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I've been told that when needing to brake quickly, throwing the gear into neutral on an automatic transmission actually allows for better/quicker braking. I understand how things work pretty well but lack automotive knowledge -- so I cannot really say if this is correct or otherwise.

If it doesn't help braking, are there any other actions happening that are positive/negative when this is done?

Thanks!

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Think of it this way.

If you need to brake so incredibly hard that you're worried the automatic transmission is getting in the way, you're better off worrying about things besides whether or not you're in neutral. You're probably about to crash or lose control, so train yourself to concentrate on steering, or making sure you're arms are out of the way of the air bag, etc. Worry about anything but the transmission.

The amount of power being delivered to the wheels by an automatic transmission is so tiny and insignificant when your foot is off the gas that you should never have to pay it any mind. In theory it might make a difference, but not in real life.

  • I totally agree that addressing the gear in a situation where a crash might be avoided due to concentration on steering is what you want to do. I'm just more curious to is happening during this process and if there is any overall net effect in taking the action of changing the gear into neutral. – shaselton Jan 29 '13 at 19:43
  • There's not any magic going on behind the curtain. You are either in neutral or you are in gear. Automatic transmissions use a torque converter allow you to come to a stop in gear, so the forces of your idling engine are negligible. If you want a lot of detail you should read up on torque converters. – Scott Hillson Jan 31 '13 at 5:50
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    As an alternate view on use of the transmission in stopping situations, I once avoided a bad rear-end collision scenario via shifting an automatic. I didn't realize it, but my left tires were right on a metal joint in heavy rain, and the ABS system seemed to detect that as slipping and refused to brake hard. So I threw the transmission in park and stopped almost instantly, with a horrible noise from the transmission and (possibly unrelated, I dunno) busted transmission seals and lost gears a year or so later. – R.. Jan 31 '13 at 6:03
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    good answer. Whenever these "when braking for an emergency, should I..." questions come up, the safe answer is always: Use. The. Brakes. To. Stop. The. Car. Also, know ahead of time whether or not your car has ABS. If it does, simply mash the pedal as hard as you can, then push it a little harder. Find the safest exit and point the car there. – mac Feb 1 '13 at 15:34
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This sounds like utter nonsense to me. Try this experiment. Get up to a set speed (e.g. 60 mph) and then let your foot off the gas pedal and time how long it takes you to get down to a low speed (e.g. 20 mph) "coasting" in gear. Now, repeat the experiment putting the automatic transmission in neutral at the same time you take your foot off the gas, so that you're really coasting, and see how long it takes. I'm betting it's a lot longer.

  • You bring up a good point. The forces in the engine might even slow your car down faster than simply coasting in neutral! – Scott Hillson Jan 31 '13 at 5:44
  • this will depend on the transmission and whether or not it is in "lockup" mode when coasting. Also, just because it's in lockup while coasting doesn't mean it's in lockup while braking (or vice versa). – mac Feb 1 '13 at 15:29
  • Even in non-lockup, the torque converter will slow the vehicle down. – R.. Feb 1 '13 at 15:39
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Putting it in neutral will somewhat reduce your available braking force, as you're removing engine braking from things.

No power is going to go from the "idling" engine to the wheels in anything relatively modern. Basically every fuel injected engine since the 80s implements deceleration fuel cutoff (DFCO), which stops injecting fuel if the wheels are in motion and your foot is off the gas. As a result, the engine is only kept spinning by pulling energy in backwards, somewhat braking the vehicle.

The benefit is that by removing the engine from the equation, you get much more controllable and predictable braking, which is important in extreme conditions, like stopping on ice, especially if one doesn't have anti-lock brakes.

  • Engine braking on an automatic will only occur if it has locked the torque converter. Otherwise the engine can only help the car accelerate, but the drivetrain is "free" so to speak. – Nick Jan 31 '13 at 14:49
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Shifting into neutral CANNOT, and DOES NOT allow for more controllable nor more efficient braking. Torque converters "Lock-Up" when the car is being Driven by the engine. NOT upon deceleration. When the brake pedal is depressed enough to cause the "Brake-Lights" to illuminate, the torque converter is taken OUT of the equation. Also, if operating properly, the torque converter will, in most applications, ONLY "Lock-Up" when the transmission is in Top Gear, and you are no longer accelerating, or barely accelerating when your vehicle speed is around 38-42 mph, and the reduced "Load" on the drivetrain, determined by measuring (engine vacuum, throttle position, current gear, and vehicle speed), then the "Lock-up" occurs. If you continue accelerating moderately to hard, you may find that you're up to 125 mph or so, and the torque converter still will not have "Locked-Up". So.....having to make an emergency stop from this speed, you'd be wise to leave the shifter completely alone. Engine braking will automatically take place and greatly improve your stability during the stopping process.

"Locking-up" of the torque converter is equivalent to having a taller Top Gear, or another speed. (I won't use the term "Overdrive" because there"s a too universal mis-understanding as to just what overdrive actually is)

As always, there's exceptions.. My old van was equipped with a 4 spd automatic, but the torque converter would frequently "Lock-up" while in 3rd. gear causing the engine to lug. I had the wrong size tires on it.

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If you can hit the breaks hard and quick enough to lock the wheels or activate the ABS with no discernible delay then the engine and transmission is no issue.

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I have been shifting cars into neutral and braking, in neutral for 10 years now. Braking in neutral does provide better braking than in gear. In gear, you car is being pushed by your transmission. So as you brake you are fighting with the transmission to stop the car. The Transmission's main job is to push you forward/or Reverse, whether you are on the gas or not. I guarantee you that I can stop my car which does not have ABS brakes, shorter than someone with ABS brakes all by putting my car in neutral and theirs in drive. This may not be seen with a soft braking, this will be noticed in hard braking. I also noticed the tires don't not screech in neutral with a hard braking. I think it is because it activates the rear brakes so instead of the front two locking up now there is 4 wheels braking.

Also keep in mind that in neutral, your engine idle goes down, as a result vacuum pressure goes up. Since car brakes function on vacuum pressure the higher the vacuum pressure the better the brakes function. The brake fluid only gets fluid to the brake caliper, it requires Vacuum to bring the fluid back to the Master Cylinder.

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    None of this is even remotely correct. The amount your car is being "pushed" by the transmission, through the torque converter, closed throttle, is negligible. The amount of traction available when exceeded causing tire squeal, is solely dependent on the braking torque vs the traction at each wheel. All four wheels brake on a properly operating system, and no transmission state "activates" rear or any other brakes. Car brakes do NOT function on vacuum. Vacuum is used to assist pedal travel, but has no consequence on the hydraulic fluid. – SteveRacer May 18 '18 at 1:53
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    The fluid is returned to the master cylinder by springs and/or square-cut oring deflection in the case of caliper pistons. I've owned several older vehicles which had no vacuum booster, but still braked fine - albeit with more pedal effort. – SteveRacer May 18 '18 at 1:55
  • Well said SteveRacer. The only point I'll add is when you are coasting around the countryside in neutral and suddenly you need to apply power to avoid an accident, guess what? You're in neutral and you need to find the stick and put it in drive or reverse or whatever is handy in an emergency evasive situation. – Tim Nevins May 18 '18 at 21:17

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