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Old 10-25-2010, 02:06 PM   #57
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The key number is lbs of Torque vs lbs of Mo Ho

All these stories about gas vs diesel boil down to lbs of torque from the motor pulling how many pounds of bus and two. I have owned both gas and diesel motor homes and both type of motors get the job done. The main issue is that if you need more pulling capacity than a big gasser will give you the next step up is a diesel.

I had a GMC motor home that was gas and very fast up hills, but the towing capacity from the factory was 1,500 lbs.

I now have a very heavy motor home with a C12. The gas motor was fine for my 12,000 lb coach, but completely unsuitable for something that weighs 40,000.

IMHO a diesel give you much less drama when you have a steep hill in the mountains. About 8 years ago I was going up Rt 12 in Utah and the grade went to 15% for a while. We were in a 32' class a with a Ford V10. Even in first gear I thought it was not going to make it. When I got to my camp spot that night a guy pulled next to me in a large DP and said he went over with "no problems". I was in a 31' class C going up 395 north from Mamouth about seven years ago and that was also a Ford V10. The motor struggled up the grade and then blew a fan belt off about 7 miles up the grade. My diesel has far less problems with hills. That said this summer I had to pull over for maybe three minutes in 100F heat on an extremely long uphill going to Lassen this summer. Basically I turned off the dash air and went up the hill.

Bottom line. Gas if fine if you don't have a huge heavy rig. If you want a big rig and tow a big car you need diesel.

B Bob
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Old 10-27-2010, 05:46 AM   #58
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Did the rule use to be not to go over 34/35 foot with a gas class a??? Has that changed now due to better/more powerful power plants??? You seem to see a lot of 37 to 40 foot gas coachs. I understand the other rule is not to purchase a 34 foot pusher or less, due to a poor ride??? I really want the diesel power, air brakes and air ride, but would have to purchase at least 5 years olds. Nice new gas class a's, but wife only seems to like 38-39 feet...Just not sure if that gaser will make it over mountains to FLA or westbound from Michigan...

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Old 10-27-2010, 06:39 AM   #59
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What it all boils down to is the difference in torque curves between gas and diesel engines. The diesel builds its torque at a lower RPM which means that, if 2 otherwise identical rigs, one gas and one diesel, hit the bottom of a 6% grade running 60 MPH, both of them in top gear @ 2000 RPM, the available HP to climb the grade is given by the equation:

HP = Torque (ft-lb) x RPM / 5252

Although it's not a MH, let me give a real world example with 2 trucks we owned in the past to pull 5th wheels. One was a 8L V-10 gas engine rated at 300 BHP @ 4000 RPM and 450 ft-lb torque @ 2800 RPM; the other was a 5.9L diesel originally rated at 245 BHP @ 2700 RPM and 505 ft-lb torque from 1600-2300 RPM (torque limited by the computer to protect the drivetrain).

At 2000 RPM, the V-10 was only making around 400 ft-lb of torque while the diesel was making 505 ft-lb torque. Further, the V-10 was on the front side of its torque curve which means that, as the engine RPM drops on the grade, torque drops as well, so HP (see equation above) is falling like a rock. The diesel, on the other hand, has 25% more torque at 2000 RPM (thus, 25% more HP AT THAT RPM), and torque doesn't drop as RPM drops - in fact, it's typical for a diesel that isn't computer controlled to see torque increase as RPM drops (aka torque rise) since peak torque may be as low as 1600 RPM or so. The actual numbers shake out like this:

V-10 HP = 400 x 2000 / 5252 = 152.32 HP

diesel HP = 505 x 2000 / 5252 = 192.31 HP

The real world result of this was that, towing the same 5th wheel on the same 6% grade, the V-10 was running 3500 RPM in 2nd gear (4 speed automatic) to hold 55 MPH while the diesel would pull the grade at 60 MPH at 2000 RPM in 6th gear (6 speed manual).

So, yes, torque is important, but the key diesel advantage is more torque at a lower (more usable) RPM, and usually bonus torque (torque rise) in reserve as RPMs drop. That's why the gasser has to go to a lower gear and rev on the steeper grades when the diesel doesn't.

As an aside, the diesel in the example above was subsequently modified with a computer "box", bigger injectors, an upgraded intake and exhaust system, upgraded clutch, etc. to generate 347 BHP @ 2700 RPM and 762 ft-lb torque at 2000 RPM at the rear wheels as measured on a Dynojet chassis dyno (roughly 400 BHP & 900 ft-lb torque at the flywheel), so if you run the math, you'll see that about 342 flywheel BHP would have been available in the example above. With these mods, a downshift to 5th gear was almost never required, even after we moved to our current 5th wheel which is 2500 lbs heavier than the one we pulled with both of the trucks in the example.

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Old 10-28-2010, 08:52 PM   #60
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Originally Posted by FLYING BUTCH View Post
I have had both. I had heard that diesel would pull better and get better gas mileage. After our first diesel (just last year) no compairson-the diesel will way out pull the V-10 and do it with MUCH better gas mileage just my two cents
I agree. I have owned both a 275 HP V10 in a 32' Coachman (18,000 lb chassis) and a 300 HP Cummins ISB in an Itasca 32' DP (28,500 # chassis). The Cummins outperforms the V10 in every way I can think of. And it gets double the fuel mileage. You always remembered the last place I fueled the V10 because it was still visible in the mirror. I have actually had times when I couldn't remember the last time I fueled the Cummins.

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