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Old 01-27-2023, 10:27 AM   #57
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^^^ It would probably be nice if a new 45000 lb motorhome could keep up with a new 30,000 lb truck/trailer. 20 years ago that likely would have been no problem.
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Old 01-27-2023, 12:25 PM   #58
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Tag axle motorhomes will blow the doors off most other RV'ers.
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Old 01-27-2023, 12:55 PM   #59
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Tag axle motorhomes will blow the doors off most other RV'ers.
They also blow the doors off a lot of RVer's monetary capability.
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Old 01-27-2023, 01:52 PM   #60
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Tag axle motorhomes will blow the doors off most other RV'ers.

20 years ago that was true, but a new 600 hp 45,000 lb motorhome wont be keeping up to a new 500 HP, 30,000 lb truck/5ver combo going up a 7% grade. Unless you can still put an aftermarket power tune on the motorhome? . I think youd need close to 700 hp in the coach to keep up.
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Old 01-27-2023, 02:23 PM   #61
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Tag axle motorhomes will blow the doors off most other RV'ers.
What am I missing, a 40,000 lbs tag axle Dutchstar with 450 hp has 88 lbs/hp power to weight while a 20,000 lbs Baystar 350hp gas has 57 lbs/hp. The Baystar will be faster getting up to speed on a on ramp and climb grade at a higher speed.

Torque does not make something go up a grade or accelerate, power does, the the more power you have per weight the faster you can accelerate and the faster speed you can hold up a grade. This is assuming proper gearing to create the proper torque and rpm at the wheels for the given speed.

CAT published a great document on RV performance, all performance data are based on power not torque, here is a excerpt:

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Old 01-27-2023, 03:28 PM   #62
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What am I missing, a 40,000 lbs tag axle Dutchstar with 450 hp has 88 lbs/hp power to weight while a 20,000 lbs Baystar 350hp gas has 57 lbs/hp. The Baystar will be faster getting up to speed on a on ramp and climb grade at a higher speed.

Torque does not make something go up a grade or accelerate, power does, the the more power you have per weight the faster you can accelerate and the faster speed you can hold up a grade. This is assuming proper gearing to create the proper torque and rpm at the wheels for the given speed.

CAT published a great document on RV performance, all performance data are based on power not torque, here is a excerpt:

Attachment 385153

With a gas powered rig you should probably assume you don't have the proper gearing. Where as the high torque diesels are much more apt to make their rated HP over a large enough rpm range to be able to put close to peak HP to the rear wheels at all times.
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Old 01-27-2023, 05:29 PM   #63
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With a gas powered rig you should probably assume you don't have the proper gearing. Where as the high torque diesels are much more apt to make their rated HP over a large enough rpm range to be able to put close to peak HP to the rear wheels at all times.
Not sure I agree, large diesels have very narrow power bands, here is a Ford 7.3 gas vs a ISL 9 Cummins, they both have a distinct hp curve that climbs across the rpm range. The ISL only has about 1000 rpm range total from 250hp to 450hp. If you keep the 7.3 in a 1000 rpm range from 3000 to 4000 rpm you get a pretty flat curve from 250 to 350 hp which is where the RV engine is limited or say 4000 to 5000 you get 350-430hp which is the pickup limit. So the gas has a similar power band to diesel it's just at a higher rpm.

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My gas MH on an on ramp or steady climb would pretty just rev to peak HP rpm, if the grade where steep enough it would slow down leading to lower rpm lowering power causing its to slow further until it hit a shift point and revved back to peak HP again and held speed.

My diesel MH does the same thing, except it weighs 8000 lbs more and has 20 less hp even though it has double the torque so the speeds at which it does this is lower. However the diesel is much more pleasant because the rpm is 2500 rpm instead of 4500 and the engine is in the back instead under my feet, and the engine braking is much better going down.

I think both the gas and diesel could benefit from say a 10 speed transmission vs the 6 to allow keeping both engines in their peak hp band in more situations.

I have a hard time believing a 450hp Dutchstar is faster up a grade than a 350 hp Baystar weighing half as much, its doesn't match my experience or physics, I would still take a diesel after having one even if slower, its so much better in other ways, but I will give credit my gas MH was faster up a grade and accelerating in general even towing a jeep, it simply had more power to weight and had no trouble revving to 4200 rpm to make that power with the 6 speed.
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Old 01-27-2023, 06:49 PM   #64
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I have a 7.3 gasser with toad we are 25,000 # I kept up with almost all the semis going up the Grapevine heading south in CA. I could even accelerate around a few rigs doing 25mph more than once. Of course I got passed on the down slope by almost everyone.

Its the down hill that the DP win.
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Old 01-27-2023, 07:33 PM   #65
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Try comparing "Apples to Apples"
ie: take Your "500hp" [any brand, any Liter] gasser and a "500HP" ISL-8.9 [450hp stock,hopped up to 500 hp/1300 torque] or a pure stock ISM 500,[11 liter, 1550 torque] diesel.
Then Put BOTH under a 30,000lb gross load and go up a 6% grade..Cabbage [OR], Lewiston/White Bird [ID] or NW bound MountEagle [TN] comes to mind and then compare performances.

You will have NO Valid Data unless they are pulling the same weight, same distance same weather conditions and the same % grade.
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Old 01-27-2023, 08:35 PM   #66
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... However the diesel is much more pleasant because the rpm is 2500 rpm instead of 4500
Unless its 502 with headers and straight through mufflers. Then 4500 RPM sounds really sweet, and 5200 RPM sounds even better.
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Old 01-27-2023, 08:55 PM   #67
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I think the tag axle motorhomes handle and steer better. I had a dually truck towing at 72 to 74 mph and a tag axle motorhome must have been going 78 mph as it passed me. Others passed me when I was doing 70 mph and they were going 74 mph.

My observation - I passed all single axle motorhomes and SRW trucks towing. The only RV's that passed me were all tag axle motorhomes. Probably about a dozen times, all were tag axle motorhomes.

You guys have to notice the tag axle motorhomes too?
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Old 01-28-2023, 06:27 AM   #68
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Then Put BOTH under a 30,000lb gross load and go up a 6% grade..Cabbage [OR], Lewiston/White Bird [ID] or NW bound MountEagle [TN] comes to mind and then compare performances.
If the gas is actually putting out 500hp and so is the diesel, they will make the grade at the same speed, this is physics.

Are you trying to make some point about NA vs forced induction at altitude causing a the gas engine to put out less than 500 hp or the gearing or something? They make turbo charged gas engines as well. The hard part is finding one in a heavy vehicle to compare directly.

Torque is simply the rotational force, it does nothing without rpm, both together is what we call horsepower which actually moves the vehicle. You can increase torque through gearing, you cannot increase power after it has been made by the engine.

A 500 hp gas and diesel both creating their peak hp the gas at say 5000 rpm with 525 lbs/ft and the diesel at say 2000 with 1313 lbs/ft would both have the same 5,836 lbs/ft torque and 450 rpm at the wheels at 55 mph with say 40 inch tires, what will be different will be the gear ratios that multiply the torque from the engine the gas will be 10:1 while the diesel 4.3:1.

The gas engine being generally lighter and less robust and operating at much higher rpm to create the same power would not survive as long as the much heavier more robust and lower revving diesel engine, this is why heavy vehicles use diesel and not gas engines along with how unpleasant the high rpm is to the operator and the efficiency of the diesel engine requiring less fuel to create the same power. However in the end power is power and torque is one component of it that is easily modified by gears along with rpm.

If I can't convince you maybe Caterpillar can, this is their explanation about taking grades at speed and the power requirements and what power actually means:

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This document is a very good read:

Cat_RV_Performance.pdf
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Old 01-28-2023, 06:30 AM   #69
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Not sure I agree, large diesels have very narrow power bands, here is a Ford 7.3 gas vs a ISL 9 Cummins, they both have a distinct hp curve that climbs across the rpm range. The ISL only has about 1000 rpm range total from 250hp to 450hp. If you keep the 7.3 in a 1000 rpm range from 3000 to 4000 rpm you get a pretty flat curve from 250 to 350 hp which is where the RV engine is limited or say 4000 to 5000 you get 350-430hp which is the pickup limit. So the gas has a similar power band to diesel it's just at a higher rpm.

Attachment 385161

Attachment 385162

My gas MH on an on ramp or steady climb would pretty just rev to peak HP rpm, if the grade where steep enough it would slow down leading to lower rpm lowering power causing its to slow further until it hit a shift point and revved back to peak HP again and held speed.

My diesel MH does the same thing, except it weighs 8000 lbs more and has 20 less hp even though it has double the torque so the speeds at which it does this is lower. However the diesel is much more pleasant because the rpm is 2500 rpm instead of 4500 and the engine is in the back instead under my feet, and the engine braking is much better going down.

I think both the gas and diesel could benefit from say a 10 speed transmission vs the 6 to allow keeping both engines in their peak hp band in more situations.

I have a hard time believing a 450hp Dutchstar is faster up a grade than a 350 hp Baystar weighing half as much, its doesn't match my experience or physics, I would still take a diesel after having one even if slower, its so much better in other ways, but I will give credit my gas MH was faster up a grade and accelerating in general even towing a jeep, it simply had more power to weight and had no trouble revving to 4200 rpm to make that power with the 6 speed.

You’re right the 7.3 gasser isn’t going to be a whole lot slower on a hill than that 9 liter diesel. A 15 L Cummins has much higher torque. A 430 hp version of the 15 L is rated at 1650 lbft of torque.
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Old 01-28-2023, 06:46 AM   #70
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You’re right the 7.3 gasser isn’t going to be a whole lot slower on a hill than that 9 liter diesel. A 15 L Cummins has much higher torque. A 430 hp version of the 15 L is rated at 1650 lbft of torque.
No one puts a 430 hp ISX in a MH as far as I know, it would be pointless because you can get the same power out of the ISL. The 430 ISX would make sense in an application where you don't want the peak 605 horsepower but are running a really heavy load all the time requiring the 430hp and you want really long service life.

In diesel motorhomes at this point its ISB 300-360 hp, ISL 380-450hp, ISX 12 500 hp, ISX 15 605hp.

A 430hp ISX 15 is slower up a grade than a 450hp ISL and the same as a 430 hp gas 7.3 (wish they would allow 430hp in MH application) given everything else the same and assuming the engines are creating their peak hp. The ISX will last much longer than any in that duty cycle though, RV are not driven hard enough for that to matter which is why you don't see that engine configuration.
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