Downhill on motorways

[dudload]

Evening all!

Just a general wonder, when going downhill at speed on motorways (which I generally try and avoid) is it bad to let momentum pull the car down the hill, or do you disengage the gear? I always think the engine doesn't sound happy when the foots off the gas downhill, but wonder what other people's view are?

[philthehill]

When the car is moving you should always be in a gear except for when changing gear.
Freewheeling down hill is very bad practice as you are not in total control of the car.
Phil

[myoldjalopy]

Haven't we been here before? As Phil says, its considered bad practice to coast downhill out of gear. I can't see why the engine 'doesn't sound happy' when going downhill, in gear, with your foot off the accelerator. I do it all the time as there are loads of hills here in Cornwall. If you get the right gear for the gradient, the iconic Morris Minor 'burble' can be maintained for a long time, depending on the length of the hill. In summer I like to treat other drivers with open windows, going uphill, to the trumpeting!

[399NRO]

Is this a diff wear on the overrun not engine noise?

[les]

In the distant past I was told, that on the overrun oil lubtication is more plentiful. All this talk of overrun has bought this memory back. The only reason I could ever think of, for it being true, is the lack of petrol being used. Something I've had spells of wondering about from time to time ! True or not. I don't see any downside to doing it.

[ianmack]

I’ve heard the extra oil on overrun theory, I think it goes that when the throttle is closed the vacuum increases and sucks oil up past the rings or down the valves. If your engine is that worn I don’t think a bit of extra oil is going to make much difference.

[les]

Well that could be a better reason than my 'absence of petrol' thought. Good to hear someone else has heard of this.

[MikeNash]

Hmmm,
"Freewheeling downhill is very bad practice . . ." say my m'learned friend Phil with whom I'm very loath to disagree, but I must point out that they've been original equipment even in recent times, especially on Saabs. And you may not know they were occasionally fitted to Morris's, for instance my 1935 Morris Oxford has it, and it reduced the fuel consumption by about 10%, enabled smooth gear changes at any speed (including with the non-sync'd bottom and second) and it worked well with the Bendix vacuum automatic clutch changing system. (I must get it on the road again!)
So I suspect that if you're not speed orientated and use your brains to lock them up when you need to, then freewheeling may not be so bad as you might think.
Merry Xmas to you all, MikeN.

[philthehill]

Mike

https://en.wikipedia.org/wiki/Freewheel

Some Rovers also had a free wheel facility which was operated by turning a wheel on the dash.

The link above also refers to oil passing the pistons on overrun (see comment above).

Whilst a built in freewheel is acceptable because you are still in control of the vehicle as the drive can be reconnected instantly - just knocking the gearlever into neutral and freewheeling is not - as you are not in total control of the vehicle.
Once you are in neutral you loose engine braking and have to rely on the brakes for control - not so good in hilly country and especially so if the vehicle is not fitted with a servo.
It was also a problem to get the vehicle back into gear on gearboxes without synchromesh a real problem.
If the engine and transmission speed are not synchronised when the drive is reengaged damage to the engine can occur if the engine is accidently over revved.
Phil

[biomed32uk]

Some modern cars now do it as well, I have a Mercedes with a 9 speed auto box, if I put it into E mode it will disconnect the engine and transmission, calls it glide mode. As soon as it feels the need to reconnect the two for reasons of control it does so.

Dosent actually seem to make much difference to economy according to what the car thinks so I don't bother with it, or E mode in general.

[JOWETTJAVELIN]

Like Les says its a good idea and to be recommended at high speed as oil is sucked up to the top of the cylinder bores. I always have a few seconds off the throttle every few miles on the motorway.