The Allison starts with big internal parts and good ratios, which is why it works so well behind a Duramax. The problem at higher power is not the basic layout. It is clutch pressure, shift timing, and the hard parts that take the load once slip is gone. The fix is to get full pump pressure to the clutch packs when the engine is under load, add clutch capacity, and control the apply timing so the transmission holds power without banging shifts or forcing engine derate. After that, the next weak links are the input shaft, hub, planet, and output shaft, which is why this build steps up to billet hard parts for 1,200-horsepower durability.
At ATS Diesel in Arvada, Colorado, Gale Banks met with owner Clint Cannon to discuss the Allison transmission being built for the supercharged Duramax-powered Lockjaw project. Cannon explained that ATS originally stood for Automotive Transmission Specialists, reflecting the company's roots in transmission building long before it expanded into turbos, controllers, and other diesel performance products. Transmission work, he said, was the foundation of the business, and torque converters became a major part of that early development.
Cannon described how ATS became known for its torque-converter work during the early diesel performance era. In 1997, the company introduced its Triple Lock torque converter, which he said was the first triple-disc converter available for years. He also designed and patented the square-tab clutch pattern used in ATS converters. According to Cannon, that square-tab design improved longevity by preventing the clutch components from rattling, reducing wear and noise.
That durability mattered because the torque converter absorbs much of the engine's torsional abuse. If the converter's internal clutch system wears or sheds material, contamination circulates through the transmission because the oil path runs from the pump through the torque converter, through the cooling system, and back into the transmission. In other words, the converter not only has to hold torque, but also has to survive long-term without generating debris that can damage the rest of the unit. Cannon tied that design philosophy to ATS's five-year warranty.
He also recalled that diesel performance was controversial in those early years. Some people believed focusing on diesel applications would hurt the company, but Cannon continued because diesel engines were producing exceptional torque. At the time, the transmissions themselves were not always the primary limitation. Engines were often only making around 400 horsepower, perhaps 450 on a strong setup, and roughly 800 lb-ft of torque, but even that was enough to destroy stock torque converters. The original single clutch pressed against a stock cover simply could not survive. ATS responded by developing a billet cover, a billet piston, and a stacked clutch-pack arrangement similar in concept to a motorcycle clutch. That work required extensive testing in oilfield, over-the-road, and higher-horsepower truck applications as diesel performance rapidly grew.
Banks also asked about ATS's signature purple finish. Cannon said the color choice was originally a way to distinguish ATS parts from everything else on the market. The first attempts unintentionally came out closer to pink, but once the company settled on a deep royal purple, it became the visual identity for the brand.
For the Lockjaw build, ATS is starting with a late-model Allison based on the LML-era six-speed configuration. Cannon said the team specifically wanted six gears. The Allison originally appeared as a five-speed in 2001, and by 2006 sixth gear had been enabled along with hydraulic changes. Later versions became more refined, and the late-model valve body offers better control.
He noted that the newer valve body moved away from trim-valve-based clutch-pressure control and instead uses EPC control, which makes the system cleaner, easier to tune, and more precise in achieving the desired shift quality. The project will also use a late-model controller.
Banks pointed out that the Roadster Shop chassis is being designed tightly around the Allison, with the truck intended to lay frame. Because of that packaging constraint, they cannot use ATS's deep transmission pan and must instead run the shallowest Allison pan. Even so, Cannon emphasized that fitting the Allison into the build is worthwhile because of its gear spread. It offers a low first gear for launch and double overdrive, with both fifth and sixth as overdrive ratios, which helps bring engine rpm down significantly at speed. ATS is not changing the gear ratios because Cannon considers the factory Allison ratios ideal for this application.
Cannon described the Allison as a fundamentally strong transmission from the factory. One of its biggest advantages is simply the size of its internal parts. The clutch packs are much larger than those found in many other transmissions, roughly twice the size of some competing units. That gives the Allison an inherently durable foundation.
Its main limitation, however, is low clutch-apply pressure. Cannon said the pump can make about 300 PSI, but only around 160 PSI actually reaches the clutch packs in stock form. That was acceptable for the Allison's original intended power level, roughly 300 horsepower, where it could live reliably all day. But once power climbs into the 400, 500, 600, or even 1,000 horsepower range, the transmission needs more clutch capacity, different shift timing, and much greater clamping force.
He also explained that the Allison is a clutch-to-clutch, non-synchronous transmission. One clutch releases while another applies, and the computer manages that process hydraulically. The hard parts and planetary gearsets establish the ratios, but the clutches do the actual work of transferring power during shifts. That makes clutch control central to both durability and drivability.
ATS's first major upgrade is hydraulic. Cannon said the company redesigned the hydraulics and added an electronic package that allows actual pump line pressure to reach the clutch packs. Instead of being limited to roughly 160 PSI of clamping force, the upgraded system can deliver as much as 300 PSI. Importantly, ATS only commands that higher pressure under engine load. When torque demand drops, pressure is reduced again.
That strategy preserves drivability and efficiency. Lower pressure when full clamping force is unnecessary reduces parasitic drag, avoids excess heat, and allows more oil volume to move through the converter and cooler for lubrication and temperature control. Then, when the engine is making serious torque, the system diverts pressure where it is needed to clamp the clutches.
Beyond hydraulics, ATS increases clutch capacity directly. Where a stock clutch pack might use three or four frictions, ATS builds may use six or seven. Cannon explained that each added clutch increases the mechanical torque-holding ability. In some cases, the result is double or even triple the holding capacity. That added capacity means the transmission does not need an excessively harsh shift to survive. Instead, ATS can tune the clutch application timing so the shift is quick enough to avoid slipping through the oil film and glazing the clutches, yet still smooth enough to maintain good street manners. In Cannon's view, the correct combination of clutch-pack upgrades, hydraulic modifications, and electronic control is what allows a transmission to hold power without becoming unpleasant to drive.
Once clutch slip is reduced and the transmission is able to transfer power efficiently, the next weak links become the hard parts. Cannon pointed to the Allison's large shafts and noted that even though they are physically massive, they can still fail at high enough torque levels. Around 800 horsepower, he said, stock shafts can begin to break. Allison did not need to manufacture those parts from exotic materials such as AerMet or 300M in stock form because the original application did not require it; the factory relied on size rather than premium metallurgy.
As power rises and slip is removed from the system, the torque has to go somewhere. If it is no longer being lost in the converter or clutch packs, it is transmitted directly to the rear wheels and through the shafts, hubs, and planetary components. Under those conditions, fatigue and breakage become real concerns. Cannon said ATS sees a progression in failure points: the input shaft tends to fail first, the hub second, and the output shaft third, although the exact order depends on the application. In a traction-limited street truck, the output shaft may be the last part to fail. In a heavy over-the-road truck pulling 30,000 to 40,000 pounds with little tire slip, the output shaft may fail first because the driveline is loaded continuously.
Banks raised the question of whether the project would remain around 600 horsepower or potentially move toward 1,000 horsepower. Cannon's answer was that ATS would simply build the transmission to the higher standard. He explained that ATS stages its builds partly to save customers money. A 600-horsepower truck does not necessarily need expensive billet shafts, billet hubs, or upgraded planetary components. But as power climbs, those parts become necessary.
For Lockjaw, ATS is building the full version. That includes the heavy-duty clutch packs, the larger pump and hydraulic modifications, the electronic control strategy, and upgraded hard parts throughout. Specifically, Cannon said the transmission will receive a billet input shaft made from 300M, an upgraded hub, upgraded planetary components, and an upgraded output shaft. In that configuration, he expects the Allison to be capable of roughly 1,200 horsepower while still operating all day at that level.
Cannon repeatedly emphasized that ATS is not trying to create a harsh, crude transmission. He contrasted ATS's approach with aftermarket methods that simply raise pressure, drill holes, and force the clutch packs to apply as quickly and firmly as possible. That can help hold power, but it often produces an unpleasant shift and is better suited to specialized competition vehicles than a truck intended to drive well.
Instead, ATS focuses on preserving drivability while increasing torque capacity. Modern electronically controlled transmissions constantly monitor engine speed, input speed, and output speed. The controller calculates ratios and watches how quickly the turbine speed drops during a shift. Manufacturers typically rely on engine derate during shifts: when the transmission commands a gear change, the TCM asks the engine to reduce power momentarily so the clutches can apply without slipping. That is why many vehicles exhibit a soft or interrupted-feeling shift under load.
ATS's goal is to give the transmission enough clutch capacity, hydraulic authority, and control precision that it can complete the shift without needing that derate. If the transmission can pull the engine down correctly and make the gear change cleanly, the TCM never has to request a power reduction. The result is a smoother and more direct transition from gear to gear, especially under heavy load or elevated power levels. Cannon said that combination of smoothness and holding power is one of the defining characteristics of an ATS build.
With the transmission plan settled, the next step in the series is a return to Roadster Shop for an update on the Spec Series chassis that will carry the supercharged Duramax and its fully built Allison.