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1980 AM General M920 8x8 - A Desert Storm Veteran Turned RV

Posted in Project Vehicles on May 1, 2005 Comment (0)
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1980 AM General M920 8x8 - A Desert Storm Veteran Turned RV

When it comes to towing, there is a growing number of four wheelers who feel you shouldn't ask a pickup to do a truck's job, and with ever-heavier trailers and more trail toys to haul, it's probably a good idea to pull it all with something that actually has the strength, weight, and stopping power to do it safely. And, in this case, then some.

One believer in larger tow vehicles is Soni Honegger of Chama, New Mexico, who after breaking a few NV4500 trannies in his trusty Dodge Ram with its Banks-enhanced Cummins, decided to go with a stouter setup-strong enough to forever end any worries about breakage. Most would be content with a regular used and dirt-cheap class-8 Freightliner, but Soni had much bigger plans-big enough to get an Army surplus '80-vintage AM General 8x6, aka an M-920. Homely (to put it mildly) in stock form, these truck tractors were a Medium Equipment Transporter, an apparatus used to tow bulldozers on equally heavy trailers. While its GVWR is a relatively low 75,000 pounds, the 920's 99,730-pound towing capacity is certainly adequate for recreational use, and noticeably better than that of a so-called Super Duty. Maybe a standard- or medium-duty truck isn't all that bad after all?

With a driven Rockwell steer axle capable of supporting 20,000 of the M-920's 31,270-pound curb weight, just planting the air-suspended lift axle would support the whole thing on only four of the load-range-L Michelins, with over 8,800 pounds to spare. With the rear Rockwell tandem setup having a 58,000-pound capacity, Soni doesn't really need the lift axle and simply uses it as a convenient place to carry two spares, which can be lowered to the ground by flipping a dash-mounted switch. Considering that each 22.5x13 steel wheel and 46.3-inch-tall tire weighs a combined 420 pounds, having them there is probably a really good idea. Factory Detroit Lockers (technically, No-Spin differentials) turn 2 1/2-inch axleshafts in the rear axles, while the front axle is slightly weaker, and has an open diff.

129 0505 02 z+1980 am general m920+rear view

Getting this mass moving is the job of a Cummins Big Cam 1 with just 400 ponies, so it's really the torque (1,250 lb-ft at 1,350 rpm) of the straight-six diesel that does the deed. With a single-speed transfer case, the engine needs other gearing to succeed, and with a 16-speed tranny, it does. Original axle ratios were 6.17:1, but even with the taller tires, Soni could barely speed in school zones, so he swapped in three sets of 4.88s. Now, the lumbering giant is able to reach a blistering 82.3 mph while bouncing off the governor at 2,100 rpm, and that's with the top gear being a 0.83:1 overdrive. Even after the gear change, in First gear that max rpm nets only 3.2 mph, so a low-range is, well, not essential. That air-shifted 16-speed Caterpillar tranny also deserves mention-it's a very uncommon transmission, and one of the few automatics you can actually grind gears with. Also, it uses a fair amount of the contents of the seven air tanks on the M-920 to complete a single gear shift. Its centrifugal clutch is more ordinary, a setup also found in Citroen 2CVs, for example. (Which, by the way, is another vehicle that's quite capable in the dirt, but unlike an M-920, because of its very low weight and supple suspension.)

Another notable feature is that the semi-ancient Oshkosh transfer case is operated by a switch on the dash (which also activates the inter-axle power divider), so that not-so-great idea isn't all that new.

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.

129 0505 07 z+1980 am general m920+front view

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.

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

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