Prev Article Next Article

Sonnax TH400 Smart Tech Drum Module

Posted By: Steve Turner
Sonnax TH400 Smart Tech Drum Module
Written by Steve Turner
Photography courtesy of Sonnax
In the intense and challenging world of drag racing, driveline components have the unenviable task of performing and a high level and enduring the ultimate rock-and-a-hard-place durability test. When a race car launches all the pieces in between the engine and the track surface are wedged between a near irresistible force and an immovable object. 
Surviving this duty can be achieved with bulk and brawn, but maximizing performance usually means reducing weight with smaller parts and lighter materials. In the case of the storied TH400 three-speed automatic transmission, myriad aftermarket upgrades can enhance and fortify these transmissions for a performance mountain.
Racers choose these transmissions because of their durability and that robust aftermarket support, which includes a wide range of gear ratios, even though they are heavier and less efficient than a Powerglide. A key reason for that inefficiency is the design of the direct drum, which counter-rotates. In first gear, it spins backward then completely halts during the 1-2 shift before suddenly matching input rpm on the 2-3 shift. Not only does this arrangement rob power, but its abrupt nature of engagement can result in breakage.
A solution that eliminates this weak link and improves performance on the drag strip is Sonnax’s TH400 Smart-Tech Drum Module, which reorients the operation of the drum with one complete upgrade. 
“Like many other three-speed transmissions of the era, the direct drum spins backward in first gear when you leave the starting line on the one to shift that comes to a complete stop. And then the two-three shift that goes from a complete stop the input rpm, and it's a big waste of energy and contributes to the inefficiency of the 400,” Gregg Nader, Product Development Manager at Sonnax, said. “…There are far fewer parts that are counter-rotating, stopping, and restarting, so there’s about a 3.3-pound weight reduction there, and that’s the goal we’re after, reducing that counter-rotating weight of the direct clutch assembly.”
While there are other aftermarket upgrades for the standard TH400 drum that reduces mass by reducing the size of the drum or using lighter materials, these solutions don’t re-engineer the operation of the drum the way the Sonnax unit does. 
“When you go from an iron drum to an aluminum drum, you gain efficiency. If you go from a large aluminum drum to a small aluminum drum, you gain more efficiency and all the module is doing is extending those gains into new territory that hasn't been accomplished before. And the benefits, which we knew in advance would be quicker reaction time, quicker acceleration quicker shifts, smoother driveshaft speed, improved G-meter response, increased wheel horsepower, and improved elapsed times. It also increases the durability on input shafts, main shaft sprags, and direct clutches,” Nader said. “So compared to an iron drum, it's 68 percent less energy, you know, to spin it up, stop it restarted compared to aluminum, direct drum, it's 53 percent and then even the small drum module is 33 percent more efficient than that.”
This complete, all-new upgrade can operate in applications ranging from street/strip rides with 500 horsepower to No-Prep and Pro Mod machines with 3,500 horsepower. Once in place, it can yield elapsed-time gains of .015 to .020 seconds while yielding more durability to go round after round.


A complete replacement for the factory-style forward and direct drums from the intermediate sprang to the pump, the H400 Smart-Tech Drum Module includes a larger-diameter, 36-element sprag, and sprag race compatible with stock-style intermediate clutches. The kit includes a six-friction forward clutch pack and a seven-friction direct clutch pack.

With the factory-style arrangement, the forward drum mates to the turbine in the torque converter and won’t spin too fast, but the direct drum is directly connected to the geartrain, so if something breaks the direct drum can spin past a critical rpm and explode. Since the Sonnax Smart-Tech Drum Module doesn’t have a separate direct drum and its parts are smaller in diameter and mass, they won’t explode even if other parts overspeed.
While there are several upgrades available to improve the TH400 drums, including the Killgore Superlite Drum (right) that also moves some functionality to the input side and reduces the counter-rotating componentry, the Sonnax TH400 Smart-Tech Drum Module multiplies the clutch capacity and features all-new, robust internal hardware. 

While replacement aluminum direct drums can shave off a few pounds, the Smart-Tech Drum Module further reduces the weight of the counter-rotating hardware by more than 3.3 pounds. Moreover, this arrangement reduces the energy required to stop and restart during the 1-2 and 2-3 shifts by 53 percent, which delivers more horsepower, improved acceleration, and enhance durability due to the reduced shock loads during shifts. 
(800) 843-2600

join our

email list

You’ll be first to know about NMCA events, race results and so much more!