Hypermiling techniques that are invalid (potential mechanical wear/cost)

[alohaspirit]

I ride the brakes going downhill in my automatic


They squeak when cold and I want an excuse to
change them tho they have a lot of pad left


I admit thats not the only reason
When I downshift down a hill, my engine brakes
so much I occasionally fall below speed limit

[swoody]

I hate riding the brakes going downhill/mountain. I had to do this quite a bit on my most recent roadtrip. The only problem is the Jeep I was driving only has 2 and 1 on the gear selector. I figure 2nd gear would be way too low to shift into on 50mph roads. Is this true, or would it be fine? What else could I do to avoid having to ride the brakes the entire time?

[Christ]

Originally Posted by swoody

I hate riding the brakes going downhill/mountain. I had to do this quite a bit on my most recent roadtrip. The only problem is the Jeep I was driving only has 2 and 1 on the gear selector. I figure 2nd gear would be way too low to shift into on 50mph roads. Is this true, or would it be fine? What else could I do to avoid having to ride the brakes the entire time?

You can actually almost redline the engine without damaging it as long as you're not on the throttle.

If you're worried about RPMs, put the transmission in 2 and run the Jeep up to 50, see what your RPMs are.

If you have a 3 speed, you're fine. I use 2nd gear on the highway to slow down.

If you have a 4 speed, you'll be closer to overspeed with 2nd gear, but I'd imagine you'd still be fine. First thing to check is that you can't manually disengage OD. If you can, do it first, before downshifting.

[swoody]

Originally Posted by Christ

You can actually almost redline the engine without damaging it as long as you're not on the throttle.

If you're worried about RPMs, put the transmission in 2 and run the Jeep up to 50, see what your RPMs are.

If you have a 3 speed, you're fine. I use 2nd gear on the highway to slow down.

If you have a 4 speed, you'll be closer to overspeed with 2nd gear, but I'd imagine you'd still be fine. First thing to check is that you can't manually disengage OD. If you can, do it first, before downshifting.

Ah, great info, thanks! I guess I'll have a run on it, and see how high it goes in 2nd. If I can manually disengage OD, should I just disable it, or would I have to disable it and then downshift to 2nd? Will just disabling it give me any engine braking?

[Christ]

Originally Posted by swoody

Ah, great info, thanks! I guess I'll have a run on it, and see how high it goes in 2nd. If I can manually disengage OD, should I just disable it, or would I have to disable it and then downshift to 2nd? Will just disabling it give me any engine braking?

Gear skipping won't hurt an auto, it's just a bigger speed change for the engine. There is talk of more wear, since the transmission has to cope with higher engine speeds without "stepping up", but I don't buy it. I've done this stuff for years now, and any additional wear is apparently not appreciable.

Anyway - If you want to disengage OD first, do it. You will get some engine braking, but not much, as you'll only grab another maybe 500 RPM. It might be enough to slow you down if someone pulls out a little too close, to prevent you from having to touch your brakes to drop 3 MPH until they can get up to speed... Only you'll know how you're comfortable with your transmission though.

You don't need to disengage OD first, though. Your transmission will lockout OD electronically if it's an electronic OD setup, as soon as you change gears.

If you have an electronic transmission, almost any that I've heard of that have a "real" TCU will actually shift for you before you get to redline, so you can't blow up your engine by overspeed.

My wife's '94 GP (Granny) will do this. I put it in 1st gear if I want to make a run with it, and don't shift at all. It automatically shifts each gear @~5500 RPM. I've never gotten out of third and into OD. The car doesn't have enough power to go much over 130, if even that fast without a tail wind.

[Clev]

When I need to brake more or less continuously downhill, I "pulse and glide" my brakes. For instance, my downhill morning run is a 60 mph zone. I'll "glide" to about 70, and then brake fairly firmly down to 50, and repeat. I figure that this will give the brakes time to cool since the pad isn't always applying pressure like it is when "riding" the brakes down.

[Christ]

Originally Posted by Clev

When I need to brake more or less continuously downhill, I "pulse and glide" my brakes. For instance, my downhill morning run is a 60 mph zone. I'll "glide" to about 70, and then brake fairly firmly down to 50, and repeat. I figure that this will give the brakes time to cool since the pad isn't always applying pressure like it is when "riding" the brakes down.

This is what you're supposed to do.

When you ride the brakes, you have to continuously apply more pressure to get the same braking effect. The more they heat up, the more pressure you have to apply. The more pressure you apply, the more they heat up, etc.

Eventually, they fail. If your hill isn't long enough to make them fail, you're still wearing them faster than you would be if you were doing it correctly, using a firm pedal until you're somewhat under speed, then allowing yourself to coast with engine braking down the hill until you NEED to brake again. You don't necessarily have to go over the speed limit, but noone would fault you for going slightly under, if that's what it took to maintain safe braking ability.

[MadisonMPG]

Is engine braking more fuel efficient than regular brakes?

[Christ]

Originally Posted by MadisonMPG

Is engine braking more fuel efficient than regular brakes?

Is that a serious question?

Brakes convert forward motion to heat, while allowing the engine to idle. In essence, you're burning fuel to slow down.

In FI cars, the ECU cuts fuel at a given RPM when there is no throttle input, which means you're slowing down/maintaining speed, but NOT burning ANY fuel at all. As soon as you touch the accelerator again, you're now spraying fuel, and you're capable of accelerating without problems.

In carb'd cars, this isn't the case, except where aftermarket modification has been made to shut off fuel inlet to the jets from the bowl when there is no throttle.

[Clev]

Originally Posted by Christ

In FI cars, the ECU cuts fuel at a given RPM when there is no throttle input, which means you're slowing down/maintaining speed, but NOT burning ANY fuel at all. As soon as you touch the accelerator again, you're now spraying fuel, and you're capable of accelerating without problems.

I can't confirm that that applies to all FI cars.

In my downhill run, the exhaust on the Clunker burbles slightly when engine braking and the vacuum increases slightly (and engine braking increases slightly) with the key off. This leads me to believe that a small amount of fuel is flowing. This is a '90 with PGM-FI.