Other specs include the 57- and 78-degree approach and departure angles, the former partially due to Soni's custom PCCC-type bumper (Pesky Commuter Car Compactor). Wheelbase is a comparatively short 211 inches, overall length is 315.3, width 98.75, and the height a towering 136 inches, including the added roof-mounted A/C unit. In other words, an M-920 isn't garage-able in the normal sense. Minimum turning diameter is a shopping-mall-unfriendly 81.6 feet. In case the 5mpg fuel consumption becomes overbearing, there are factory towbar mounts up front, and glad-hands for the air supply and brakes. Then again, whatever is capable of flat towing an M-920 would likely use at least as much fuel.
With a 99,730-pound towing capacity, does Soni's M-920 take the adequate-capacity tow rig concept too far? Of course not. It just sets a new standard. There's not even anything heavy, super, or extreme-duty here. This tow rig is simply an example of good, strong standard equipment, and with a few visual tweaks that make it infinitely better-looking than stock.
A 15,000-pound Warn winch right behind the 80-inch sleeper (actually a "communications box" off of an M-880, now equipped with a propane heater, toilet, stove, and bedding) serves as an aid when loading the trailer, or for self recovery. A fifth Optima battery is mounted near the winch as a booster of sorts. An Onan 4 kW generator provides 110 volts for hairdryers and other trail necessities. The Holland fifth wheel pivots in two planes-perfect for uneven terrain. While the towing capacity is high, hitch weight is limited to about 40,000 pounds. But the lift axle can be used to add to that, for a more respectable 60,000 pounds.
Common and reliable, the NTC-400 Cummins has a 5.5-inch bore and 6-inch stroke-that's 14 liters in modern-speak. This one had only 27,653 miles on the ticker when purchased by Soni, so there are probably at least 400,000 to go before an in-frame rebuild is needed. Ironically, many parts for these motors are even less expensive than for a small-block Chevy. | 
With stock Detroit No-Spin differentials (Detroit Lockers), 4.88:1 gears and 2 1/2-inch axleshafts, the tandem rear Rockwells are sufficiently strong for the low (by today's standards) torque output of 1,250 lb-ft, even after the 17.23:1 First gear reduction in the tranny. |
While not particularly heavy-duty, the 20,000-pound-rated front axle is certainly heavy. Just the knuckles are a pain to lift, even with a floor jack. Oddly enough, the tag specifies this as an axle from the Rockwell trailer axle division. | |
GENERAL
Owner: Soni Honegger, Chama, New Mexico
Vehicle/Model: '80 AM General M920 8x6
Estimated value: Built with tax dollars, but with 10 gallons of fresh oil in the crankcase and new paint, it must be worth a few bucks, still
ENGINE
Type: Cummins NTC-400 14.0L I-6 turbodiesel
Aspiration: Stock fuel injection, owner-installed fender-mounted air cleaner, single 6-inch exhaust
Output, hp and torque @ rpm: 400 @ 2,100; 1,250 @ 1,350
DRIVETRAIN
Transmission: Caterpillar 7155 air-shift 16-speed automatic with two Reverse gears, centrifugal clutch
Transfer case: Oshkosh single-speed, divorced with air-operated front drive actuator
SUSPENSION
Primary: Dual Bostrom air seats.
Front: Rockwell asymmetrical leaf springs
Rear: Hendrickson RT 450 walking beam with leaf springs
AXLES/DIFFERENTIALS
Steer: Rockwell FDS-1807 20,000-pound, open diff, wedge-type air brakes.
Center: Rockwell 20,000-pound air-suspended, trailing arm drop-axle; S-cam air brakes
Drive: Rockwell SUHD tandem 58,000-pound, Detroit Lockers; S-cam air brakes, air-actuated power divider
Ring and pinion: 4.88:1
WHEELS/TIRES
Wheels: Accuride 22.5x13 reversible steel
Tires: Michelin XZL all-position 445/65R22.5; load range L; 12,300 pounds @ 120 psi capacity