The trick to clutchless shifting is getting familiar with how fast your engine loses RPM when you let off the throttle. I learned clutchless shifting on a 64 Freightliner with a Detroit 8V-71 and a Roadranger 10-speed. That one wound up and slowed down almost instantly, so shifting was simply a matter of "floating" the gear, neither accelerating or coasting, and slipping it out of gear into neutral. Then, let off the throttle, and ease the shifter into the next gear position. As the gear speeds match, it will fall into the next higher gear without clashing. It takes practice, but the average driver can learn pretty well in a few days, max. Downshifting is similar, float the gear to disengage, rev up in neutral until the gear speeds match, and poke it into the lower gear. Shifting a 2-transmission rig is a little different, but once you get the hang of one, two isn't all that much harder. One transmission or the other MUST stay in gear, to allow the engine RPM to be manipulated to speed-match the gears. The absolute hardest setup for me, personally is the 4X4, since the ratios are so close together that if I ever lose count of which gear I'm in, I have to stop, and begin all over. With a Detroit and a 4X4, I can keep the engine speed in the top 200 RPM all the time, where a "Screaming Jimmy" develops practically all its pulling power. Get one lugged down below 1950 RPM or so, and the game's over! From 1950 to 2150, they haul butt!
Cummins engines have more low-end torque, and they rev up and slow down more slowly. It takes a different shifting technique on the 4-stroke engines like Cummins or Cat.
Jerry
