Shifting to neutral when going downhill

[hans030390]

What about it you're on a flat surface? I can coast a LOT farther in my car in neutral than in gear, as my deceleration is slower (I drive a manual). I know I'm getting less MPG in neutral...but considering I have less coasting distance in gear, I have to keep my speed up (or consistent) for a longer period of time in order to reach a certain stopping point. With it in neutral, I can let off the gas much earlier and coast for a much longer distance.

In that case, wouldn't it be overall more efficient to coast for a longer distance in neutral than to be on the gas for longer and coast for a shorter time (with infinite MPG)?

In my specific case, I drive a '95 Civic DX. That should be fuel injected, correct? I know that it's common sense to keep it in gear if you're coasting downhill. Gravity takes care of that extra friction with it in gear.

 

[fleabag]

Originally posted by: hans030390
What about it you're on a flat surface? I can coast a LOT farther in my car in neutral than in gear, as my deceleration is slower (I drive a manual). I know I'm getting less MPG in neutral...but considering I have less coasting distance in gear, I have to keep my speed up (or consistent) for a longer period of time in order to reach a certain stopping point. With it in neutral, I can let off the gas much earlier and coast for a much longer distance.

In that case, wouldn't it be overall more efficient to coast for a longer distance in neutral than to be on the gas for longer and coast for a shorter time (with infinite MPG)?

In my specific case, I drive a '95 Civic DX. That should be fuel injected, correct? I know that it's common sense to keep it in gear if you're coasting downhill. Gravity takes care of that extra friction with it in gear.

This question was already addressed in the thread.

 

[antyler]

I have done it maybe once in my new vehicle, but im afraid to do it really because the "n" is right next to the "r" and I would hate to throw it in "r" going down hill or not.

 

[LTC8K6]

I would be worried abour hitting reverse too, but I believe on most modern autos it will just ignore your request for reverse. Just like it will ignore a request for first gear if you are going too fast.

 

[BassBomb]

Originally posted by: antyler
I have done it maybe once in my new vehicle, but im afraid to do it really because the "n" is right next to the "r" and I would hate to throw it in "r" going down hill or not.

Most Automatics can't push into R but can push into N D D2. That is you won't have to press the button on the shifter to shift into N D D2 and can easily swap back from N. R requires you to press the buttom to get to it.

Personally I would not try shifting to neutral going down on an auto

 

[Zenmervolt]

Originally posted by: AMCRambler
or the engine would stop firing and then you'd basically be rolling down the hill with the wheels of the car turning the engine.

That is EXACTLY what happens. Exactly.

ZV

 

[LTC8K6]

Zenmervolt, wouldn't it be beneficial for the most modern cars with fly by wire throttles to also open the throttle when coasting in gear?

 

[exdeath]

I don't think it would really matter, you're either sucking against vacuum or compressing dead air, so the drag in the combustion chamber during engine braking would be similar. If you intake against vacuum, it will cancel out it's extra load on the intake stroke by pulling back on the compression stroke, and of you compress dead air on the compression stroke it will cancel out by rebounding on the power stroke. So either would be like a spring in the combustion chamber essentially.

Air however causes drag resistance when being sucked through the induction system and intake ports at high RPM and vacuum does not, so I would say pumping empty vacuum at that time is the lesser load and even a fly by wire system should engine brake throttle closed for the longest possible coast.

 

[Zenmervolt]

Originally posted by: LTC8K6
Zenmervolt, wouldn't it be beneficial for the most modern cars with fly by wire throttles to also open the throttle when coasting in gear?

You'd lose a good bit of the engine braking though. Probably not really worth it.

ZV

 

[LTC8K6]

Some engines can keep the valves open, too...

Yeah, it's a catch-22.

You'd need to have the engine breaking available as well. Wouldn't want to get rid of that.

You'd need a COAST button.

Shut off fuel, open throttle/valves.

 

[LordMorpheus]

Originally posted by: Zenmervolt

Originally posted by: LTC8K6
As another example, you can turn the engine backwards and damage it if you push the car backwards while it's in a forward gear. This sometimes happens when a race car spins and goes backwards at high speed such as in NASCAR.

The problem there is the sudden transition from forward to backward. An engine really doesn't care which way it spins from a purely mechanical viewpoint, but a sudden change in which way it is spinning puts a lot of stress on reciprocating components.

ZV

nascars are carburated - you force that engine backwards and you'll be sucking air in the exhaust and blowing it out the carb, that's gotta be a bad plan, I'm sure that's how some of those underhood fires start.

 

[Zenmervolt]

Originally posted by: LordMorpheus

Originally posted by: Zenmervolt

Originally posted by: LTC8K6
As another example, you can turn the engine backwards and damage it if you push the car backwards while it's in a forward gear. This sometimes happens when a race car spins and goes backwards at high speed such as in NASCAR.

The problem there is the sudden transition from forward to backward. An engine really doesn't care which way it spins from a purely mechanical viewpoint, but a sudden change in which way it is spinning puts a lot of stress on reciprocating components.

ZV

nascars are carburated - you force that engine backwards and you'll be sucking air in the exhaust and blowing it out the carb, that's gotta be a bad plan, I'm sure that's how some of those underhood fires start.

Point, hadn't thought of that. On the positive side, you could make S'mores!

ZV