Cerpin Taxt
Lifer
There's no fscking way that plane is gonna take off.
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On Automatic Transmission do you save gas shifting to neutral at stop signs?
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Yes. Although it isn't very much.
When you are stopped in drive, the engine is straining against the torque converter which is stalled. As a result the ECU will boost the fuel to the engine in order to stop the engine from stalling. Even older carburettered cars would sometimes select a higher idle jet when in drive to prevent stalling.
That said, other carburettered cars would do the opposite - they would have no compensation for being in drive. They would idle high in neutral when there was no load, but idle correctly in drive. In this case, they'd probably use more fuel while in neutral.
Of course, when the torque converter is stalled at idle speed, it doesn't present much of a load - but it probably saps about 1-2 hp.
When you are in neutral, the transmission is disconnected from the torque converter, and the torque converter can spin freely, providing no load to the engine. This means the engine can run with minimum fuel consumption.
That said, you also put extra wear on the transmission by repeatedly switching from neutral to drive. While stopped in drive, the whole transmission is stalled - nothing moves. In neutral, all the planet gear sets spin freely, which means the input gears spin at the speed of the engine and torque converter. When you select drive again, the brakes in the transmission have to bring those gear sets to a stop before the gear is selected - repeatedly switching requires repeated braking of the gears - which causes the brake bands in the transmission to wear out.
The first part of your post is what I was alluding to in my post.
I'm not sure how late model cars manage things, but... I'm curious how much difference in injector pulse width there really is between no load idle and loaded idle vs. a higher RPM at no load.
I think that the difference is insignificant enough that you would never be able to determine whether it was actually helping or not, there are just too many other variables. Instead, you should operate the automobile in ways that produce the least wear on mechanical components. This will, in turn, save you the most fuel - by making sure your machine is working as efficiently as it can at all times.
Zenmervolt
Elite member
Bingo.
Unless you have a pretty crappy car, even a full-on panic-stop isn't going to heat the rotors enough to cause warping concerns when stopped at a light. You'd need to make several hard stops and then not only hold the brakes on, but also keep the pedal pressed down very hard indeed.
Not a legitimate concern on public roads and not an issue at the track.
ZV
JulesMaximus
No Lifer
They would have been warped by the time he stopped. Rotors don't warp after you come to a stop.
bobsmith1492
Diamond Member
Just because it's running at higher RPMs doesn't mean it's using more fuel. You can feel the car pushing against the transmission and brakes when in drive (obvious since, when you let the brakes off, it starts moving). Therefore, taking it out of gear allows it to rev up because there is less of a load on the engine but it's not necessarily using more or less fuel.
I'm really not sure; taking it out of gear takes the load off, but it still revs up higher. When in gear, there is a load on the engine, but it usually slows down which makes me think it might be using the same amount of fuel in both cases.
Solution: drive a manual. 😉
Hail The Brain Slug
Diamond Member
What's your reasoning behind this?
Hail The Brain Slug
Diamond Member
Thanks for the specific answer backed by logical reasoning. :roll:
JulesMaximus
No Lifer
Well, there is some solid logic by leaving your car in gear so that in an emergency you could just drive out of it without having to first realize that you need to move and then realize that you aren't in gear and then move the shift lever into gear. If you're already in gear then you just react. It takes out that added reaction time.
There really is no benefit whatsoever from shifting into neutral at a stoplight with an automatic transmission car.
Yup. Besides, when I was learning how to drive a MT, my friend told me to downshift instead of putting it into neutral. Besides the engine braking, he said that it is better for the tranny if you downshift down to at least 2nd gear when coming to a full stop. Can anyone shed some light on this?
JulesMaximus
No Lifer
Well, it's certainly better to be in gear when the light changes. When driving a manual transmission car I usually downshift on approach to a light and then put it in neutral until just before I anticipate that I need to go and then I put it in 1st. Obviously, if I were in an area where I felt that I might need to escape quickly I would leave plenty of room between me and car in front and sit in gear with the clutch in.
I do the same thing when riding my motorcycle but then I don't ever ride my motorcycle into areas I'm unsure of.
AgaBoogaBoo
Lifer
There have been long threads on this, and the simple way to do this is the following:
1. Stay in gear
2. Apply breaks
3. Keep applying breaks, as RPM's get closer to idle, bring car out of gear
You shouldn't need to use engine braking, we're just doing city driving and not on a track, right? Also, downshifting just means that you're using the clutch one extra time, that is unnecessary.
What's cheaper to replace? Brake pads or a clutch? Search around on AT, this has been covered in long threads, but basically, your brake pads are just fine for coming to a stop. Ironically, that's what they were designed to do.
How long should you wait before bringing it out of gear? It honestly probably doesn't matter a lot. You can bring it out when you feel like it - wait for it to drop to 2000 rpm, 1000 rpm, right as you apply brakes, etc.
Yes, it does, because the engine is loaded when it is in gear. It takes more gas to keep the engine idling with the trans engaged than it does when the engine is freewheeling in neutral.
Think of it from a stick shift perspective: If you are stopped on a slight incline, and you can slip the clutch and engage it just a bit to hold the car still, do you think that puts a bit of a load on the engine?
Of course it does. (also wears the clutch but that's another story)
So having the tranny in gear at a stop is the exact same thing.
The reality is, though, the gas it costs you is insignificant. To say that leaving the car in gear burns more gas it technically correct, but it's not enough to worry about.
And someone explain to me why keeping the brakes applied to a hot rotor would warp it. Because it won't. So the rotor is hot. Big deal. You're squeezing it from both sides. Big deal. That's not going to warp it, unless you have it nearly red-hot, and even then it's questionable.
he shuts his engine off, so he can coast with no fuel usage at all!!!
but only when going down a hill.
😛
Zenmervolt
Elite member
The theory is that the pads against the rotor allow the rest of the rotor to cool while the area under the pads stays hot and that the differential in cooling rate causes warping. It's possible if you heat the rotors to red hot on a racetrack or something, but it's still unlikely even then. On the street it's such an incredibly small probability that it can be ignored.
The person saying that his engine is off at low speeds most likely drives a hybrid and is being smug about it.
ZV
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