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Old 02-06-2008, 04:20 PM   #1
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Diesels and Engine Braking: Ken vs. His Transmission

BACKGROUND

The diesel engine does a fantastic job of engine braking, e.g. having it in a low gear going downhill and not having to use the brake like a maniac. For extremely long, consistent downhills, this makes total sense and saves the heck out of the brakes.

QUESTIONS

A. At what point is downshifting not worth it due to being too hard on the transmission? For instance, it seems to make sense on a long downhill. But probably not as one approaches a stop sign. Correct? Or incorrect? In other words, when is using engine braking instead of your real brakes (in non-emergency/long slowdown periods of course) a knucklehead thing to do?

B. Prior to downshifting, I'll tap off the overdrive on the shift lever as though it's my first "downshift". Do the words of advice you may give regarding lower gear engine braking apply equally to turning off the overdrive, or is something different going on there?

I have a feeling I'm not the only one with these questions out there... it should perhaps be added to that Diesels for Dummies thread.
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Old 02-06-2008, 07:35 PM   #2
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Hi Ken,

I use my OD off frequently to slow down. Unless I'm in the mountains, I almost never downshift beyond that (in my van).

Some people think downshifting causes more wear and tear - I don't think so. You're only loading the side of the gears that doesn't see a load when accelerating. Maybe you only even out the wear on the gears! A downside might be wear on the shift linkage.

In my stick-shift PU, I always downshift and rarely use the brakes, except to come to a complete stop.

If it worries you, you could either stop doing it or invest in and extended warranty.

Mike
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Old 02-06-2008, 07:54 PM   #3
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Quote:
Originally Posted by Ford_6L_E350
Hi Ken,

I use my OD off frequently to slow down. Unless I'm in the mountains, I almost never downshift beyond that (in my van).

Some people think downshifting causes more wear and tear - I don't think so. You're only loading the side of the gears that doesn't see a load when accelerating. Maybe you only even out the wear on the gears! A downside might be wear on the shift linkage.

In my stick-shift PU, I always downshift and rarely use the brakes, except to come to a complete stop.

If it worries you, you could either stop doing it or invest in and extended warranty.

Mike
Ditto. I frequently turn off OD to slow the van when descending hills. That, plus moderate braking, is almost always enough to suffice. Very seldom do I need to shift down into 3rd. I find that unless I'm going pretty slow, the RPMs can climb fast in 3rd. Only on long twisty downhills have I found downshifting helpful. I'm no expert so don't know if I'm doing this the "right" way.
Rob
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Old 02-06-2008, 08:12 PM   #4
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Last summer, I descended Dalton Pass in our SMB up in the Yaak-Kootenai country of northwestern Montana. Although a very narrow paved road, it had several extremely steep 5 mph switchbacks. I had the overdrive off, and the SMB was in 2nd or even 1st for most of the descent. I rarely had to use my brakes.

I try to avoid using lots of brakes in any descent,and 2nd and 1st gear worked great on this traverse.
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Old 02-06-2008, 09:43 PM   #5
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Engines/transmissions are designed for engine braking...brakes are not designed for continuous usage. A stuck caliper is a fire hazard

Diesels also do not run fuel when braking, so there is no cost penalty versus a gas engine, which will let fuel in
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Old 02-06-2008, 11:55 PM   #6
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OK, so far so good. But is there damage to the transmission if I were to use downshifting instead of braking on hilly windy roads? Thus... into third, out of third, into third, then second, then back to third, then back to drive. A lot. I do like the point about wearing the sides of the gears that one doesn't use for accelerating.

I use third a lot. Coming down from Yosemite, it's a great way of doing the steep winds while burning no gas, but I do also then drop to second and back to third around steep hairpins. Again, am I sacrificing my transmission only to save the brakes?
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Old 02-07-2008, 12:11 AM   #7
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Though I have no hard information here, I suspect that frequent shifting would cause transmission wear. I try to only engine brake when I know I'll be slowing for more than only a few seconds. For long descents, I always engine brake. It's only on those long descents when I worry about the brakes overheating.

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Old 02-07-2008, 12:17 AM   #8
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engine braking with the Scangauge

On a side note, my Scangauge shows I'm burning gas when I'm engine braking, but I suspect the engine is actually completely shutting off fuel to the injectors while the engine speed is above idle (or the vehicle speed is above some minimum, etc.). At the same vehicle speed, it shows more gas being consumed when the transmission is in even lower gears (higher revs). I've tried the Scangauge in a Prius, and my Jeep WJ, and it also shows fuel being consumed when the engine shouldn't be using gas.

Anyone else see this behavior with their Scangauge?

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Old 02-07-2008, 12:22 AM   #9
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Re: engine braking with the Scanguage

Quote:
Originally Posted by geoffff
On a side note, my Scanguage shows I'm burning gas when I'm engine braking, but I suspect the engine is actually completely shutting off fuel to the injectors while the engine speed is above idle (or the vehicle speed is above some minimum, etc.). I've tried the Scanguage in a Prius, and my Jeep WJ, and it also shows fuel being consumed when the engine shouldn't be using gas.

Anyone else see this behavior with their Scanguage?

-- Geoff
I get both... if I have my scangauge on the gauge setting (the screen with four readouts) it reads 9999 when engine braking. However, if I have it on the "Trip" screen and viewing "current," then it tells me I'm burning fuel. To summarize: Scangauge gives me two completely different answers depending upon which screen I'm viewing. One question would be: which of those numbers is it using for calculating MPG for the tank?
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Old 02-07-2008, 08:52 AM   #10
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Gas engines seem to inject more fuel when engine braking than they do when idling. Diesels shut off the fuel when engine braking.

Mike
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