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Old 10-12-2015, 06:36 PM   #1
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How do you drive a diesel in mountains?

Recently bought 1994 Allegro Bus, Cummins 5.9L 6BT with Allison MD3060. I don't have much mountain driving experience and have read a lot about descending the grades but my question is how much throttle to climb? If we are drive at 65 mph on the flats we will obviously start slowing as we start climbing. How much throttle is safe. With a gas motor I would probably run no more than about 1/2 throttle and accept what ever speed we slowed to. On the diesel should we do about 1/2 throttle? 3/4? WOT? I realize keeping the rpm in the 2000-2200 range to keep the fan spinning will help in the cooling.
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Old 10-12-2015, 07:00 PM   #2
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What I do is hold RPM around 1800-1900 or so and use gear selection to allow me to hold it there, nowhere near WOT even though I might be going 25mph. I think it's more than half and less than 3/4 mostly. Someone please correct me if I'm wrong but I believe top torque is at about 1700.
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Old 10-12-2015, 07:00 PM   #3
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You have the right idea, use your shift pad to manually downshift to maintain 2000-2300 RPM without having to put your foot into it. Speed is not the issue. Pay attention to RPM and engine/trans temperature.

Now, when you get to the top, use the same method to go down. Use your exhaust brake and use the same gear that got you to the top to get to the bottom. Use your service brake with firm application (do not pump the brakes as this will cause them to overheat) to bring your speed down when the transmission wants to shift up. Sit back and enjoy the ride. Never mind the traffic behind you. They will just have to wait their turn.
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Old 10-12-2015, 07:10 PM   #4
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Set cruise control and engine brake then watch the scenery. With the engine we have that's about all I need to do.


The truck industry seems to say, find a speed/gear combination that allows you to have some throttle in reserve, enough to gain speed if really pushed. Drop speed and gears if necessary.
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Old 10-12-2015, 07:35 PM   #5
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Quote:
Originally Posted by Fugitive861 View Post
Recently bought 1994 Allegro Bus, Cummins 5.9L 6BT with Allison MD3060. I don't have much mountain driving experience and have read a lot about descending the grades but my question is how much throttle to climb? If we are drive at 65 mph on the flats we will obviously start slowing as we start climbing. How much throttle is safe. With a gas motor I would probably run no more than about 1/2 throttle and accept what ever speed we slowed to. On the diesel should we do about 1/2 throttle? 3/4? WOT? I realize keeping the rpm in the 2000-2200 range to keep the fan spinning will help in the cooling.

Your B engine needs to climb at around 2500 rpm to keep both speed up and heat down. There is nothing wrong with WOT. Diesels are built for that. Down shift as needed to keep the RPMs up at that level. If heat gets up to over 200 then downshift again & back out of the throttle some until heat stabilizes.


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Old 10-12-2015, 07:44 PM   #6
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I have a similar setup, but with an 8.3 and probably a heavier coach.

The engine and trans do a good job of looking after themselves. Going up, I just drive, and the computers handle things beautifully. Never a hint of engine or trans overheating.

Going down, I apply the exhaust brake, and manually select a gear that requires at most occasional service brake application. If things get going to quickly, I apply the brake solidly and get to the next lower gear range. Change the gear on the selector, and the trans will shift when the RPM allows. I never worry about who is behind me, unless the line gets long. Then I stop and make coffee!

We have done some serious mountains, including Nine Mile Canyon Road in California. THAT is an interesting downhill.

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Old 10-12-2015, 07:48 PM   #7
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Wink

I keep my cummins in 'mode', and keep the pedal on the floor(on steeper grades) - works fine, slows to an acceptable speed and acceptable rpm automatically. Downhill, I set exhaust brake, and let off 'gas' pedal when I'm at the speed I want to descend, such as 45mph. The exhaust brake generally handles it nicely, only using the brake pedal sparingly, on steep descents. The rpm can run high, sometimes approaching 30k, but normally stay within the 20k-25k range during downgrades. I don't remember ever using a manual change of gear either way. But, then again, I'm never in a race.
30k miles, with toad, across country several times West and East, including Canada...11.4 mpg average all miles.
This is my first diesel, first toad, first rv - maybe I'm missing something, but 'nothin ain't broke yet', and cummins service at about 30k miles said everything looked just fine : )
I guess I just trust the engine and transmission to do what they were designed to do, without too much interference from me!
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Old 10-12-2015, 07:59 PM   #8
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I climb at WOT unless that leads to speeds higher than I want, in which case I generally find I don't need to worry about selecting gears unless the transmission starts to hunt.

Coming down again depends on the grade and the speed. I generally need to be in 4th to hold below 55mph on 6% grades. On some of the longer, straight grades you get driving on I-80 in Wyoming, for example, I'll let speed build to 65mph or so, so that the transmission stays in 5th and just kinds of allows 'dragging my feet', because on less than a 6% grade if the coach gets into 4th it'll slow up way more than I want.

But if in doubt on speed, I will get on the brake pedal for 5-10 seconds and bring the speed down NOW. Better to do it quickly and early -- it's always better to go "man, I could have come down that grade faster" than have your entire seat cushion sucked up between your cheeks for 10 minutes.

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Old 10-12-2015, 08:01 PM   #9
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5.9 Cummins in mountains

I have a very similar setup 1998 Holiday Rambler with a 5.9 and Allison MD3060. I hold rpm's around 2000-2200 and have never had a problem over heating trans or engine. My coach weighs around 24,000 and I flat tow a Jeep Cherokee that's 3200 lbs. This coach amazes me the way it pulls hills with the RPM around 2000. I have not been out west with it yet ?
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Old 10-12-2015, 08:19 PM   #10
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As per my previous post:

Quote:
RV Uphill Climbs
When climbing long inclines, your RV needs to be operated within its power band.
The power band is a span of engine RPMs where you have the maximum horsepower available to handle the extra load imposed when going up long inclines.
This becomes even more noticeable with diesel engines. Their power band is usually a rather narrow band of RPMs. When operated within their assigned power band, you will have a tremendous amount of pulling power. Fall below that RPM, and it will do you no good to push harder on the throttle. All you’ll get is more black smoke coming out of the exhaust.
To stay within the power band, you must downshift to a lower gear, and you may even be required to let off the throttle a bit so the engine can work more comfortably. Pushing too hard will only create more heat and increase the likelihood of overheating.
The whole point when climbing long inclines is to adjust your gearing, so you will remain within the comfortable RPM range that your engine likes. Maintaining a certain speed because the sign on the side of the road says that’s the speed limit may be impossible. Slow down and use a lower gear instead.
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Old 10-12-2015, 08:37 PM   #11
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Well said Mr D.
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Old 10-12-2015, 08:42 PM   #12
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I'm usually WOT climbing a grade. Downshift to keep RPMs at 2300 or so. Going down is more challenging as I have no engine brake. I turn off the overdrive and down shift as needed. Use the brakes occasionally as needed. I have Allison without. Lock up torque converter, so I'm probably braking more than you'll need to. I have only 190HP but it always makes it to the top.
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Old 10-12-2015, 08:51 PM   #13
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Mountain drive with Dieael

Quote:
Originally Posted by Mr_D View Post
As per my previous post:
Mr.D: that is probably the most helpful advice given,I wish I had seen this a few years ago. I definitely have it figured out now, because almost all of my driving is through the Sierras. Regards
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Old 10-12-2015, 10:19 PM   #14
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I tend to back out of the throttle a little and take it easy going up the hills, on the down hill side the exhaust brake does a good job keeping the speeds under control.
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