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enduroman · 10-20-2010, 09:23 AM

Another diesel vs gas question. Currently own a 2008 Fleetwood Southwind 37C which we use primarily traveling to motocross races in central and east central U.S., tow a 14ft enclosed trailer @ about 2500 lbs.
IMO the Ford F53 chassis is at about its limit carrying a 38 ft MH with 3 slides, hence we are looking at diesel.
One of the options we are looking at is a 2010 Discovery 40x with Cummins 350 HP isbxt. While a short test drive confirmed this MH pulls moderate hills far better than our current rig, having never owned a diesel MH I'm a little concerned the isbxt may be a little "short" for a 40footer.
My only questions is, to those who have lived with the Cummins isbxt [6.7 L] in 38'+ MH what do you think? I especially would like to know if you made the jump from the Ford v10 to the Isbxt.
Please don't let this devolve into a general "Gas vs Diesel" slamfest I've been in the automotive / light truck repair business for over 30 years and am aware the general advantages/disadvantages of both.
Thanks for your input

historyljc · 10-20-2010, 11:43 AM

enduroman,
I can't address the Ford vs Cummins ISB 350 directly. I have a 40 footer with a CAT 350 that I've pulled 5,000 pounds or more behind coast to coast over the last 4 1/2 years. I've had no problems. I believe the new Cummins 350 has more torque than my CAT so I wouldn't view it with any trepidation.

Spikester · 10-20-2010, 01:14 PM

I went from an 06 HR Admiral (30' floorplan) with a Ford V10 that had a Banks Package and a Transminder module, to a 08 Newmar Ventana (33' floorplan) powered by 350HP version of the ISBXT. What the Diesel lacks in raw acceleration, it makes up for it in torque for hill climbing, noise reduction and economy. I now climb hills in cruise control (in overdrive gears) that my V-10 would not (it would drop out of cruise then downshift out of OD gears). Also experienced a considerable increase in economy. But the really big difference IMO is in the ride and handling you get with an air suspension chassis. With my gas chassis 7 hours of driving would wear me out, now its almost like 7 hours in a BMW, but I must clarify this by adding that my MH is equipt with "Comfort Drive".

Spike

94-Newmar · 10-20-2010, 02:46 PM

Why they continue to advertise diesel engines in horsepower instead of torque is beyond me, torque is what makes things move.. lol. Having said that there is no comparison in the torque numbers of the two powerplants.

Triton V-10 (6.8L)
Max HP - 362 @ 4750 rpm
Max Torque - 457 lb ft @ 3250 rpm

6.7 ISB
Max HP 350 @ 3100 rpm
Max Torque 650 lb ft. @ 1500rpm

Truckers use a loosely followed rule of thumb for torque to weight ratio to determine decent acceleration up hills, power to get up to highway speed from a dead stop etc etc which is roughly 50 lbs of weight per lb-ft of torque. So in otherwords, at 33,000 lbs you'd want a powerplant making about 650 lb-ft of torque. That doesnt mean than something slightly underpowered won't do want you want it to do, its just that it wont do it quite as fast as you might like. The ratio above is a "preferred" range to stay within.

If the coach you're considering is going to be under 33,000 GCWR then you'll be fine with the Cummins powerplant.

Spikester · 10-20-2010, 05:43 PM

The Eng Specs for the 2010 40X Discovery show that it has the ISBXT that develops 750 FtLbs of torque. The GVW of the coach is 32,000#, so that works out to be about 43 FtLbs of Torque for every 50# of Coach. The GCVW is 42,000# so if you ever had the coach full loaded towing 10,000# you would be above the "loose rule of thumb" mentioned above, but would/could you ever get yourself in that situation? So IMO, and Fleetwoods, the ISBXT configuration in the 40X configuration has enough power to operate the coach effectively. Be aware when shopping around there are various configurations of the ISBXT, not all are the "top of the line" 750 FtLb configurations.

To me the question might arise, is there enough cooling to keep the Engine running at its optimum temperature, which opens up the ole "rear radiator vs side radiator" debate.

Spike

enduroman · 10-21-2010, 04:45 PM

Quote:

Originally Posted by Spikester

The Eng Specs for the 2010 40X Discovery show that it has the ISBXT that develops 750 FtLbs of torque. The GVW of the coach is 32,000#, so that works out to be about 43 FtLbs of Torque for every 50# of Coach. The GCVW is 42,000# so if you ever had the coach full loaded towing 10,000# you would be above the "loose rule of thumb" mentioned above, but would/could you ever get yourself in that situation? So IMO, and Fleetwoods, the ISBXT configuration in the 40X configuration has enough power to operate the coach effectively. Be aware when shopping around there are various configurations of the ISBXT, not all are the "top of the line" 750 FtLb configurations.

To me the question might arise, is there enough cooling to keep the Engine running at its optimum temperature, which opens up the ole "rear radiator vs side radiator" debate.

Spike

The Discovery is listed at 26,500# empty so based on how we're usually loaded I think we'll come in at around 31,600#GCVW. Based on the formula we should be ok.
I wish the 2010 Discovery had the ISC as the 2011's are equipped with. Checked on ordering a 2011 Discovery but they dont want to sell nearly as bad as the price came in about 34k higher and I haven't convinced myself that 1000cc is worth that much.
Thanks for the info.

Eagle123 · 10-21-2010, 06:00 PM

I have 350hp ISC 34k 2000 american eagle. Anyone know how much torque it has? I miss my old 1999 V10. I feel now I have to bring a tractor trailer with me when I go camping. It's nice when I get on the interstate, but all other roads seem to small.

Big rigs towed a 6000 lb horse trailer with no problem. My wife would pass big trucks going up hill until I yelled at her to quit wasting fuel. She would go eighty so, now I don't let her drive very ofter. For some reason she won't listen me..lol

Eagle123 · 10-21-2010, 06:01 PM

I meant both rigs towed with no problem

enduroman · 10-22-2010, 08:25 AM

Quote:

Originally Posted by enduroman

The Discovery is listed at 26,500# empty so based on how we're usually loaded I think we'll come in at around 31,600#GCVW. Based on the formula we should be ok.
I wish the 2010 Discovery had the ISC as the 2011's are equipped with. Checked on ordering a 2011 Discovery but they dont want to sell nearly as bad as the price came in about 34k higher and I haven't convinced myself that 1000cc is worth that much.
Thanks for the info.

Just a revision so as not to throw anyone, my math was a little fuzzy last night. The Cummins ISC actually gains 1600cc over the 6.7L ISBXT. Sorry for the mis-information.
Closing on the sale of our convenience store today at noon, so the Motorhome shopping starts in earnest now.

Spikester · 10-22-2010, 03:23 PM

Be careful when you are looking at the 2011 models, you maybe looking at an ISC that is 2010 emission compliant, vs a 2010 ISB that is 2007 emission compliant. New emission engines (Uric Acid Additive) come at a cost, plus loss of performance, when comparing engine to engine models. Some 2011 models are/were still being built with chassis that had 2007 emission compliant engines, IMO these are the best, and cheapiest.

Good luck in your quest for a motorhome, unless you already have, give Newmar a look, lot a bang for the $$, and they were/are building some 2011 models with 2007 emission compliant diesels. I know that when I bought our Ventana in 2007 the better deal was on a new coach ordered directly from the factory, rather than on an existing coach at the dealership. And I mean a much better deal!

Spike