It's a great engine to wow all the youngsters at the truck show. A true aural delight.
But to drive one for work was pretty scary. There were rules to driving an old DD...
DD Rule #1: Never let off of full throttle. EVER. Its a requirement that's eerily similar to riding an 80 or 125cc 2 stroke motocross bike, up a cliff.
Rule #2: Never go below 1600rpm if under any kind of load. The 6-71 5+2 '80 Brigadier I drove did not tolerate lugging AT ALL and would protest by overheating in seconds.
Seriously, in like only 5 seconds!
Rule #3: Plan uphills in advance. With zero torque rise to pull you out meant hitting the beginning of a hill at full tilt or else there was no chance. (due to what happens in #2)
Rule #4: Make sure the 4 ways were functioning. Top speed of that loaded rig (consisting of a full 7 yard dump body and a 580B Case behind it) was 48-52 mph flat out on the NJ Turnpike. Flashers on for the entire trip! Back then (early 90's) I think the 425 Cat was top dog, and as they blew by me I dreamed about doing 68
rule #5: Carry spare gallons
of oil. Not quarts, but gallons.
All that got tiring real quick.
The 300 Macks of the day were so much better. Still see many of them on the road today, which tells you something.
But the DD's are cool to show off now I guess. So unique, and full of character the trucks of today just dont have.
Well put! You said it. I think owning a piece of equipment that has one would be great fun from a hobbyist perspective today however -depending on the type of application- I would probably choose an alternative power plant to use in my profession on a daily basis for work.
You are 100% correct about their power bands and gave a good example. The comparison really is like a small 2-stroke dirt bike where there's no power down low, then all of a sudden you get a similar light switch-like effect and you get power but only for a short window of time up at the top. Because of the lack of low end torque, they were really not all that efficient to run in a manual trans config as you would have to slowly wait for the torque to build up again between each shift; it's part of the reason why you see old dump trucks and semi tractors that have these engines always going so slow (in particular, city driving situations) because they lose quite a bit steam and MPHs between gears- especially on hills.
Like you said, you really had to prepare and work around the power band because if you missed a shift or didn't climb into that power band soon enough before a hill, things were not going to be looking up for you. They're not at all like an inline-6 4 stroke such as a CAT or Cummins where they make massive amounts of torque as soon as you let out the clutch and practically drive themselves in comparison. It's part of the reason why a lot of cases drivers would get worn out from driving them because it was like a marathon operating them combined with the noise; you never really got a break. I think they work more efficiently in applications like tour buses (or big gen sets and boats for that matter) were they can run at sustained RPMs on the freeway near or in their power bands with an automatic transmission. In this type of scenario, their power band type works great and is always accessible but in a dump truck for example where a lot of the driving may be around town and you need low end torque.... I'll take an inline 4-stroke diesel variant, yes please.
The old adage for these was "slam the door on your finger"
to get you in the right mood so that you could then "drive it like you stole it."
You really felt like you were beating on them but that was the only way to drive/operate them.
They were also just as slow in heavy equipment too that had them. I ran a Clark Michigan loader with a 6-71 and it was always an effort just to get the machine rolling from a stop. You'd hit the pedal, RPMs would rise and nothing would happen, press the pedal a lot more this time and then finally you'd begin moving at a crawl and you'd think you're already half way through the RPM range by the noise output. When you'd come up to a pile to grab a load you were always standing on the pedal for all it was worth. If ground was loose enough, you might even break a tire loose going into the pile only it would spin verrrry verrrry slowly.
If you were driving it on the street and put it into the high gears it would actually get up to a pretty good clip but that lack of any low end torque for actual dirt moving work on a job site meant lots
of noise and very little speed, especially, again, to get moving or while going into a pile. I remember an operator of the same machine telling me about how one time somebody who could hear the machine from a distance complained and said "the workers are speeding around the job site!"
gauging by the noise output when it reality that was not the case at all.