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....when going DOWN HILL :

1) Staying in gear using engine braking

- or

2) Shifting into neutral and letting the engine idle, using foot-brake to control speed

DISCUSS! B)

Jules

Easy. Number one. With the car in gear and your foot off the throttle the engine management can fully shut off the fuel injectors. If you let the car idle, it goes into its idle circuit and operates the injectors at a low duty cycle.

Number two is dangerous anyway, so it's a moot point.

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I'd say staying in gear uses more fuel, but if you're on a steep down hill you want to use the engine braking, not your brakes to control speed.

Well, brakes are cheaper to replace than an engine. I say use your brakes to control speed.

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I'd say staying in gear uses more fuel

You would be wrong. It uses less fuel.

You're correct in saying that engine braking, in conjunction with the brakes, is the correct and safe way to go down a hill. It doesn't damage the engine whatsoever.

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I'd say staying in gear uses more fuel

You would be wrong. It uses less fuel.

You're correct in saying that engine braking, in conjunction with the brakes, is the correct and safe way to go down a hill. It doesn't damage the engine whatsoever.

John, but he (JeTexas) does not say "in conjunction with...".

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So long as the engine isn't allowed to exceed redline there is nothing wrong with engine braking.

Maybe true, but brakes are still cheaper to replace, then engine wear.

You are assuming that engine braking causes excessive engine wear. It doesn't. If you don't lose the motor due to IMS failure it's likely to last you longer than you have the car.

Better to replace neither the engine nor the brakes prematurely, no? :P

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Brake pads are one 'ellva lot cheaper than trans. and engines. If youy spent 25% of your time free wheeling up to stops, down hills and the like, that would be almost 25% less friction & load on your transe and engine every year.

Regards, Pk

P.S. Of course I'm not saying coasting down long hills rideing the breaks all the way.

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Brake pads are one 'ellva lot cheaper than trans. and engines. If youy spent 25% of your time free wheeling up to stops, down hills and the like, that would be almost 25% less friction & load on your transe and engine every year.

Regards, Pk

P.S. Of course I'm not saying coasting down long hills rideing the breaks all the way.

Oh brother. :rolleyes:

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I'm not sure I understand what's going on here. My observations have been different. If I coast down a hill, clutch in but also a gear selected, I typically accelerate down the hill due to gravity, and make up quite a lot of ground using fuel at idle. There's a hill on the way to the track where I have to keep touching the brake to keep from hitting 90+ since there's occasionally a trooper at the bottom waiting for people like me. :D

For curiosity's sake, I've reset the fuel mileage reading on the computer on two consecutive days going down that hill. The difference I saw was impossible according to John V's comments. The difference was about 30 m.p.g. on the whole of the hill. It's about two minutes to go down it, and the reset was at what appears to be close to the same spot. Leaving the car in 5th gear, I saw low to mid 60s. Coasting, low 90s. I've done this twice, switched the order the second time, and got a similar reading. I try to keep about the same speed going down the hill. The car in 5th gear is running a few thousand RPM, and coasting is about one thousand. I understand the logic for both arguments, but my completely unscientific testing is pointing hard to the opposite of John V's argument.

Where is my test going wrong? Is the hill too big for what this thread is trying to convey?

-Michael

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I'm not sure I understand what's going on here. My observations have been different. If I coast down a hill, clutch in but also a gear selected, I typically accelerate down the hill due to gravity, and make up quite a lot of ground using fuel at idle. There's a hill on the way to the track where I have to keep touching the brake to keep from hitting 90+ since there's occasionally a trooper at the bottom waiting for people like me. :D

For curiosity's sake, I've reset the fuel mileage reading on the computer on two consecutive days going down that hill. The difference I saw was impossible according to John V's comments. The difference was about 30 m.p.g. on the whole of the hill. It's about two minutes to go down it, and the reset was at what appears to be close to the same spot. Leaving the car in 5th gear, I saw low to mid 60s. Coasting, low 90s. I've done this twice, switched the order the second time, and got a similar reading. I try to keep about the same speed going down the hill. The car in 5th gear is running a few thousand RPM, and coasting is about one thousand. I understand the logic for both arguments, but my completely unscientific testing is pointing hard to the opposite of John V's argument.

Where is my test going wrong? Is the hill too big for what this thread is trying to convey?

-Michael

Your test is invalid because you're not keeping an important parameter constant between the two tests: speed. If you use the brakes to hold yourself to the same speed coasting as would be provided by engine braking alone, your fuel mileage will be better using the engine braking.

The engine should use essentially zero fuel under engine braking conditions. I've yet to see a fuel injected car that fires the injectors in an engine-braking scenario (assuming the revs are above idle). The injectors are absolutely firing if the car is at idle or the idle circuit is engaged (< 1300 RPM or so).

J

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I'm not sure I understand what's going on here. My observations have been different. If I coast down a hill, clutch in but also a gear selected, I typically accelerate down the hill due to gravity, and make up quite a lot of ground using fuel at idle. There's a hill on the way to the track where I have to keep touching the brake to keep from hitting 90+ since there's occasionally a trooper at the bottom waiting for people like me. :D

For curiosity's sake, I've reset the fuel mileage reading on the computer on two consecutive days going down that hill. The difference I saw was impossible according to John V's comments. The difference was about 30 m.p.g. on the whole of the hill. It's about two minutes to go down it, and the reset was at what appears to be close to the same spot. Leaving the car in 5th gear, I saw low to mid 60s. Coasting, low 90s. I've done this twice, switched the order the second time, and got a similar reading. I try to keep about the same speed going down the hill. The car in 5th gear is running a few thousand RPM, and coasting is about one thousand. I understand the logic for both arguments, but my completely unscientific testing is pointing hard to the opposite of John V's argument.

Where is my test going wrong? Is the hill too big for what this thread is trying to convey?

-Michael

Your test is invalid because you're not keeping an important parameter constant between the two tests: speed. If you use the brakes to hold yourself to the same speed coasting as would be provided by engine braking alone, your fuel mileage will be better using the engine braking.

The engine should use essentially zero fuel under engine braking conditions. I've yet to see a fuel injected car that fires the injectors in an engine-braking scenario (assuming the revs are above idle). The injectors are absolutely firing if the car is at idle or the idle circuit is engaged (< 1300 RPM or so).

J

I'll chime back in now.

Looks like I got you all thinking! So it wasnt a daft Q after all...

John V,

I had a suspicion that the fuel injectors DO fire when the engine is on the over-run - I thought it was possibly to keep the top end / valves lubricated and also possibly to keep the catalytic converter up to temp ??

I think the above test by Michael just about proves this theory since the MPG reading on the OBC is directly linked to the pulse width of the injectors from the DME.

Jules

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Your test is invalid because you're not keeping an important parameter constant between the two tests: speed. If you use the brakes to hold yourself to the same speed coasting as would be provided by engine braking alone, your fuel mileage will be better using the engine braking.

The engine should use essentially zero fuel under engine braking conditions. I've yet to see a fuel injected car that fires the injectors in an engine-braking scenario (assuming the revs are above idle). The injectors are absolutely firing if the car is at idle or the idle circuit is engaged (< 1300 RPM or so).

I understand your point, but the difference in speed wasn't more than a few m.p.h. I was trying to keep it at around 70-75, and true, the car accelerated in both cases due to gravity. There were points where the car was going faster when coasting, and points where it was going faster when in gear. Overall, the speed was fairly constant.

I suppose I should try this again, but maintain one speed the whole way down the hill. I can use the cruise control to hopefully maintain the speed while in gear. I'll have to be very careful to use the brakes to keep the car at that speed on the second run.

I'm going to the track again this Friday, but it's only a one day event. If I have time, I'll go down the hill, turn around, go back up the hill and try again. I'll let you know what I find.

-Michael

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I understand your point, but the difference in speed wasn't more than a few m.p.h. I was trying to keep it at around 70-75, and true, the car accelerated in both cases due to gravity. There were points where the car was going faster when coasting, and points where it was going faster when in gear. Overall, the speed was fairly constant.

Your earlier post stated:

Leaving the car in 5th gear, I saw low to mid 60s. Coasting, low 90s. I've done this twice, switched the order the second time, and got a similar reading.

Which is it?

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Fact - When on the overrun no fuel is injected until revs drop to tickover speed

Fact, fuel used to contain an upper cylinder lubricant - lead, unleaded still has lead its just that extra lead is not added, valves, valve guides are manufactured to run on unleaded and require no additional lubricant than is added by unleaded and the oil that swirls around the camshaft.

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I understand your point, but the difference in speed wasn't more than a few m.p.h. I was trying to keep it at around 70-75, and true, the car accelerated in both cases due to gravity. There were points where the car was going faster when coasting, and points where it was going faster when in gear. Overall, the speed was fairly constant.

Your earlier post stated:

Leaving the car in 5th gear, I saw low to mid 60s. Coasting, low 90s. I've done this twice, switched the order the second time, and got a similar reading.

Which is it?

I see the confusion. The "low to mid 60s" and "Coasting, low 90s" numbers were referring to m.p.g., not m.p.h. The previous sentence to the second item you quoted refers to m.p.g. and I thought the reference would carry through since it clarifies what was meant my 30 m.p.g. When I wrote it, I thought there might be confusion, but decided against it because of the proximity to the reference. Hopefully this clarifies what I meant:

In gear, speeds around 70-75 m.p.h., I was seeing mid 60s in m.p.g. Coasting, maintaining around 70-75 m.p.h., I was seeing low 90s in m.p.g.

-Michael

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In my brain it still seems more logical that a car uses less fuel while coasting at idle than it does in gear. I mean, if you check any of the hypermiler websites, they always emphasis coasting as much as possible to increase fuel efficiency.

But this discussion obviously pertains to coming down gentle/occasional slopes, not coming down a mountain in New Mexico/Colorado as I would never attempt that coasting.

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