Diesel power

In general, is a 400hp Cummings diesel better or more effective than a 350hp Cummings diesel? These will be in a 40ft unit.

Jjemc said:

In general, is a 400hp Cummings diesel better or more effective than a 350hp Cummings diesel? These will be in a 40ft unit.

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I've got a Cat not a Cummins, but it's not really a Cummins question. When I was towing a Cherokee my 38 footer did fine with 350hp. Now, with a very heavy toad I'd say I'm a little under-powered. So yes, 50 more hp will be "more effective" at climbing hills but you'll burn more fuel - there's no free lunch. The extra HP won't really make any difference when you're at speed on flat terrain since you're not using it all anyway.

Your reply is about what I thought. I drove a 40ft unit with a 350hp and it felt like  it was a struggle to get up a slight incline. Mind is still in a fog.

That is what a 6 speed transmission is for.>>>Dan

Jjemc said:

I drove a 40ft unit with a 350hp and it felt like  it was a struggle to get up a slight incline.

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It's really about expectations. If you never want to be in the slow lane climbing a hill in a 40 footer (probably towing) you're going to need more like 450hp, maybe more.
Mine was fine until I decided to drag a 6500# truck, obviously that makes a difference (I need the truck). If all I did was hill climbing I guess I'd be more bothered, but the reality is that I might have a slow climb or two in a couple thousand mile trip. So I get to the top five minutes later, who cares? Back when I had an underpowered 460 Ford 40' gas rig, climbing long hills wide open meant worrying about overheating, red hot manifolds, etc. (plus listening to the noise). Cat & Cummins are perfectly happy wide open for an hour while you enjoy the scenery. And the noise is all "way back there..."

Is 400 hp more than 350?  Of course!  The Cummins ISL engine has more hp, more torque and more liters of displacement than the ISC 350, plus it produces the hp at a lower rpm.

But what you really want to compare is horsepower to weight ratio. For a given weight coach, more horsepower will always accelerate it more quickly.  In general, RV makers shoot for around 1 hp per 100 lbs of coach weight.  That gives acceptable if not blazing performance (your car probably has more than 1 hp per 20 lbs).

Jjemc said:

I drove a 40ft unit with a 350hp and it felt like  it was a struggle to get up a slight incline. Mind is still in a fog.

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I have a 40ft with a Cummins 350 and mine doesn't struggle to get up a slight incline at all even towing my trailer.
But on the highway I usually drive 55-58mph and going up big hills I might slow to around 50mph or so.

Sure the 400hp will carry more.  However like said no free lunch.
It will cost more and probably get a little lower mpg and use bigger oil and fuel filters etc.
But if you feel the need to drive at 70+ then it would be the better choice for you IMO.

You can't generalize by coach length - gotta talk weight.

8.3l ISC is no longer in production. Cummins has a 6.7l ISBX and a ISL 9l for most applications.
The 9l is a redesigned 8.9 that is delivered in different HP ranges. So a 350 HP engine is the same block as the 400 HP engine.

rls7201 is right about the ISC being discontinued, but in a used coach the odds are strong that a 350 hp diesel is an ISC Cummins.  If talking about a newer coach, the ISBX is usually employed if the hp need is under 360 and it comes in 300, 340 and 360 hp versions (in recent years).    The ISL is available (for motorhomes) in 330, 350, 380, 400 and 450 hp versions, but in practice only the 380, 400 and 450 hp versions are offered by the major RV chassis builders.

My Dynamax comes with the Cummins? 8.9L ISL Turbo Diesel Engine (350hp/1,000 lb.-ft. of Torque), with a Allison? 3200 TRV Six-Speed Automatic Transmission. I've not weighed the coach yet, the GVWR is 33,000#.

I cannot comment on its performance yet either, as our maiden voyage is coming up on the 17th, which includes towing my Jeep wrangler Unlimited and so it will be my first opportunity to actually experience what the coach will do. 

John,
Thanks for that info, John. I should have been more specific!  Since the big "super C" motorhomes are built on a medium or heavy truck chassis rather than a Class A motorhome type, they employ different spec engines and trannys.  The ISL9's used for the medium truck application are available in configurations as low as 260 hp and 720 lb-ft of torque.

As usual Gary you are spot on.

ISL9 for Medium-Duty Truck: Specifications Advertised Horsepower: 260-380 hp, Peak Torque: 720-1250 lb-ft, Governed Speed 2100-2200 rpm, Number of Cylinders 6.

Gary is easily the most perspicacious person on this forum, I can?t recall his ever being in error. He might well be the most well informed person on a certain subject of anyone I?ve encountered on internet forums.

Gary RV_Wizard said:

But what you really want to compare is horsepower to weight ratio. For a given weight coach, more horsepower will always accelerate it more quickly.  In general, RV makers shoot for around 1 hp per 100 lbs of coach weight.  That gives acceptable if not blazing performance (your car probably has more than 1 hp per 20 lbs).

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With all due respect, horsepower (which is a measure of energy per unit time) defines the speed at which you can climb any particular hill, but it is torque that determines your acceleration.  Torque is the rotational analog to force.  Most of us are aware that Force= mass x acceleration.  For rotational physics, torque governs the rate of change of angular velocity.  In simple terms, it's how quickly the spinning driveshaft will speed up (that is, how quickly the vehicle will accelerate.)  To provide a specific example, if you lose momentum on a long hill climb, your ability to recover any of that lost speed is a function of how much torque you have available.

MH manufacturers are reluctant to talk about torque because diesel engine torque is pretty much a direct function of engine displacement.  No amount of tuning can change that.  Many of today's "affordable" MH's are based on the Cummins ISB engine with its ~800 ft-lbs of torque.  It gets pretty pricey to jump to an ISL with ~1,200 ft-lbs or to one of the really big block ISX's with >1,600 ft lbs.  I love my CAT C-12 with its 1,550 ft-lbs of torque but there's no way I could afford a new MH with an engine of that size.

docj is correct, but I will add that simply quoting peak torque numbers doesn't help all that much. Torque is not the same over the RPM operating range, and peak torque is almost always at a different rpm than peak horsepower.  The transmission will shift to maintain peak horsepower under load, not peak torque, and for good reason.

One of the advantages of a diesel is that it delivers torque at much lower rpms than a gas engine, so you get more horsepower in the rpm range used for accelerating from a standing stop. Further, any big displacement engine delivers more torque than a smaller one, all across its rpm range, so it always has more hp available at rpms below the hp peak.

Gary RV_Wizard said:

One of the advantages of a diesel is that it delivers torque at much lower rpms than a gas engine, so you get more horsepower in the rpm range used for accelerating from a standing stop.

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For those who have never examined the torque vs rpm curves for a diesel, they are quite a bit different from what we are used to for gasoline engines.  A gas engine develops its max torque in some "power band" usually found at mid-RPM's.  The exact location and width of the torque band depends on the particular engine, but suffice it to say that peak torque will probably be in the 3,000-5,000 rpm range and it will fall off significantly below ~2,000 rpm.  At an engine idle speed of ~500-900 rpm it has rather low torque which is why it is so easy to stall a gas engine with a manual transmission if you're new at it!  ;D

In comparison, a big block diesel will have a fairly flat torque curve which may even increase with falling rpm until only slightly above the idle speed.  This gives diesels their reputation for being able to slog through most anything.  As long as the engine manages to turn it is developing close to its maximum torque.  For my CDL I learned on a straight truck powered by an International diesel with a 6-on the floor manual transmission.  Learning to handle that transmission seemed, at first, to be the most difficult part of the course.  But I can say that in several weeks of driving it, I only stalled it once.  It was nearly impossible to stall!  ;D

I've attached the torque and hp curves for my C-12 engine.  Notice how torque increases with decreasing engine speed. Coupled with a 34,000 pound MH this engine produces fantastic off-the-line acceleration.

That's a beast you've got there Joel. You're probably using a little C7 like mine as a starter motor  :-\

The Cummins ISL (8.9L) is nowhere near that flat, so that gigando C12 (and the Cummins ISX15) have an advantage there.  I've attached the 2017  Cummins ISL9 torque/hp graph for comparison.  This is the 450 hp version.  It's worthwhile noting that the very same engine tuned for  370 hp instead of 450 produces its max horsepower at 1650 rpms instead of 2100.  In modern electronic engines, the engineers can adjust performance fairly radically.

Sun2Retire said:

That's a beast you've got there Joel. You're probably using a little C7 like mine as a starter motor  :-\

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What I find awesome is watching it start in cold weather.  It doesn't have any glow-plugs nor does it need an engine heater unless temps go below ~15F.  Just crank and it fires up.  Rough for a moment then it settles in.  Quite a monster!