What RPM when climbing a mountain

[dix39]

And then there is the diff. gearing which further adds to the complication. If one is really interested in following the gearing thing there are a number of calculators online to do that with.

I would be comfortable with just saying that the different gears are there to mate engine RPMS to MPH as well as possible.

Steve

[docj]

Originally Posted by TonyMac

In a gas engine like the big, beautiful 8.1, you can see from the chart, that max torque occurs close to max hp. Not so in my Cat C7 and most diesels. As rpm goes up, so does hp, but torque drops off dramatically.

One of the advantages of a "big block" diesel is that the torque and HP curves are significantly flatter. I've attached the relevant curves for the C12 I have in my MH. I often drive in 4th gear on hilly, winding two-lane roads. That puts me in the relatively flat 1600-1900 rpm range which delivers peak HP and lots of torque. It also provides near maximum braking from my Jake Brake.

[TonyMac]

"the torque and HP curves are significantly flatter."


They are drawn that way, but the numbers say something drastically different. Your peak torque is 1550 at 1200 rpm. It drops to 1050 at 2100 rpm. That's a 33% drop over 900 rpm, very steep and much steeper than the C7 at 12% across a similar range. However, your low torque is higher than my high torque, and you really should share with us less fortunate.


Likewise, your hp starts at 355 at 1200 rpm, rises to 423 at 1800 rpm, then drops slightly to 410 at 2100 rpm. This is not flat at all, but a curve with 16% on the frontside, and 4% on the backside of a 900 rpm range. However, this is more advantageous, as peak torque and peak hp are not just higher, but more importantly, closer. You've got a nice sweet spot between 1200 and 1800. My peak hp is twice as far away at 2400.

[docj]

Originally Posted by TonyMac

However, your low torque is higher than my high torque, and you really should share with us less fortunate.

--not a chance! But you did correctly point out the fact that the torque/HP curves in my graphs do exhibit larger percentage changes than I had acknowledged.

Torque is something that is usually "missing" from MH literature these days because with diesels torque is directly related to engine size and there's no way that a small block engine can be "tuned" to increase its native torque output.

By way of contrast, manufacturers can tune a 6.7L Cummins ISB engine so that it gets ~350 HP. Since many customers don't understand the significance of torque, they don't consider what it means to have an engine with an output of roughly twice the torque of the ~750 ft-lbs of theirs.

I don't think it's possible to describe the feel of driving a relatively light weight MH (~37,000 pounds including toad) powered by an engine with as much torque as we have. As any hot-rodder knows, torque is what gets you moving off the line and our MH is almost car-like in its acceleration which makes for very easy driving. Very little concern about merging into traffic on the freeway. A real joy to drive. A key reason why I won't ever give up my old coach--there's no way I could afford a new one with an engine and transmission of the class we currently have.

[Bzzy]

FLavionics, Congrat's on your purchase...sure glad you chose the DP vs. the Gasser!

We just put about 6K miles in a couple of road trips through some very hilly terrain and my Fleetwood has the identical engine/trans set-up as you have. What I found is that contrary to what some report. I believe the Allison 3000 does a very good job of selecting gears on uphill grades when I am in cruise control mode. I spent a very significant amount of time watching exactly what RPM the motor was at on grades, in which gear it would be in, when the trans would downshift on its own, etc. My observations were that 98+% of the time there was no reason for my intervention because it would typically not let the RPM's get much below 1800 RPM before automatically downshifting....if I tried to manually intervene above this RPM the RPM's would jump to around 2500 RPM in the next lower gear and actually slow the vehicle down quicker rather than almost maintaining speed. I'd say that the Engineers got the Fleetwood/ISB 6.7/Allison 3000 combo dialed in very well. I'm sure those with more experience might be about to tweak an extra 1 or 2 MPH up the grade but for all of the fussing I'm not sure it's worth the effort on these particular MH's. Other makes/models may not be so lucky.

And as others have said the "old wives tale" of same gear up and down a Grade is a crock with todays DP's! Many a grade our DP will drop to 5th gear and cruise the hill maintain speed within 1 or 2 MPH....coming down that VERY same grade I have had to hit the retarder and watch it actually drop to 4th gear to hold the speed from accelerating....so much for that wives tale lol!!!

[Dav L]

"same gear up and down a Grade"
Yeah, that is hog wash. Never saw a hill that was symmetric in the real world. And I won't remember on the way back.
It's much simpler...use what it needs.

[FLavionics]

Thanks Bzzy,
I should have posted on my original thread "diesel vs gas" but all it took really to decide on diesel was a back to back test drive. I first drove a new 38' Winnebago then the Fleetwood Expedition. WOW what a huge difference. The drive alone convinced me " Half the fun is getting there"

[hrw]

Just set the cruise and let it do the rest. Trying to keep rpm at a particular level is silly. Let the cruise do the thinking while climbing and especially descending. That will provide a much smoother ride on your driveline. Manually trying to control speed is only necessary in heavy traffic and icy roads. Cruise will keep a steady amount of power or braking at your set speed. Don’t stress. Enjoy.

[Dav L]

"Cruise will keep a steady amount of power or braking at your set speed"
Cruise control does auto braking? Not in many vehicles.
Some engine braking (no accel applied = engine braking) but not wheel braking.
But I get and mostly agree with your larger point.

[Jake21]

Why is keeping a rpm at a particular level silly? Eg: climbing vail pass and leaving the coach on cruise control rarely works. Cars in the left lane going 70, trucks in the right lane going 30ish. I stay in the right lane, find the speed needed to stay steady with the other slow moving vehicles and manually choose the gear to keep me around 1700 rpms. That’s silly?

[docj]

Originally Posted by hrw

Trying to keep rpm at a particular level is silly. .

With all due respect, I strongly disagree. When climbing steep hills, manually selecting a lower gear permit me to run my C12 in the 1800-2000 rpm range which improves engine cooling and keeps the engine at roughly peak HP.

Coming down a mountain, manual gear selection allows me to keep the rpm's close to engine red-line which significantly increases the braking HP of the Jake Brake.

In contrast, the Allison, even without cruise control engaged, will repeatedly attempt to upshift to 5th (even 6th) going uphill which forces the engine to run in the~1200-1400 rpm range which results in it getting significantly hotter. Going downhill, my transmission is set to automatically shift to 4th when the Jake is applied, but going down some of the steepest hills this summer I've put it into 3rd a couple of times.

[Domo]

Freightliner, Cummins and Allison all say to simply turn on your cruise control and let the vehicle figure it out.

I don't want to overheat, so I don't push to get over the mountains - I simply turn on my flashers and join the line of semi's that are in the far right lane.

I'd rather save my equipment than try and save a few minutes of time going uphill.

[docj]

Originally Posted by Domo

I don't want to overheat, so I don't push to get over the mountains - I simply turn on my flashers and join the line of semi's that are in the far right lane.

If that works for you, then, by all means, do it. But my MH can climb grades at speeds significantly faster than the typical speeds at which heavy trucks would climb the same grade and I see no reason to climb at a speed slower than it is capable of.

Having driven over 60k miles in my MH in the past 8 years, I think I am well qualified to know how to maintain proper temperature control of my engine. As multiple others have already posted, the key to temperature control on steep hills is to keep engine revs up near redline, depending on what that might be for your engine. That maximizes the flow of coolant through the radiator.

With all due respect, I can't imagine climbing a steep grade with the cruise control engaged, but I'm in no way trying to convince you to not do something you feel is recommended by your manufacturer. But I sure wouldn't do it in my MH.

[A Traveler]

Originally Posted by hrw

Just set the cruise and let it do the rest. Trying to keep rpm at a particular level is silly. Let the cruise do the thinking while climbing and especially descending. That will provide a much smoother ride on your driveline.

Wow.

That is some of the absolute WORST advice I have ever seen posted here! PLEASE don’t drive like this!

It is so bad that I don’t know where to start in trying to explain why it’s bad advice. So I won’t...[emoji53]