Is 360hp/800 torque enough for a 40' DP?

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Hilltopr76

Hilltopr76

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Joined
Aug 6, 2017
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15
Location
Tulsa, OK
I'm considering a Thor Aria 3901 that comes with the cummins ISB XT 6.7L. Is that enough power/torque for that size (about 35,000 GVW). Supposed to tow another 10,000 lbs.
 
You'll be in the slow lane with the trucks on the big hills, but you'll be OK.

I have the 8.9 liter, 400 HP ISL in our coach, which also weighs about 35,000 pounds. On the steepest interstate grades I can stay above 55 if I shift down to 5th and keep the RPM up on the horsepower peak. You'll be about 5 to 10 mph slower.
 
Nothing entry level about it. 360hp is a good amount, you won't be competing in the Indy 500 with it but it will get you up and down the mountains. Just drop a gear and enjoy the view! keep your RPM's in the peak torque range and you would do just fine.
 
captsteve said:
Nothing entry level about it. 360hp is a good amount, you won't be competing in the Indy 500 with it but it will get you up and down the mountains. Just drop a gear and enjoy the view! keep your RPM's in the peak torque range and you would do just fine.
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Thanks for that encouragement. I love the coach, just wanting some experienced motor coach drivers to tell me I won't regret the lack of power/torque.
 
My American Coach Revolution was 350hp (not a bargain basement coach) and I never had any issue. It would slow on steep grades but not overly so. So, like I said, sit back and enjoy the view. If you were in a hurry you would have bought airline tickets!
 
My year old Newmar Ventana is a 360 HP unit with a max gross wt. of 36,400 (right in the 100 lbs/HP area) and is a slightly better performer than the Beaver I used to have (525 HP, 50,000 max wt). Either unit will scoot right up shallow, short hills, but will lose speed as the hills get steeper and longer. That's going to be true of most any coach, gas or diesel, though some are a little better than others. Don't expect to find one that can go 65 mph up a 5%-6% grade, though -- 'taint gonna happen.
 
The motorhome power rule of thumb is 100 lbs per horsepower. That is what most RV manufacturers provide and it yields adequate though not car-like performance. Slow on grades and start-up from a dead stop, but cruises fine. By comparison, your car probably has less than 20 lbs per hp, so the coach will be a laggard in comparison.  Comparing at the other end of the scale, heavy semi's often run 40k-80k lbs with 400-500 hp engines.  It's not a race!
 
You can move 80k lbs with a one horsepower motor. You just need great gearing and some patience!  ;)

Big diesel train locomotives are typically around 4000 hp and they pull hundreds thousands of tons. That's not exactly apple-to-apples with coaches and trucks, but you get the idea.
 
Hilltopr76 said:
Thanks for that encouragement. I love the coach, just wanting some experienced motor coach drivers to tell me I won't regret the lack of power/torque.
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Most of the time your speed over the hill is going to be governed by traffic. I have been over all the big grades in the west and never had a problem with my 300hp 5.9. The standard rule of thumb is if you can't accelerate in a gear you need to manually downshift till you can. Increasing rpm will also keep you from overheating on hills. I will run 2000-2500 rpm in whatever gear I can accelerate in some times 5th sometimes 4th and on one stretch of 12 degree hill down to 3ed but that was more because the slow truck ahead of me.
I have added a EGT (exhaust temperature gage)  and boost gage to tell me exactly what is going on.
Any questions just ask.
Bill
 
captsteve said:
...keep your RPM's in the peak torque range and you would do just fine.
Click to expand...

This is a common misconception. It is not torque that gets you up a hill at speed. It is Horsepower.

The peak torque rpm in a typical diesel is going to be down in the 1,400 to 1,600 rpm range. Lug up a long hill at that rpm and you will go very slow and probably overheat the engine.

You must shift down to a lower gear and get the rpm up to the HORSEPOWER peak for the best performance on a grade. The horsepower peak rpm will be in the 2,000 to 2,300 rpm range. At that rpm the engine develops more power and the water pump runs faster, keeping the engine cooler.
 
I used to drive an expedited freight truck, 40 foot with a 96 inch sleeper, and a315 cummins, loads maxed out at 33000 lbs, went all over the west up and down steep grades. Always let the transmission pick it's own gear, never had any problem , with overheating. Never won any races either, but always got the load delivered on time, let the engine and transmission do their work and the motorcoach will be just fine. Take this advice and any others with a grain of salt.
 
Have 41ft. Fleetwood Diso 40X with 6.7, 350hp, 800tor, Cummins. We tow a '13 Jeep GC. Have no problems with mountain passes. Just have to learn when to down shift to keep the motor in it's sweet spot. Oh, you won't be the fastest thing out there either.
 
The Allison 3000 tranny will keep the engine running in its "sweet spot" without any manual downshifting. It is in constant communication with the engine to produce the power needed at the most effective engine rpms.
 
Gary RV_Wizard said:
The Allison 3000 tranny will keep the engine running in its "sweet spot" without any manual downshifting...
Click to expand...

Gary, I respectfully disagree. The fallacy in letting the transmission do all the shifting by itself is that it cannot anticipate anything. It can only react to something that has already happened.

When approaching a steep grade, I will shift down to 5th gear just before actually starting up the grade. I anticipate that I will need more power in a few moments and I set myself up for that. By doing that at 65 mph, I get the Cummins up to 2,000 rpm (its HORSEPOWER peak rpm) just before I start up the grade. Most of the time the rpm and speed will remain the same and I roll up the grade at 65, with all 400 horses on line and working for me.

If I just let the transmission shift itself, speed will drop about 5 mph before the shift takes place. Then, when the transmission does finally shift down to 5th, the rpm will be about 1,800, well below the horsepower peak. In other words, I'm already behind the power curve, figuratively and literally, by letting the Allison shift on its own.

I have always maintained that you cannot drive these big diesel motor homes like they are a car. You can't just put it in "D" and forget about it. Driving them properly requires some thought and input from the driver.
 
I agree with Gary.

On my unit I only sometimes will downshift and do it more going downhill than uphill.

Going up a hill it may (or may not) shift to 5th - I'll bet one big difference is I drive 55-58mph most all the time on highways.
So my coach doesn't lose more then 2-3mph most of the time before it either downshifts or just pulls more torque to maintain my cruise speed.

Speaking of torque I didn't see anyone mention that the 800 ft lbs of torque the OP stated is a little low, for the size of unit he's looking at.
I have 1050 ft lbs of torque and a computer program running to show me the torque all the time and I have seen my unit maxed out
at 1050 many times going up hills not mountains and not always pulling anything either.  Also my GVWR is 31K - so less that what the
OP was looking at

So that is why I think 800 is a little low especially
since the OP mentioned towing up to 10,000 lbs was listed on the vehicle he was looking at.

If the OP can drive 55 (or close to it) then he will probably be alright as many other said it's not a race.
IF he is the type that says the speed limit is 70 so I will drive that or more if I choose too - well then I think he will be disappointed, Just MO
 
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