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      07-28-2012, 11:03 AM   #1
kdbryce
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6mt going down hills

so the other day i was driving with my dad in his car. we were going down a hill, so he put it in neutral (6mt). just as conversation, i told him "when i'm going down a hill i noticed that my mpg's are higher when i leave it in gear than when i put it in neutral for some reason."

he told me that someone had told him that on some cars it's actually bad to let it coast in neutral.

now he knows absolutely nothing about cars, and it doesn't really sound like that can be true.. but has anyone heard anything like that ever?

have i been ruining my car lol?
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      07-28-2012, 11:05 AM   #2
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Fuel injection is shut off above a certain RPM at 0% throttle (which equates to coasting in gear). MPG is almost always higher in this scenario than coasting with the engine idling (and burning fuel). I find that my N52KP doesn't have a whole lot of engine braking compared to other engines I've had (probably partially because of Valvetronic).

Performance driving school will also teach you that you never disconnect the wheels from the engine like that for any period of time. They teach that you should ALWAYS have some kind of gear selected ready to apply power if necessary.
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      07-28-2012, 11:32 AM   #3
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Quote:
Originally Posted by Surly73 View Post

Performance driving school will also teach you that you never disconnect the wheels from the engine like that for any period of time. They teach that you should ALWAYS have some kind of gear selected ready to apply power if necessary.
this i was taught when i was around 10 year old driving LADA
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      07-28-2012, 11:47 AM   #4
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Quote:
Originally Posted by Surly73 View Post
Fuel injection is shut off above a certain RPM at 0% throttle (which equates to coasting in gear). MPG is almost always higher in this scenario than coasting with the engine idling (and burning fuel). I find that my N52KP doesn't have a whole lot of engine braking compared to other engines I've had (probably partially because of Valvetronic).

Performance driving school will also teach you that you never disconnect the wheels from the engine like that for any period of time. They teach that you should ALWAYS have some kind of gear selected ready to apply power if necessary.
This

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      07-28-2012, 11:48 AM   #5
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Huh I coast in neutral all the time I don't think it's bad
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      08-16-2012, 12:39 PM   #6
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Originally Posted by P3rsian Pr0digy View Post
Huh I coast in neutral all the time I don't think it's bad
Its about the control, If you coast in neutral on down hill, the car will gain more speed and can go out of control.

You need to keep it in some gear depending on the speed.
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      08-16-2012, 12:50 PM   #7
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+1. The only time your car is out of gear is when it's stopped.
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      08-16-2012, 12:52 PM   #8
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No... Always in gear so you have engine power to move the vehicle on a pinch.
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      08-16-2012, 01:09 PM   #9
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On the subject of gas usage

Its got and gas usage gauge on the display .
There is better control of the car when not in neutral
which is one of the main reasons for having a manual.
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      08-16-2012, 01:10 PM   #10
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perhaps engine idling itself on neutral is bad for the engine.... ive heard this also. dont have a exact mechanical explaination... anyone?
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      08-16-2012, 01:27 PM   #11
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Quote:
Originally Posted by kbryce View Post
he told me that someone had told him that on some cars it's actually bad to let it coast in neutral.

now he knows absolutely nothing about cars, and it doesn't really sound like that can be true.. but has anyone heard anything like that ever?
Of course. However, it is more of a dfriving safety issue than "ruining the car" issue.
  • Firstly, for downhill driving specifically, trying to coast in neutral is a very serious and dangerous error since it puts the entire thermal load on the brakes, leading to potential brake fade with disastrous consequences. It is surprising anyone would ask something like that, since it is actually common knowledge. This is the reason even basic AT cars have manually selectable "low gear" modes in their transmissions.

  • Secondly, in pre-ABS days braking in neutral (even in horizontal surfaces) was considered less safe then braking in gear. When you are braking in gear, at least part of the braking power comes from engine braking. In this case the open differential installed in the drive axle works as "poor man's ABS": if one wheel loses traction, the other wheel will also lose "engine braking" component of the braking force. This keeps the braking force more balanced between the sides of the car.

    If the entire braking force comes from brakes (as it happens when you brake in neutral) any difference in wheel traction can lead to braking force disbalance, causing the car to swing to the side.

    With the global introduction of ABS, this factor became significantly less important. But old-school people often keep quoting it as an argument against braking in neutral.

  • Thirdly, there's that "you should be always ready to switch from braking to accelerating" argument. It is one of those "sounds right" things that don't really seem to play any significant role in real life, but it is still worth mentioning.

  • Fourthly, sometimes you can hear people say that keeping the clutch disengaged for extended periods of time puts more wear on the throw-out bearing, causing it to fail prematurely. While it is possible that the throwout bearing might experience additional wear when the clutch is disengaged, it is more along the lines of "instead of serving for 100K miles it will serve for 99.5K miles" kind of thing. I.e. in any modern clutch the throw-out bearing does not experience any statistically detectable wear in disengaged mode. If it fails because of that, it means it it was defective in the first place.

Last edited by AndreyT; 08-16-2012 at 01:38 PM.
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      08-16-2012, 01:28 PM   #12
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I have learnt that coasting with clutch in will cause wear on your throw out bearing.

As for coasting in neutral... That is just poor drivin habits. Always be in control of your vehicle. Like it has already been said, coasting with modern efi in gear shuts off fuel flow.
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      08-16-2012, 01:54 PM   #13
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Quote:
Originally Posted by gds52 View Post
Its about the control, If you coast in neutral on down hill, the car will gain more speed and can go out of control.

You need to keep it in some gear depending on the speed.
Quote:
Originally Posted by talisman311 View Post
+1. The only time your car is out of gear is when it's stopped.
Quote:
Originally Posted by talisman311 View Post
No... Always in gear so you have engine power to move the vehicle on a pinch.
Quote:
Originally Posted by ctuna View Post
Its got and gas usage gauge on the display .
There is better control of the car when not in neutral
which is one of the main reasons for having a manual.
Quote:
Originally Posted by bmwpride335i View Post
perhaps engine idling itself on neutral is bad for the engine.... ive heard this also. dont have a exact mechanical explaination... anyone?
Quote:
Originally Posted by AndreyT View Post
Of course. However, it is more of a dfriving safety issue than "ruining the car" issue.
  • Firstly, for downhill driving specifically, trying to coast in neutral is a very serious and dangerous error since it puts the entire thermal load on the brakes, leading to potential brake fade with disastrous consequences. It is surprising anyone would ask something like that, since it is actually common knowledge. This is the reason even basic AT cars have manually selectable "low gear" modes in their transmissions.

  • Secondly, in pre-ABS days braking in neutral (even in horizontal surfaces) was considered less safe then braking in gear. When you are braking in gear, at least part of the braking power comes from engine braking. In this case the open differential installed in the drive axle works as "poor man's ABS": if one wheel loses traction, the other wheel will also lose "engine braking" component of the braking force. This keeps the braking force more balanced between the sides of the car.

    If the entire braking force comes from brakes (as it happens when you brake in neutral) any difference in wheel traction can lead to braking force disbalance, causing the car to swing to the side.

    With the global introduction of ABS, this factor became significantly less important. But old-school people often keep quoting it as an argument against braking in neutral.

  • Thirdly, there's that "you should be always ready to switch from braking to accelerating" argument. It is one of those "sounds right" things that don't really seem to play any significant role in real life, but it is still worth mentioning.

  • Fourthly, sometimes you can hear people say that keeping the clutch disengaged for extended periods of time puts more wear on the throw-out bearing, causing it to fail prematurely. While it is possible that the throwout bearing might experience additional wear when the clutch is disengaged, it is more along the lines of "instead of serving for 100K miles it will serve for 99.5K miles" kind of thing. I.e. in any modern clutch the throw-out bearing does not experience any statistically detectable wear in disengaged mode. If it fails because of that, it means it it was defective in the first place.
Quote:
Originally Posted by avocet View Post
I have learnt that coasting with clutch in will cause wear on your throw out bearing.

As for coasting in neutral... That is just poor drivin habits. Always be in control of your vehicle. Like it has already been said, coasting with modern efi in gear shuts off fuel flow.
thanks for the help guys

guess i was just being stupid.
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      08-16-2012, 02:09 PM   #14
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Speaking of the throwout bearing

When you are at a stop put the car in neutral and let the clutch out
This does two things
It reduces the wear on the throwout bearing and its safer as if you were
in gear and accidentally had your foot slip off the clutch you would rear end
the next guy or slip into the intersection on a red light.
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      08-16-2012, 02:32 PM   #15
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Quote:
Originally Posted by kbryce View Post
thanks for the help guys

guess i was just being stupid.
seeking answers is never stupid.
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      08-16-2012, 03:06 PM   #16
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Good to know!
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      08-16-2012, 05:30 PM   #17
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Basically all has been covered. But yes, coasting (with the engine not connected to the wheels) is from a safety point of view not good, and does often put more load on the brakes. From the point of view of mechanical troubles, only the release bearing takes a pounding if you are sitting with your foot on the depressed clutch pedal - even sitting at the lights or resting your foot on the clutch lightly while driving is not advised. Engine will be using more fuel too since it is not shutting the fuel supply off while running under compression alone. On the 330i instead of the temperature gauge we have the econometer, and it shows exactly how this functions.

Under engine braking approaching a stop it will be on zero fuel usage, then clutch in so as to not stall, and from the car still moving the needle will briefly climb until you are stationary. Coasting out of gear the needle sits at a low-ish consumption which is obviously lower as the speed increases, but why waste the fuel when you can be safer using engine braking...
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