With all the performance mods available for diesels, producing big power can be a fairly straightforward job. What's not so obvious, though, is how all that extra torque affects the drivetrain components. The factory automatic transmissions are up to handling stock power levels, but when you start adding engine upgrades that double its torque you're asking for trouble from your transmission.
North American Diesel Power (NADP), a company known for both diesel engine and transmission upgrades, has built a reputation for making big power with diesel engines and building transmissions that will stand up to the colossal torque. As a drag-racing enthusiast, NADP's Barry Voltner knows firsthand what a bad-boy diesel can do to a weak-sister transmission and, thus, developed the Heavy Hauler Series. He feels that, in most cases, the stock unit is incapable of handling extreme power loads and demands of drivers putting high horsepower to the wheels.
The Heavy Hauler Series of transmissions are not merely for racing trucks. They're also able to meet the demands of daily-driven vehicles, trailer towers, and delivery trucks. They are upgraded with quality parts to tolerate extreme performance loads, each one labeled as either a "300," "450," or "Race." These designations mean that the 300 can handle up to 300 hp at the wheels, the 450 can handle up to 450 hp at the wheels, and NADP claims the Race can handle as much as 750 hp. Since we were in the market for a transmission that would hold up behind our 860 hp common rail Cummins engine in Project X Diesel, NADP spec'd out one of its 48RE Race transmissions to take the beating.
To get a general idea of just how much goes into an NADP upgrade, these are all the internal pieces used in the Heavy Hauler Race version of the 48RE unit backing up the Cummins engine. Every component that will be reused is inspected, cleaned, and/or polished. | 
Here's an assembled overdrive section with an upgraded steel output shaft, which has been heat-treated. This billet output shaft is only available for four-wheel-drive applications, so our two-wheel-drive Project X transmission will have to get by with the original factory piece. | 
An NADP transmission technician assembles the gear train, fitting the forward drum with the hardened billet input shaft. |
NADP builds the valvebody to proprietary specs. It's not just an aftermarket shift improvement kit but a custom-tailored unit for improved shift quality for our particular driving and power needs. Our transmission has been smooth enough for over 3,000 miles of on-road driving, reliable enough for dyno testing, and strong enough for sub-11-second quarter-mile passes. | 
The forward drum is then fitted with high-friction performance-grade clutches and steels that can handle 750-plus hp! | 
The billet accumulator is being installed in the transmission case. It handles transmission line pressure better than the stock unit because of the extra set of seals. |
After years of breaking factory pieces, NADP's input shaft, main shaft, four-wheel-drive output shaft, accumulator, and servo have all been upgraded to aftermarket billet steel in the Race version of the transmission. | 
After the clutches and seals are installed, clearances on the direct drum are checked with compressed air and a dial indicator gauge. | 
The reverse piston was also upgraded to steel instead of plastic. |