3000 RPM. But unless you're running a line pressure of near 200psi you don't typically need to worry about it.

Ok did it again today, and I think it will be my preferred method to coasting (before I'd let off the gas only). I tried shifting into neutral and compared the RPMs between holding the cruise control down and neutral, neutral did result in lower RPMS of 800RPM though this was done at around 35mph only. However inspite of the neutral having lower RPMS, I found that once again, holding the cruise control does help prolong the glide from a higher speed say 55mph all the way till 20mph much much better than just letting off the gas since I usually have more engine braking. So I guess people were right, it never went into neutral, it just disengaged somewhat, resulting in RPMS of 1000/1200 all through the glide vs a gradual decline from 1700RPMs on the highway, resulting in engine braking.

I need to do a lot more experimenting to understand this transmission better, I'm just annoyed I have to use an automatic at all since from what I understand, manuals perform universally the same with the only differences being with how the clutch is handled and whatnot. There are far too many quirks to document with these automatics it's mind numbing.

*************, guys, thanks for this thread.

I wanna try this technique and see if the cruise control can be of use for P&G.

Other ideas:

- Have you tried P&G, say between 30 and 40MPH by setting your cruise control at 30MPH, accelerate to 40 with the pedal, then coasting back to 30? Would that yield a good glide with your Toyota with the same drop to 1000RPM? If so, there's no need to hold the "decel" button.

- if this N-D can be electronically controlled, maybe we could also make a convinient "N glide" button on the steering wheel. Maybe better than the "decel" button if that one doesn't really achieve "N".

- Would precing "Decel" for long periods of time cause problems to the car computer (ECU)? It's not designed to be used like that and maybe it could lead the car to enter a weird mode... Like disable cruise control, throw a code, go open loop or something.

Anyway, I'm kind of surprised at your findings.

My AT car when in cruise control mode holds its speed even in a downhill (not too steep I guess). So I assume it uses engine braking and throttle to stay a constant speed (but no @!$# fuel cutoff from what the Scangauge says so no gas savings).
So it would mean that when you press "Decel", the car will just coast instead of engine braking... Hmmm.

Other AT P&G problems:

- Shifting gears is awkward, and maybe even dangerous (P and R are a sort distance away)

- Shifting gears annoys passengers not used to this (I went into an argument about that - pfff), the shift clunks are noisy and cause the driver to make a lot of movement.

- Shifting gears is slow and impractical because the levers are not designed for doing that constantly.

So if I can find a more practical and quiet solution, I'm all for it...

BTW, friends that are car "savvy" but not hypermiling savvy claim that I'll destroy my tranny shifting back and forth. So yeah, I'd want to make sure that P&G on an AT doesn't end up costing me a nasty repair in the middle of nowhere (I already lost a car in the middle of nowhere, and don't want a repeat).

I wish I understood how AT trannys work. If you guys know of a good simple online tutorial, post a link! I'll start with googling :-)

https://en.wikipedia.org/wiki/Automatic_transmission
https://auto.howstuffworks.com/automa...ansmission.htm