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Old 08-30-2014, 11:27 AM   #1
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RPM climbing hills

I have a 2006 Holiday Rambler with the 8.1L engine and Allison Trans. and I'm trying to find the RPM's for the Max horsepower. We are getting ready to head out west for an extended trip and I'm trying to figure the best way to climb steep grades. From what I've been reading, trying to keep the right rpm for the max horsepower is the way to go. I'm guessing just let the tranny do its thing or should I downshift into a lower gear. East coast grades aren't to steep and long to practice.
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Old 08-30-2014, 11:37 AM   #2
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First- it is not a race to go up a mountain. Stop the cruise, and let the tranny do its downshifting. If the rpms get low, manually down shift. Watch you oil and water temps. Higher rpms makes all the fluids stay cooler
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Old 08-30-2014, 01:12 PM   #3
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Your transmission and engine ECMs will figure it out for you - they have the numbers programmed in and now exactly when to shift to keep in the optimum power band. Just push down the pedal and go.

However, you may find the engine noise objectionable at the peak horsepower RPMs. The 8.1L produces its rated 340 hp at 4200 RPMs. Most people think it is "screaming" at that RPM, but it is actually working as designed. Peak torque is at 1600 but stays pretty much constant until around 3500.
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Old 08-30-2014, 01:12 PM   #4
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If we are talking about tackling a 6% grade, manually down shift to keep the RPM between 3,500-4,000. When the RPM drops to 3,000, downshift. If you let the trans. do the downshift you have lost valuable momentum. Watch you water temp if running at 4,000 RPM, following trucks. Normal grades can be taken by dropping out of Overdrive and watching your RPM. I would suggest purchasing the ScanGauge - Trip Computer + Digitial Gauges + ScanTools to accurately monitor engine performance.

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Old 08-30-2014, 01:24 PM   #5
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Quote:
Originally Posted by Fred and Bonnie View Post
If we are talking about tackling a 6% grade, manually down shift to keep the RPM between 3,500-4,000. When the RPM drops to 3,000, downshift. If you let the trans. do the downshift you have lost valuable momentum. Watch you water temp if running at 4,000 RPM, following trucks. Normal grades can be taken by dropping out of Overdrive and watching your RPM. I would suggest purchasing the ScanGauge - Trip Computer + Digitial Gauges + ScanTools to accurately monitor engine performance.

Fred
+1

This is what I found with my previous Ford gas MH.
It only had the 275hp, with a 4spd tranny, and on long 8% grades, if you waited for the auto to downshift, it was too late to get the motor back up near peak HP.
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Old 08-30-2014, 02:20 PM   #6
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It's is not a contest to see who can get over the top first. I never let the tranny determine which gear to be in on steep grades. By the time it has figured out that a downshift is necessay and it performs the function you could lose 10mph. Which you will not be able to recover. Keep your RPMs near peak hp and you will ease the strain on everything. Higher RMPs also mean that you will run cooler.

As a side note whatever gear you used to climb the grade is the same one you should use to descend the grade.


Best of luck and Happy Trails!!
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Old 08-30-2014, 05:38 PM   #7
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Chances are you will experiance your first transmission "Limp Mode" on your mountain trip. It can be quite unsettleing if you don't expect it. You will only have enough power to pull over to the side. When the Allison transmission decides it is safe to resume (can be 15 minutes or more) power will be restored. Once experianced your climbing will become less aggressive.
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Old 08-31-2014, 06:56 AM   #8
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So how do you determine what gear to downshift into, is it just by experience? I have a dump truck with a trailer with a manual tranny, I can tell when to shift down by the feel and sound, but it seems different with the allision auto tranny. I first experience with this setup, I was in cruise control and we started going up a fairly steep incline and the tranny downshifted, boy did it scream, engine and tranny. After that I shut off the cruise and drove myself, but never manually downshifted.
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Old 08-31-2014, 09:59 AM   #9
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Old 08-31-2014, 12:41 PM   #10
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Quote:
Originally Posted by Rocky Larson View Post
Chances are you will experiance your first transmission "Limp Mode" on your mountain trip.
This sure isn't normal, we have crossed across the Rocky Mtns. on I-70, and numerous western mountains, without experiencing "Limp Mode"

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Old 08-31-2014, 01:19 PM   #11
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If you experience "limp mode" when climbing grades, you have problems that need attention. Perhaps cooling, or perhaps ignition.

I don't see the point of manual downshifting with a modern automatic - I think it is mostly ego that makes people think they can manage RPMs better than the engine & tranny computers cooperating with each other. Push the go-peddle to the floor and the two will arrange thing to apply the best combination of fuel & air to the engine for max horsepower and the tranny will shift to keep the RPMs in that range while delivering power to the rear wheels. Computer controlled automatics aren't slow to up or down shift like in the old vacuum-controlled days - they immediately react to changes in demand.

I know I won't convince any gear heads of that, but it's my opinion, supported by powertrain engineers. If you want to shift gears, get a manual tranny.
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Old 08-31-2014, 01:32 PM   #12
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I think I will let the vehicle take care of itself before I start messing with changing gears. If my engine and tranny seem to be having problems I can always adjust things but it make sense to let my high tech power unit sort things out. 340 hp and weighing around 23,000 it should fly up the hills!! I'm no rush though.
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Old 08-31-2014, 03:21 PM   #13
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Very good question one that I also struggled with. I have crossed the rocks 4 times and always used the low rpm theory ( I am one of those who can NOT stand the screaming high RPMs). The last time I came across on Wolf Creak pass EAST BOUND I did not like the results. I stopped at the contently divide to rest the unit the temp gauge never went up but the water in the reservoir was bubbling. I think next time I will disconnect the toad.

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Old 08-31-2014, 04:54 PM   #14
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That RPM/torque chart Tom posted should answer every question about mountain grade driving. Keeping the accelerator "floor-boarded" is not the correct answer. Keeping a gas engine in the upper-to-middle of its torque curve is a better option regardless of road speed.
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