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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