Re: [A/S] Fw: Airstream Pullers

[Jim Dunmyer <jdunmyer@xxxxxxxxxxx>]

Harvey,
 There's a difference between torque (twisting force) and HP (twisting
force, applied at a high speed). The main difference between gas and
Diesels in this respect is that the gas engine develops its torque at a
far lower RPM than where the max HP rating is. Your 350 Chevy is
developing max HP at 3500, but I'll bet the torque peak is closer to
2500 RPM. 

The Cummins develops max torque at under 2000 RPM, max HP is not a lot
higher, perhaps 2500 or so. 

What all of this nonsense means is that a Diesel will seem to "dig in"
and simply pull when the need arises. Because of the narrow RPM
operating range, it's necessary to do a bit more shifting than with a
gas engine, that's why the 5- and 6-speed manual transmissions are so
popular. As you probably know, over-the-road trucks have 10- or 13-speed
transmissions; their optimum operating range might be as narrow as 500
RPM.

My normal crusing speed when towing is just over 60 MPH, putting the
engine at about 1700 RPM. This is actually below optimum as far as power
is concerned, it does better at 2100-2200 RPM. Most minor grades on the
Interstates won't affect it, but a steeper hill will cause it to lose
speed and finally downshift if it's on Cruise Control. However, if I
force the shift to 3rd before the truck gets around to it, I can easily
maintain speed; if the truck shifts by itself, it'll pick the speed back
up to normal. However, if I were to run at 65+ MPH, most of those hills
won't require a downshift because the engine is closer to its optimum
operating speed; I just don't like to tow that fast.

I don't think it much hurts the engine to wind it up, but anything above
about 2400 RPM won't help the acceleration much. You don't really want
to short shift it at 1800 RPM. Just like with a gas engine, you'll find
where your engine is happy and learn to drive the truck to keep it that
way. 

As far as longevity goes, change the oil, keep the fuel clean, don't
shut it down immediately after a hard, hot pull, and pay attention, and
it should last a long, long time. Fuel economy is just like a gas
engine: put your foot in it and pay at the pump, it's that simple. Take
it easy when starting out, keep the speed reasonable, and you can go
lots longer on a tank.

I can offer this additional experience: Our first tow vehicle was a 1994
Dodge 1/2-ton truck with the 360 V8, auto trans, 3.54 axle. The manual
recommends towing with the OD locked out, so that's what I did. If I
forgot to push the button, I'd get a reminder at the first overpass when
it kicked down to Direct by itself. The engine turned about 2200 RPM at
60 MPH and it handled most hills "OK".

The Dodge w/Cummins mostly stays in OD and normal overpasses will not
cause it to downshift. Hills have been no problem at all so far, but
I'll admit that we've not been out West and into any real serious stuff.
Maybe this coming Winter.

                                          <<Jim>>