Long grades with my 2005 400hp Cummins?

[oldryder]

Rookie DP owner.  2005 Winnie Vectra.  Trip home will include some long grades.  Had to learn to let the V10 in my last RV rev.  Pretty sure (actually absolutely sure) thats the wrong approach for the diesel.

I don't care if I have to slow down just don't want to abuse the drive train.  For  most powerplants the most efficient option is to operate them at the RPM where the engine torque output peaks.

What is the recommended approach for long grades with a big diesel and what gauges should I be watching.?

thx to anyone taking the time to educate a rookie.

mark in MN

 

[Oldgator73]

I can?t speak from experience with motorhomes but I had a Dodge with the Cummins motor pulling a 37? 5th wheel. I would get it serviced at the Cummins garage and was told those motors were meant to be run run wide open. I drove up some pretty long and steep grades, one in Utah where I had traverse in 2nd gear screaming all the up.

 

[Larry N.]

Going uphill, the transmission can be left alone, or on long, steep grades you may wish to downshift a bit, but the key is to not lug the engine (tranny shouldn't let you) and keep the RPMs in the higher torque range, which varies somewhat with the engine. On my Beaver with the CAT C-13 (525 HP) engine 1400-1700 RPM were best, and it's a little higher on my Ventana with the 360 HP Cummins. On both the transmission did pretty well but occasionally I felt it was a little better with manual shifting.

Presuming you have an engine brake (Jake or otherwise), the first thing on a downhill is to engage it at a low speed. It should come on almost anytime you let off the go pedal. The (presumably) Allison transmission should handle the gearing for you, but the engine brake will probably select a low gear (it was 4th on my Beaver and is 2nd on my Ventana), so there's not much to do with those, but a lot varies with the engine, transmission, coach weight, brake type, etc. but, as with a gasser, the goal is to keep the speed down and minimize the use of the service brakes (brake pedal) to avoid overheating them, perhaps fading and or catching fire.

A description of which Vectra model, engine, transmission, engine brake type (compression, variable turbo, one, two or three stage, etc.) would help give better advice. Of course these big diesels are, in some respects, not the same as those in pickups, but a lot of the same concerns are there.

 

[Kevin Means]

We live out west and are always climbing and descending 6 to 7% grades. The engine temp gauge in our previous motorhome barely moved when we climbed them. (Ford V-10)

When we got our current DP with a Cummins 450, I was a bit surprised at how much it heated up when climbing those grades in the summer, but it has never overheated. We've climbed long steep grades in the summer, in temps exceeding 100 degrees many times (once when it was 109) and never overheated.

I use an engine temp of 221 degrees as my personal benchmark. Overheating occurs at 240 IIRC, at which time the engine will start shutting down to protect itself. If the engine temp hits 221 degrees, which it has a few times, I manually downshift to increase the engine RPMs. That increases the speed of the radiator fan, and drops the engine temp fairly quickly. Don't worry about damaging the transmission. It won't let you downshift if it doesn't want you to. And FWIW, our transmission temp barely moves at all.

So yes, you will see a difference in engine temp behaviour, especially on hot days when climbing grades, but as long as your cooling system is working properly, it's easily manageable.

Kev

 

[Gary RV_Wizard]

Push the go-pedal down and let the transmission & engine computers sort out the right gear and rpm range. It will probably end up between the torque peak and the HP peak and it certainly will NOT be overstressing the engine.

 

[Skookum]

The biggest thing with turbo diesels is exhaust gas temperature. Like Gary said, you can run it wide open, and because it's electronically managed, and with an automatic transmission, the programming will be mindful to keep EGT's within a range and you'll generally be fine. It will select the appropriate gear as well.

I prefer to manually intervene here and there to keep EGT's out of the higher end of that range, if they don't need to be there. If you have an EGT readout or pyrometer on the dash, it's helpful to see how your inputs change EGTs. It's worth reading about if there's any interest, otherwise, foot to the floor and let the computer do the rest.

 

[Tom]

Ditto what Larry said.

FWIW when the factory-trained CAT mechanics come to our boat, they run both engines wide open to get them warmed up, but that of course is under no load.

My other half does most of the driving of our coach, but I cringe when the Cummins RPM get "high". As others have said, the transmission will shift when it decides it's time.

Ditto on the use of exhaust/PacBrake (or Jake/engine brake) coming downhill.

 

[A Traveler]

I have a 2006 coach with the exact same engine. They key to climbing steep grades is HORSEPOWER, not torque. And the way you get maximum HP is to shift down and shoot for 1,900 to 2,000 RPM on the climb. The 400 HP ISL horsepower peaks at 2,000 RPM. The other advantages to operating at a higher RPM is that both the water pump and the fan run faster at higher revs.

I take issue with the ?Let the transmission figure it out? crowd for one important reason. The tranny can only react to something that has already happened. It cannot anticipate anything. When I approach the bottom of a steep interstate grade, I shift down to fifth gear BEFORE I get into the grade. That puts me ahead of the curve, with the engine right on the HP peak as I start up the grade. I don?t have to wait for the speed to sag 5 to 10 mph before the transmission figures out that it?s time to go down a gear.

 

[Utclmjmpr]

Diesels "grunt" for a reason,, they are built to do just that better than any other option.. They also heat more for a reason,, diesel fuel contains more BTUs than gasoline..
So it's all about leaning and understanding each method of power.. >>>Dan

 

[Larry N.]

So it's all about leaning and understanding each method of power.. >>>Dan

"Leaning" is for airplanes, Dan...  : ;D 8)

The biggest thing with turbo diesels is exhaust gas temperature.

Beyond what Skookum says above, you also want to let those turbos cool down for a bit before shutting off the engine. If shutting down shortly after exiting the interstate (or other high speed or high power run) I like to let it idle for 3 minutes or so, but if I had a little driving at slow speeds where the turbos didn't kick in much for several minutes then I may cut that to one minute or so.

 

[Utclmjmpr]

I did not realise I had written lean and it should have been learn,, comes from typing at night.>>>Dan

 

[Gary RV_Wizard]

The tranny can only react to something that has already happened. It cannot anticipate anything.

True enough, but the reaction time is no more than a second or two. And you could get the same anticipation by pushing the accelerator down at the bottom of the hill.
It offends some people to think that a transmission computer can do as good or better job of power management than they can. I don't expect to convince any of that ilk, but maybe the rest will relax and let the computers do the job they were designed for. 

 

[Larry N.]

I did not realise I had written lean and it should have been learn,, comes from typing at night.>>>Dan

That's what I suspected, Dan, but I couldn't pass it up...  ??? : ;D

And you could get the same anticipation by pushing the accelerator down at the bottom of the hill.

But with certain rigs you have to be careful. The manual on my 2007 Beaver with the CAT C-13 engine very specifically cautioned to not just automatically floor the pedal as it would waste fuel:

Ascending a Grade:
When approaching an uphill grade, assess the grade and length before beginning the climb. Prepare early for
long climbs. Unlike gasoline engines, diesels do not necessarily produce more power by pressing further on the
accelerator! A gasoline engine will happily work at full throttle (at least for a short period of time), but a diesel
usually just wastes fuel at full throttle. The power output from a diesel engine is dependent upon the following:

? Fuel/Air mixture - At a given RPM, the engine, even with the help of a turbo-charger, can only ?pump? a
given volume of air into the combustion chamber. This volume of air can efficiently combine with only so
much fuel; so it follows logically that feeding more fuel to the fire will simply waste fuel.

Determine ranges where the motorhome works best by driving long grades when temperatures remain stable
for the duration of the climb.

Another spot in the manual says:

? Avoid using full throttle when ascending a long hill. This wastes fuel and increases engine operating
temperature from incomplete combustion. Manually shift to a lower gear and use less throttle. Fuel will
burn more efficiently.

Another operating note for that specific engine is:

Keep the engine at a low to mid operating range of 1100 to 1500 RPM. This will use less fuel than
operating at higher RPM.

That's really too low for many engines, but that C-13 was redlined at 1800 RPM. Of course leaving it to the computers would keep things where they belonged, but there was a very noticeable speed loss starting up all but the mildest hills if I let them do it all, thus the shift to 5th when starting up a steeper hill, especially a long one. There was often a 5mph+ speed difference at the top of the hill  (same hill) between using the downshift or letting the computers handle it all.

The computers on other units may take care of that, but I've always kept that in mind, especially on that 2007 Beaver which did well to hit 6 mpg (usually lower- often under 5).

 

[Utclmjmpr]

The "experts" continue to preach electronics,, but many including mine do NOT have a computerized system,(altho the trans is,).. Back to experience and training..>>>Dan ( AND I enjoy an average 10 MPG as a result).>>>

In my past I've driven 18 wheelers at 75 thousand pounds using a 250 horse diesel engine and thirteen speed transmissions in the western US and me and the engine survived to do it again.  ( Prior to 18 speeds and automatic clutches ) This is NOT my first rodeo.>>>

 

[8Muddypaws]

I've found that with my puny little 300hp C7 that it's best if I anticipate the grade and manually shift.  Otherwise I'll loose inertia and have to work hard to get back up to speed.

I programmed computers for 45 years.  I'm smarter than they are. (Most of the time.  OK, maybe once in a while!)

 

[AStravelers]

True enough, but the reaction time is no more than a second or two. And you could get the same anticipation by pushing the accelerator down at the bottom of the hill.
It offends some people to think that a transmission computer can do as good or better job of power management than they can. I don't expect to convince any of that ilk, but maybe the rest will relax and let the computers do the job they were designed for. 

There is nothing magical about the transmission and engine computers.  They  have no knowledge or intelligence.  Computer ONLY do what a HUMAN programed them to do.  I have never found one that downshifts as soon as I want it to in mountain driving.

One of the things I love about the 6 speed Allison transmission, found in many diesel pushers, is it allows you to select the gear you want to be in and it will hold that gear and keep it from upshifting.  The computer will still downshift if you forget to downshift to keep your RPM's up. 

When approaching a long uphill or somewhat steep uphill is to cause the transmission to downshift.  Either by pressing a little harder on the go-peddle or manually downshifting.  Once I am in 5th gear I usually want to stay there.  I like to keep my RPM's in the 2000-2250 range.  I find there are times, if I let the transmission computer make the decisions, it will upshift a gear if I let up a little on and accelerator and then in a few minutes as the hill steepens it will down shift again. 

I much prefer to drive my rig like I would if I had a standard transmission and not depend on whatever parameters a human programmed the computer to do.

And yes I feel I can watch my engine temperature, RPM's, speed and how hard hard I want to push the engine/transmission than the human that programmed the computers.   

 

[ArdraF]

Elevation hasn't been mentioned and that can have a big impact on performance.  One time on I-70 westbound from Denver we were so busy chatting that no one thought to take it off cruise control.  About the time we got to Eisenhower Tunnel our big Cummins 500hp ISC darned near shut down.  The engine was trying very hard to keep up its speed and was burning fuel like crazy.  Lesson learned.  Take it off cruise and downshift at high altitudes where there's less efficient fuel usage.  When we approach the 17-mile Baker Grade on I-15 south of Las Vegas we move over to the truck lane and drop to 55 mph.  The engine and components don't work as hard and less fuel is wasted.  The rpms also stay in the "sweet spot" recommended by the manufacturer.

Heat is another factor.  If it's 110 degrees outside we definitely slow to 55 mph on a long upgrade.  I don't recall the engine temperature ever going over 212 and as soon as you start the downhill the engine cooling begins.

ArdraF

 

[Larry N.]

Take it off cruise and downshift at high altitudes where there's less efficient fuel usage.

Isn't that Cummins turbo-charged Ardra? Engines that are NOT turbo-charged certainly lose a lot of performance with altitude, but unless it's not running right a turbo-diesel should maintain full power, or nearly so, to well over 10K. The engine having trouble trying to keep it's speed up could be due to the grades getting steeper, or just simply because the grade is so very long that it finally gets down to very low speed on that steep grade, but unless I'm missing something (always possible) the elevation, as opposed to a steep grade, shouldn't reduce performance much, if any. That steep grade would also account for the extra fuel burn.

All that being said, manually downshifting (done at the proper time) does keep the speed up a little better on longer grades (at least on the Beaver and Ventana) because it doesn't lose quite as much speed/momentum/whatever at the beginning of the grade, and keeps a little more oomph available in the later stages, as well.

 

[Utclmjmpr]

Anytime when climbing ,particularly a long climb, speed is not a priority.  Getting to the top with everything still in good working order is that priority.
  And usually other traffic will determine some "adjustments" also..>>>Dan
  Larry, Got your PM,, No problem and I have thick skin,,, been around the patch a few times..>>>

 

[Gary RV_Wizard]

Computer ONLY do what a HUMAN programed them to do.

Right, but the computers in this case were programmed by professional power train engineers who presumably know a thing or two about power management and efficiency. And they programmed for that specific engine & transmission.

I have never found one that downshifts as soon as I want it to in mountain driving.

That doesn't make the computer decision wrong - it merely says it has different priorities than you do.

There is far too much focus on fuel efficiency and emissions these days to tolerate improperly programmed powertrain computers.