Hot-Rod Semis: The 20,000-LB Monsters Of The Dirt

• April 23, 2021
• Story By
  Mike McGlothlin

They’re the headlining act at the events they’re invited to. The heavyweights of the truck and tractor pulling world. They are the hot-rod semi’s—the chassis-tweaking, wheel-lifting torque monsters that never disappoint. Cummins, CAT and Mack power, single, compound and four-turbo configurations, and everything from late-model day-cabs to old-school cab-overs populate this elite class of sled pullers. As spring turns to summer, you’ll find these behemoths lighting up the night at select venues across the Midwest and East Coast—and we can’t wait to watch them thunder down the track. Below, we’ll examine the physical makeup of these earth-moving machines through the sphere of the Lucas Oil Pro Pulling League rule system. Enjoy!

Stock-Appearing

In addition to being able to weight up to as much as 20,000 pounds, all trucks in the hot-rod semi class must maintain a stock appearance for their given model year, specific truck model and make of truck. Tube chassis frames are permitted, but most retain the factory foundation, which with these trucks originally intended to gross 80,000 pounds is more than sufficient for handling the strain. Oberlin, Ohio’s Mitch Cloudt campaigns the Mack shown above, coined “Rubber Duck.”

Popular Power Plants

While the diesel engines lurking under their massive tilt hoods can be made bombproof, internally, any engine campaigned must have been available in road-use trucks. Due to its immense size, the KT Cummins—an 1,150 ci (19.0L) inline-six offered in over-the-road trucks back in the late 1970s and early 1980s—is a natural choice for the hot-rod semi class. One exception to the rule is the allowance of the Cummins QSK19, a modern descendant of the KT series. Caterpillar power, typically in the form of the 3406, is employed in the class, too, as are E9 Mack V-8’s. Regardless of engine choice, key rules dictate that a 3/8-inch cable must surround the head(s) and block, and that the maximum deck-plate height can measure no more than 1.25-inches.

Two-Stage Turbocharging Limit

Like Super Stock diesel truck and Super Stock diesel tractor categories, two-stage turbo arrangements are permitted in Hot-Rod Semi—and the setups run the gamut. Single turbochargers that rival the 5-to-6-inch inducer sizing found on Pro Stock tractors and larger-than-life compound arrangements are par for the course for these 20,000-pound monsters. And, there is even a quad-turbo Mack V-8 sporting a compound arrangement on each bank! See Dale Francis’s Macksimus Prime (I and II) trucks for that wild display of engine bay porn.

Fuel & Water

With diesel being the only source of fuel allowed in the hot-rod semi class, mechanical injection dominates the landscape. This old-school form of injection has yet to be matched in terms of fuel volume delivered by modern, electronic and higher-pressure systems. Water injection is permitted, and is arguably vital for keeping these engines from burning down, but the only additive allowed in these systems is a water-soluble oil.

Driveline

To both level the playing field and maintain adequate safety, a hot-rod semi’s transmission and rear end must be commercial truck factory components, but no cast clutch components are allowed. Steel flywheel and clutch components are mandatory and the bell housing has to be covered by an approved scatter shield. To help keep a consistent hitch height while hooked to the sled, the rear drive axle must be rigid to the frame in all directions.

Hitch Height

Per Lucas Oil Pro Pulling League rules, each semi’s maximum allowable hitch height for competition is 18 inches. The drawbar must be frame-mounted and can be adjustable between 16 and 20 inches. The latter rule provides for trucks that compete in different sanctioning bodies (which commonly call for a 16-inch hitch height) to get in on the action at PPL events. By comparison, most pickup trucks are allowed to run a 26-inch hitch height.

DOT Tires

The rules on tire usage are very clear: no cut tires and DOT-approved only. Further tire regulations dictate that their maximum size can be 11x24.5-inches or 10x22-inches. Maximum tire width is not to exceed 10-inches and all trucks must have dual tires and dual wheels on both rear axles. In addition to the body rules, these tire regulations help preserve the road-going look these monsters exhibit out on the track.