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Old 07-07-2015, 08:35 PM   #1
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Why am I climbing a 6% grade at 35 mph?

We bought a diesel pusher for the power to climb over passes throughout the country. We got a 360 HP Cummins in our Tuscany 34st with 10K towing capacity so we could tow our 2007 Cherokee. We were frugal in our packing and I don't believe we are overloaded (and planning to get tire-by-tire weighing done asap).

So why are we climbing these 6% grades on Hwy 101 on the Pacific Coast (2700 feet is the tallest point) at 35 mph? I try to let the transmission do its thing but it seems to wait until I'm crawling at 15 RPM before downshifting. I'm in performance mode, not eco.

Is this normal? Is there a secret climbing button I don't know about?
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Old 07-07-2015, 08:40 PM   #2
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Because you don't have 650 hp and 1950 lbs ft torque?

You need to be proactive and downshift BEFORE the engine looses rpm. You need to have the rpms up close to the HP peak rather than the torque peak. That will help but even the mighty DP's have a limit

What engine do you have? An ISB, ISC, CAT?
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Old 07-07-2015, 08:53 PM   #3
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Looks about right to me. You weigh more than mine and you only have a little more horses then a 8.1 L Vortex. Climbed that hill a few times over the past 20 years.
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Old 07-07-2015, 09:01 PM   #4
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Quote:
Originally Posted by Jodee Gravel View Post
We bought a diesel pusher for the power to climb over passes throughout the country. We got a 360 HP Cummins in our Tuscany 34st with 10K towing capacity so we could tow our 2007 Cherokee. We were frugal in our packing and I don't believe we are overloaded (and planning to get tire-by-tire weighing done asap).

So why are we climbing these 6% grades on Hwy 101 on the Pacific Coast (2700 feet is the tallest point) at 35 mph? I try to let the transmission do its thing but it seems to wait until I'm crawling at 15 RPM before downshifting. I'm in performance mode, not eco.

Is this normal? Is there a secret climbing button I don't know about?

Mr_D is exactly correct in the post above. The transmission doesn't know about the long grade you are starting up and likely if you let it do the shifting, you will get behind the horsepower curve and never recover.

Your best climbing performance will require you to keep your RPMs up over 2400 while climbing. If you have the Cummins ISB, your governed RPM is 2600, so when you see your RPMs dropping below 2200 you should be shifting. You can shift earlier if you want. The shift won't occur until RPMs drop to a safe level.


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Old 07-08-2015, 07:33 AM   #5
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Originally Posted by RandyKathyM View Post
Looks about right to me. You weigh more than mine and you only have a little more horses then a 8.1 L Vortex. Climbed that hill a few times over the past 20 years.
When towing, torque is king. You really shouldn't compare the HP of a diesel to the HP of a gas motor, apples and oranges.

2700 ft. is nothing. When I tow through the Rockies, we hit about 9200 ft. above sea level, and I'm towing a Cherokee or a Jeep Wrangler, and I'm towing on a 2,000 lb. car hauler, not flat towing.
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Old 07-08-2015, 07:42 AM   #6
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Old 07-08-2015, 07:45 AM   #7
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Quote:
Is this normal? Is there a secret climbing button I don't know about?
Secret is to manual downshift to keep RPM above 21-2200.
Your transmission can't see the hill coming ahead but you can
Use your secret climbing button on the shift pad.
It should look like
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Old 07-08-2015, 07:53 AM   #8
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Also, I know this sounds silly, but you do have the gas pedal floored, right? Not "nearly floored" or anything, but truly at 100%? In my coach, that definitely makes a difference -- at 100% the Allison's logic is that if the engine is at the torque limit (which should be the case as you are climbing) and the rpm slows to where it can shift down to the next lower gear without over-rev, it does so.

But I have noticed that I had to coach my wife to keep it floored, because it you are even just at 90%, it will definitely bog down a lot more.

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Old 07-08-2015, 09:07 AM   #9
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Also, I know this sounds silly, but you do have the gas pedal floored, right? Not "nearly floored" or anything, but truly at 100%? In my coach, that definitely makes a difference -- at 100% the Allison's logic is that if the engine is at the torque limit (which should be the case as you are climbing) and the rpm slows to where it can shift down to the next lower gear without over-rev, it does so.

But I have noticed that I had to coach my wife to keep it floored, because it you are even just at 90%, it will definitely bog down a lot more.

Steve

If your intention is to climb the grade at max mph and keep engine heat down, then you will need to keep your RPMs up close to HP peak. Torque peak is down 600 to 1000 RPMs below that. It's true that the greatest fuel economy will be achieved at a couple of hundred RPMs above torque peak but you will not be able to control heat rise and you will be creeping up the grade in the far right lane with your flashers on. I prefer to bite the bullet on MPG and get up at max MPH.

If with your best efforts, the heat continues to rise above 220, then the only answer is to down shift as far as it takes to keep your RPMs up close to max at about 1/2 throttle. When that turbo is at max boost and glowing cherry red, it's a very effective heat engine. If the cooling system can't cope then you just have to back out of the throttle but keep RPMs as high as you can with the transmission.


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Old 07-08-2015, 09:07 AM   #10
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Don't know squat about a Cummins.... Can read a HP / Torque graph. Torque and HP are inversely related by RPM. Higher rpm = less torque more HP. Lower RPM = more torque less HP.

Your engine has a narrow efficient operating range. ~ 1100 - 2200 RPM. The torque curve is not flat... It's \. Not ----. Your acceleration & pulling power comes from torque. Your best pulling power is at the low RPM range. The transmission is doing its job by keeping the RPM in the low RPM high torque regime for the speed & load on the drive train. The result is a bleeding off of road speed.

You have computer controlled variable vane turbocharger. You let that bad boy load up and pump in the air by letting the trans stay in the highest gear possible for as long as possible.

I'm also guessing that the engine is not a "turbo normalized" design...meaning that it will put out sea level power and performance at high altitude. Which may explain its lowered performance as you gain altitude. This may be the root cause of your performance issue.

That is an 8L inline engine. They are not noted for turning high RPM and their power falls off great above 2200... Hence the rev limiter. Think a Packard straight 8! Very conventional. Very unlike the high rpm 3L Mercedes diesels.

I expect your expectations are exceeding the RVs abilities on hill climbing. Big pushers always seem to slow down on hills... Even small ones.. IMHO. Interesting, if not amusing, to have a monster DP with a TOAD smugly rocket past my Sprinter based Class C View on the flats and disappear into the distance. Come the hills... I slowly reel them in and eventually easily pass them on a steep grade as they struggle to maintain speed and are boiling black soot like a WW2 destroyer laying smoke screen.
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Old 07-08-2015, 09:17 AM   #11
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Hmmm...
have had gassers and deezles for a long time...
I can tell you my large deezle trucks and current dp will recover faster than my little gas jeep that jeep will downshift at an ant hill ! but still great lil vehicle - just have to get used to a small naturally aspirated gas engine that likes to go from 5th to 3rd and SCREAMS rpms...


But, last week was my first trip into 'hills'... small, but the little 'hills' (2000 ft - 5% grades) here in Texas towing our 4200 lb wrangler certainly did beat the economy mode

tried regular mode and it still would slow down more than I wanted (two lane roads and cars would pile up) so, it was either wait for pull off lane every 5 miles or so or still set my cruise and just anticipate (and USE) the hills...

what goes up must come down... so left the cruise where it was and I would use the last bit of the downhill to pick up momentum based on how long a grade coming up...

5 or 10 mph for a short bit and liberal supply of throttle fixed it most of the time !!!
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Old 07-08-2015, 09:25 AM   #12
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Don't know squat about a Cummins.... Can read a HP / Torque graph. Torque and HP are inversely related by RPM. Higher rpm = less torque more HP. Lower RPM = more torque less HP.

Your engine has a narrow efficient operating range. ~ 1100 - 2200 RPM. The torque curve is not flat... It's \. Not ----. Your acceleration & pulling power comes from torque. Your best pulling power is at the low RPM range. The transmission is doing its job by keeping the RPM in the low RPM high torque regime for the speed & load on the drive train. The result is a bleeding off of road speed.
WRONG! Power is power, and torque is torque. Power is a product of torque x rpm. The fastest way up a hill is at peak power rpm, not the peak torque rpm.

The engine runs more efficently at lower rpms, so that's why the transmission wants to stay in a higher gear.
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Old 07-08-2015, 09:27 AM   #13
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I have an 360 hp ISB in my Allegro RED. It weights 28,500 and pulling my Honda Element has no problem maintaining 50~55 up a 6% grade. If I get balked by a slower vehicle though, I don't try to make the speed back up. Just ride it out then.
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Old 07-08-2015, 09:43 AM   #14
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If your intention is to climb the grade at max mph and keep engine heat down, then you will need to keep your RPMs up close to HP peak. Torque peak is down 600 to 1000 RPMs below that.
Yes, but my point is, at least with my Allison 3000, if you are at 100% throttle, the end result is that it will be in the lowest gear possible for a given speed. The fact that declining rpm and full throttle results in high/max torque is an interesting fact (that I like as a gearhead)

Obviously, my ISC spins a lot slower than the ISB's -- my max rpm is 2200 and I can hit max torque at 1500 or so, and the gearing of my setup is such that when the engine gets down to that 1500 or so, it can downshift to the next lowest gear and pull right around 2100rpm.

So the only time I have to back out of the power and gear down is for heat, and that only happens when I have a cooling issue, fortunately. (or unfortunately, as my bank account would say after replacing the hydraulic pump that drives the radiator fan)

In normal mode, on a 100 degree day, I can climb the Grapevine starting at 177 degrees and be only at 195 at the top of the mountain, which about as tough as climbs get for generating heat.

Steve
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