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-   -   What RPM when climbing a mountain (https://www.irv2.com/forums/f258/what-rpm-when-climbing-a-mountain-407669.html)

FLavionics 08-22-2018 12:32 PM

What RPM when climbing a mountain
 
So my new to me DP is rated 360HP @ 2400RPM
and 800 ft-lbs @ 1800RPM.

When climbing a mountain would it be best to stay at maximum torque by manually shifting which would be 1800RPM?

And what about descending

Skip426 08-22-2018 12:59 PM

Many engines rated at 360 HP over the years.
Is yours a Cummins ISC because the max HP @ 2400 seems low RPM if it's an ISB.

Adding the year would be a help too .:bow:

Keep track of the speed you're able to maintain , driving up the hill to the summit. That speed will be your " control speed " for the decent .
If your going faster at the summit than you were on the climb; less grade near the top; slow down !
Using your engine brake ( compression brake / exhaust brake ) watch you engine RPM in the gear that the engine brake system has selected for your speed . As the RPM climbs to 2500 , apply your service brakes fairly hard to drop the RPM to 2,000, then release to allow the service brakes to cool.
If you find your having to repeat the application of the service brakes , in a short period of time , your going too fast , apply the service brakes to slow you to the point where the system , shifts the trans to the next lower gear and , then release the service brakes to allow them to cool.
There are different downshift programs with the engine brake systems , some will only down shift to forth , others second , some require you manually down shift. These things will become apparent as you get used to driving your coach.
REMEMBER , it's much easier to speed up than slow down , so as you drive setting your "control speed " low, at the top of your decent is preferred to setting it too high.

beenthere 08-22-2018 01:13 PM

I think with 800lbs torque it would be an ISB, I thought the isc does 950-1000lbs.. max rpm im sure is more but max HP sounds right at 2400, not?
But the description of what to do in hill situations, you described right on. so really don't matter what engine... cept the tranny on some like you mentioned.. jeff

bob91yj 08-22-2018 01:18 PM

Old school rule...you go down the hill in the same gear you went up it in.

Jake21 08-22-2018 02:13 PM

Quote:

Originally Posted by bob91yj (Post 4360737)
Old school rule...you go down the hill in the same gear you went up it in.



That never made sense to me. Up hill grade is 6%, down hill is 3%. I will be in a different gear.

J

Finance 08-22-2018 02:20 PM

You want to be between torque peak and hp peak. So 1800-2400.

Which the Allison will do quite nicely on its own. :)

No need for manual intervention unless the grade is such that it repeatedly shifts up and down.

Old Scout 08-22-2018 02:43 PM

….to the contrary, I have rarely found the Allison to "do nicely" on hills over a few % grade----tranny tends to hunt gears....anyway, engines tend to vary on RPMs but around 2000 to 2100 is a nice neighborhood to be in--RPM-wise--for ISCs and ISLs, apparently a bit more on an ISB...the other consideration is to keep RPMs up so cooling fan [direct-drive or hydraulic motor] runs at max cooling capacity on long, hot grades.....

TonyMac 08-22-2018 02:58 PM

Quote:

Originally Posted by Finance (Post 4360817)
You want to be between torque peak and hp peak. So 1800-2400.

Which the Allison will do quite nicely on its own. :)

No need for manual intervention unless the grade is such that it repeatedly shifts up and down.

Agreed. My Cat C7 produces max torque (860) at 1440rpm, and max hp (350) at 2400rpm, also redline. I like to stay a hundred or two above max torque, but I don't let the tranny "hunt". If it keeps upshifting and downshifting, I select the lower gear, I select an rpm above max torque, and accept whatever safe mph that gives. I never use much above 2000rpm without upshifting to drop the rpm back towards 1500. At 1800rpm, my torque has dropped to 830, at 2000 it's 810, and at 2400 it's 760, 100 lb-ft below max according to Cat's charts.

FLavionics 08-22-2018 03:17 PM

The engine in question, cummins ISB 6.7 driving a 2013 Fleetwood Expedition GVWR 32,400.
This is our first diesel!!!

A Traveler 08-22-2018 09:25 PM

It is not torque that gets you up a grade at speed. It is horsepower. You must shift down and get your RPM up near the HORSEPOWER peak, not down at the torque peak.

As good as an Allison transmission is, you cannot just put a big motor home in D and drive. The fatal flaw in doing that is that the Allison cannot anticipate anything, it can only react to something that has already happened! You can anticipate a grade by shifting down to a lower gear and getting your RPM up near the HP peak BEFORE your speed drops and you get behind the power curve.

Go up a hill in the same gear you go down is an old wives tale that just wont die. If I end up at 50 mph in 4th gear going UP a 7% grade on an interstate where the speed limit is 70, why in the world would I want to come DOWN that same grade at 50 mph in 4th gear? I let the coach run back up to the 65 or 70 mph speed limit and ride the Jake Brake in 5th or 6th gear to maintain speed going down. Works every time.

Hoosier14 08-22-2018 10:03 PM

Go up a hill in the same gear you go down is an old wives tale that just wont die. If I end up at 50 mph in 4th gear going UP a 7% grade on an interstate where the speed limit is 70, why in the world would I want to come DOWN that same grade at 50 mph in 4th gear? I let the coach run back up to the 65 or 70 mph speed limit and ride the Jake Brake in 5th or 6th gear to maintain speed going down. Works every time.[/QUOTE]


While this advise might work for lighter motor homes, this is very dangerous advice for many class A drivers. This is exactly what should not happen as it may / will result in the loss of your service brakes going down. With a 7% grade, my very powerful 2 stage Jake Brake would not hold my 25 ton coach at 70mph. This speed would be a death trap. The old saying of going down in the same gear as going up is not perfect (depends on the grade on each side going up and down), but it is a reasonable guide. If Im going up at 45 mph, Im definitely not going down a 7% grade any faster. It is important to find the speed in which the Jake can hold the Coach without much need for the service brakes. As we came down Wolf Creek Pass a couple months ago, this speed for our coach was 25-30 mph for nearly 9 miles. One should never start out at the speed limit and slow down if needed: it may be too late. One should start slow and pick up speed if the Jake brake allows for more speed. Stay safe.

G jeffers 08-22-2018 10:05 PM

Quote:

Originally Posted by Jake21 (Post 4360811)
That never made sense to me. Up hill grade is 6%, down hill is 3%. I will be in a different gear.

J

The same hill, not the other side of a hill.

1320Fastback 08-22-2018 10:51 PM

How does one remember what gear they go up in 3 weeks later when they are going back the other direction? And what if I go back a different way and have never climbed said hill in the first place?

As to OPs question, stay above peak torque.

Dutch Star Don 08-23-2018 01:54 AM

"Flavionics"......Take all the information you receive here and experiment with it. One of the best statements is that your peak climbing power is somewhere between your peak torque and peak horsepower.

My previous coach had a Cummins 400 ISL which had some tuning done that brought it closer to 450HP. It was light for a diesel pusher at 36'. It could climb ANY grade out west at 55+ mph, even while towing. The Allison transmission is smart, but it doesn't know it's climbing a grade until it's often too late and has lost momentum. If you previously drove a gas rig, stomping on the accelerator would downshift it and make the engine rev, giving you power. If you do that with a diesel pusher, it will fall on it's face and take time to recover. A diesel pusher doesn't accelerate, it gains momentum!

On my last coach and current coach, I always downshifted to 5th at the base of 5% or steeper grade. On the previous coach, I set the cruise and never had to do much to complete the climb. Descending the grade was a little more work since I had an exhaust brake and it was marginal at keeping my speed down. That coach really made the old wives tale about speed up and down a grade, just that, a wives tale.

Fast forward to my current coach. Its 5K heavier, pushing more wind with a larger front end and dragging a tag axle. It won't climb like my last coach, even though it's the same motor. This coach I have to do more driving. I still drop to 5th at the base of a grade and often will get a little bit of a running start, speeding up about 5-6 mph over my average speed of 62-63 mph. As I start to slow down, I eventually have to downshift manually to 4th to hold about 48-50 mph.

Now going downhill is just the opposite of my previous coach. With a two speed engine brake and integrated cruise control, I can set my cruise at 60 mph and hold that speed. Again disproving the same speed/gear up and down a grade.

You can see that two different coaches worked differently climbing. Sooooo…...try different things, try manually shifting, try dropping a gear at the base of the grade. Find where your coach settles in on a steep grade....it maybe 45mph in 4th and you're just wasting fuel trying to push it past that point.


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