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BucketList · 02-07-2012, 05:54 PM

I am considering getting a 42' to 45' DP motorhome. Depending on the model line of the motorhome manufacture, they typically have either a ISL9450hp, ISM500hp or ISX600hp. Assuming that the gross weight of the coach is the same for the purposes of my question ...........Is there significant throttle response differences between these 3 engines? I do understand that the torque ratings are substantially different at 1250, 1550 and 1850ft lbs respectively. And so I would naturally assume that there would be a substantial difference in the feel of power between each engine setup but I am not sure if this really is the case since I have never driven a diesel bus or towed behind one. When is this power difference most noticed? Accelerating from a standstill up to freeway speed? In hills? Towing? All of the above? I am sure to those that are familiar with these engines in this application, my questions sound a bit amusing

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tylerdmoore · 02-07-2012, 06:18 PM

Hmm, well if we assume the same weight, frontal area etc. then you will see a huge difference between these engine choices.

ISL = 8.9 litres
ISM= 10.8 litres
ISX= 14.9 litres

The best analogy would be 4 cyl, 6 cyl and 8cyl in a gas application. You will notice a significant difference from one end to the other. The torque is what you need to move heavy weight, not horse power. The ISM and ISX are better suited to the size and weight of unit you're considering.

The ISX is actually a bit over-kill. This is the same motor we use in our HEAVY dump trucks pulling 105,500 pounds.

FlyingDiver · 02-07-2012, 07:22 PM

Quote:

Originally Posted by tylerdmoore

The ISX is actually a bit over-kill. This is the same motor we use in our HEAVY dump trucks pulling 105,500 pounds.

True, but I can pass truckers going up significant grades, if I want to. But they're all nuts, and they pass me on the downside, usually going 75-80 mph.

joe

Coached · 02-07-2012, 07:30 PM

I can't give you a literal comparison but I can give you a relative one. We traded a 2008 Tiffin 42' tag on our current 42' AC. They weigh pretty much the same and the "Liberty" chassis we currently have is a modified Spartan "Mountain Master" chassis, the same chassis we had on our Phaeton. However, our Tiffin had the Cummins ISC 360/1050 while this coach has the ISL 450/1250. Once underway I don't think there's much difference but getting underway happens quite a bit sooner. Uphills there is a very noticeable difference, towing or not. Interestingly enough I found the engine braking on the Tiffin to be much more noticeable and quicker acting than on my current ISL? AC assures me that our current set up is working within spec but I am going to pursue it with Cummins as I preferred the braking action on the Tiffin. Perhaps someone can chime in here with any experiences they have with the engine brake on various Motors? Based on this experience I would think you would see a noticeable difference if you stuffed an extra hundred or so horses into my motorhome; it already works pretty darn good. I've said for a long time now that my ideal coach would be a 40' tag with about 450 hp and a "reasonable" price tag.

Cat320 · 02-08-2012, 12:14 PM

The bottom line is get the coach with the floor plan you like. There are very few if any engine options on these rigs...not like a pickup where you can get a V6 or V8, it will come with one engine and no options for a larger or smaller engine. For the size MH you are considering, I think you'll find the engine installed will do the job.

lonestarace · 02-08-2012, 01:14 PM

"Back In The Day" there were a few engine options. I've seen Monaco Signatures that were built around 2000 that had either the ISM, N14 w/9sp trany, ISX, or DD60. But most had the ISM.

edgray · 02-09-2012, 01:31 PM

Quote:

Originally Posted by BucketList

I am considering getting a 42' to 45' DP motorhome. Depending on the model line of the motorhome manufacture, they typically have either a ISL9450hp, ISM500hp or ISX600hp.......

I think I read somewhere that the ISM did not meet 2010 EPA emmision requirements and was no longer available for on-highway applications.

Regardless, here is a paste of the info CURRENTLY available on the Cummins website for motorhome engines:

QUOTE
EPA 2010 Overview
Premium motorhome buyers don't settle for second best. That's why the number-one choice for diesel-powered motorhomes is Cummins. We offer a broad range of power for every size motorhome on the market. Plus, our 2010 engine and aftertreatment system reduces particulate matter by more than 90% without compromising performance, reliability or durability.

ISX15 The XPI fuel system delivers a precise quantity of fuel at ultra-high pressures. This, together with more robust electronic engine controls, enables multiple injection events per cycle. Flexibility in injection timing increases fuel economy and performance while decreasing exhaust emissions.

ISL9 The ideal engine for larger luxury motorhomes. With power up to 450 hp and 1250 lb-ft of torque, the ISL9 can accelerate at any speed and quickly climb the steepest grades even when towing. The ISL offers a very strong engine brake for extra braking capacity and added driver control. With 20,000 mile (32,000 km) oil change intervals, the ISL saves money and time when it comes to maintenance.

ISC8.3 Built on a reputation of delivering additional performance when a little extra is needed. With up to 380 horsepower and 1050 lb ft of torque, the ISC8.3 is designed to provide the performance to deal with heavier loads and steeper grades.

ISB6.7 The ISB6.7 features a High Pressure Common Rail (HPCR) fuel injection system that enables multiple injection events per cycle for cleaner, quieter and more complete combustion. The proprietary VGT™ Turbocharger has been improved as well for strong throttle response and dependability.
END QUOTE

BucketList · 02-19-2012, 07:14 PM

Thanks to all who have replied. Yesterday I drove my friends America 45' Eagle with a 500hp cummins. It was a 2011 with about 10,000 miles on it. This was the very first DP motorhomes I have driven, let alone a 45' unit. The last coach drove was a 35' fleetwood gaser years ago. So this was a DRAMATICALLY different experience. Not really knowing what to expect I would have guessed that there would have been more throttle response. So this was an eye opener for me and and made me realize that i am naive and have unrealistic expectations of thinking that a coach this heavy will have any type of throttle response. So after driving this I immediately said to myself a 500 hp is inadequate and a 600hp is a must. Then my friend basically told me that there would a small noticeable difference in power between the 500 and 600, but it would not be dramatic and that the upgrade costs about 50K.......ouch!

saydiver · 02-20-2012, 12:01 AM

Quote:

Originally Posted by BucketList

Thanks to all who have replied. Yesterday I drove my friends America 45' Eagle with a 500hp cummins. It was a 2011 with about 10,000 miles on it. This was the very first DP motorhomes I have driven, let alone a 45' unit. The last coach drove was a 35' fleetwood gaser years ago. So this was a DRAMATICALLY different experience. Not really knowing what to expect I would have guessed that there would have been more throttle response. So this was an eye opener for me and and made me realize that i am naive and have unrealistic expectations of thinking that a coach this heavy will have any type of throttle response. So after driving this I immediately said to myself a 500 hp is inadequate and a 600hp is a must. Then my friend basically told me that there would a small noticeable difference in power between the 500 and 600, but it would not be dramatic and that the upgrade costs about 50K.......ouch!

I think the upgrade cost is more like 100k at least it is between the Newmar Essex and King Aire. There are a few minor differences in the interior, but most of the difference in cost is the chassis and engine. The Essex MSRP with options is around 700k+ and the King Aire is around 800k+.

Sue

J Walker · 02-20-2012, 06:52 AM

On any diesel engine you get torque when in the power band of RPM and it falls off a lot when you are outside of it. So when accelerating from a stop you don't feel the power until getting into the power band. A more powerful engine won't change that much. The first thing to do is get used to the idiosyncracies of a diesel engine. I would say that you should judge an engine by its ability to cruise down the highway and pass other vehicles. Also the ability to climb grades will show if you have enough HP. If you can maintain 40MPH and and occasional 35MPH on a grade that is good. Some want to go faster on grades but I feel that is an overkill. When buying a MH the HP is usually matched to the vehicle weight. So a 500HP MH will usually weigh more than the 400HP MH and the difference in power will not be very noticeable.

Steve Ownby · 02-20-2012, 08:15 AM

Most of us develop our performance expectations driving cars and light trucks powered by gasoline engines. These vehicles typically have one horsepower per 10/20 pounds. Adequate power in a motor home is defined at one horsepower per 90/100 pounds.

Diesel engines are not designed with acceleration systems built in as gasoline engines are. I have heard it described at a diesel "gathering momentum" and the key to performance is to maintain momentum.

Shardan1 · 02-20-2012, 01:14 PM

Most over the road trucks don't have 450 hp engines and gross at 80,000 lbs. Except for a few Owner operators you'd be lucky to find any engines in the big trucking companies that have more than the above mentioned 450hp. And then most are limited/governed to 65mph. the only way they go faster is on a big downhill run.

Ann n Gene · 02-20-2012, 11:19 PM

I think the throttle response you are looking for is a product of a bygone era. With the new emissions regs and the increased mileage expectations, diesels are prevented from over fueling (black smoke and quick response) so you will notice a little lag, controlled by the computer to allow the engine to efficiently use the diesel it does get and to ramp up the turbo. It is frustrating to some of us old time truck owners but a fact of life with new diesels. You will get torque and top end performance but no quick acceleration. All just my experience and opinion.

lonestarace · 02-22-2012, 07:05 AM

Quote:

Originally Posted by Shardan1

Most over the road trucks don't have 450 hp engines and gross at 80,000 lbs. Except for a few Owner operators you'd be lucky to find any engines in the big trucking companies that have more than the above mentioned 450hp. And then most are limited/governed to 65mph. the only way they go faster is on a big downhill run.

I don't know what world you've been livin on?, but here in TX the speed limit is as high as 80mph. We just got back from Big Bend in W. TX and we had 100s of OTR trucks pass us doing 80mph.
We go 60mph