Trans Mods To Handle Different Power Levels

Trans Mods To Handle Different Power Levels

The increasing popularity of GM’s Duramax powerplant backed by an Allison automatic has prompted a diesel subculture of D-Max enthusiasts. You can find good, used trucks for a decent price these days, and with minimal modifications to an LBZ, you can make an easy 400-500 horsepower. Unfortunately, as is the case with most diesel trucks, the transmission becomes the weak link in the chain. But there are several options to upgrade your Allison to handle any horsepower or torque increases you can throw at it and keep the truck reliable.


The stock Allison automatic transmission that backs the vast majority of Duramax-powered Chevrolet and GMC trucks on the road today is a great-performing five- or six-speed transmission. When the Allison transmission is used with a stock or mildly modified Duramax engine, it will handle most mods provided it’s not abused or poorly maintained.

As with any automatic transmission, if you overheat it, overload it or don’t properly maintain it and allow it to run low on fluid or with old dirty and burnt fluid, it will not last long no matter what power level you’re at. Heat is the primary enemy of any automatic and slipping under high power will cause a lot of heat buildup in the transmission.

For GM owners who have stepped up the performance of their Duramax engines to near 500 horsepower, a performance rebuild to keep the Allison reliable is the smart choice. And there are several levels of upgrades, depending on usage.

We stopped by Diesel Technology Source (DTS) in Monroe, Georgia, to find out what constitutes the proper upgrades for a street-driven Allison. David Browning of DTS recommended his Street build for trucks making up to 800 horsepower. For trucks making more than 800 horsepower, he suggests their Race build, which is available with billet input and output shafts for up to 1,000-horsepower builds. And for trucks with more than 1,000 horsepower to the rear wheels, DTS offers a Full Billet Race Allison transmission build that adds a billet intermediate shaft, planet and hub for the ultimate in strength.

1 The transmission is completely stripped down before it’s cleaned, painted and reassembled. DTS mounts the case to an engine stand to secure it while working on it and to allow it to be rotated into whatever position is needed.

2 The factory C3 clutch pack (on the right) shows burning and heat damage from slipping. DTS replaces them with Raybestos GPZ clutch discs adding two extra discs for more power handling capability.

3 The original C4 clutch pack is also showing signs of damage and will also be replaced by Raybestos GPZ clutch discs. Using a shorter apply piston allows DTS to use one addition clutch in the pack.

4 The C2 clutch pack on the right is the original and looks to be in decent shape but the high-performance pack on the left uses an additional clutch and better material to hold more power without slipping and overheating the clutch pack.

5 DTS replaces the factory C1 clutch pack’s six double-side clutches with a total of 14 single-sided Raybestos GPZ clutch discs to provide more heat sinking surface area and more clutch surface area to give a more positive engagement with less slippage and heat buildup.

6 David Browning loads the first set of clutches and steels into the Allison case to begin the build.

7 & 8 But before moving on to load more clutches, Browning measures the clutch play and uses the DTS machine shop to achieve the precision tolerances necessary to help the transmission hold up against big power engines.

9, 10, 11 & 12 After flipping the case over on the stand, he loads the planetaries and more clutch packs into the transmission case. He measures and verifies every step of the operation to make sure that the clutches are set up with the spacing he has determined to work best for high-power Allison applications.

Browning said the good news is that the factory shafts inside the Allison transmissions are larger and stronger than the factory shafts found in other transmissions, allowing the stock shafts to be used with trucks making up to 800 horsepower in the DTS Street Allison. Browning also feels that moving up to the billet shafts is not necessary until trucks reach or exceed the 800-horsepower threshold for the Allison transmission and even then he starts with just a billet input and output. He recommends the full billet treatment only for trucks making more than 1,000 horsepower.

All of the DTS Allison transmission builds use Diesel Performance Converters (DPC) triple-disc billet torque converters to handle the task of coupling the Duramax engine to the Allison transmission. Since it falls on the torque converter to pass the engine’s output to the transmission and ultimately to the wheels to drive the truck, it’s vitally important that the torque converter be up to the challenge.

The DPC triple-disc torque converters used by DTS in their transmissions feature a billet front cover to prevent ballooning or deflection that’s common with stamped steel covers like the factory uses. Internally, they use Torrington bearings rather than bushings for smooth operation as well as larger clutches with multiple discs for more holding power when locked than the single factory clutch. The stator is also optimized for better flow than stock and they’re even available with a billet aluminum stator for the ultimate strength.

The DTS crew also uses their in-house machine shop to modify internal transmission components to hold additional clutches for better holding power in each of the clutch packs. Taking precision measurements and machining components as necessary allows Browning to compensate for any casting-shift or machining variations from the factory to make sure that each transmission they build is individually optimized rather than just dropping a kit into it that may or may not meet their stringent tolerances for performance.

DTS provides their customers the choice of building the transmission with either Raybestos or Alto clutches. The Street builds use additional clutches and steels in the C1, C2, C3 and C4 clutch packs, while the Race builds use the same amount in the stout C1 pack and one additional clutch and steel in the C2, C3 and C4 clutch packs to hold more power without slipping. The C5 clutch pack is very robust from the factory and retains the original clutch count. The Street build comes in at $4,200 and the Full Billet Race build will set you back about $7,200.

13 & 14 While Browning is assembling the main internals of the Allison transmission, Will Rice reassembles the tail housing, installing the planetary and output shaft.

15 The output shaft on an Allison transmission is beefy, but it’s not unbreakable as can be seen with the broken shaft on the left. For applications of more than 800 horsepower, DTS recommends upgrading to billet shafts even with an Allison transmission.

16 Next, Browning lowers the tail housing down onto the transmission case.

17 Rice then torques the mounting bolts to secure the tail housing to the transmission case.

18 & 19 Throughout the assembly process, Browning takes measurements to make sure the assembly is going together the way he specifies. When needed, he machines parts to bring the assembly into tolerance.

20 Browning lowers the completely assembled drum into the transmission case, making sure that the splines properly engage.

21 The crew at DTS modifies the transmission pump to deliver additional line pressure to hold the clutches better and help prevent slipping.

22 Rice bolts the pump assembly to the bell housing and then lowers the assembly down onto the transmission case over the input shaft.

23 & 24 The valve body also receives some modifications to increase line pressures and work better in high-power applications before it’s reinstalled. All of the electronics are also tested and operation is verified to make sure everything operates properly.

25 Rice finishes off the rebuild by installing a cleaned and repainted factory transmission pan. An aftermarket deep aluminum pan can be selected to hold more fluid and provide additional cooling.

26 Looking at the worn and burnt clutch inside a factory Allison torque converter (on the right) and the huge triple-disc clutch assembly used in the DPC torque converter (on the left) it’s easy to see how it handles more power.

27 Taylor Wilder measures all of the tolerances on the converter to make sure it’s properly aligned and sized to work flawlessly with the rebuilt Allison transmission before it’s welded.

28 A computer-controlled welder is used to mate the two halves of the converter together. After welding, Wilder tests it for leaks and balances it before painting it and sending it out to the customer.

With all of DTS’ transmission builds, the crew completely tears down the transmission removing every nut, bolt and piece of hardware before thoroughly cleaning the case and all of the hardware. After everything comes out of the industrial parts washer, the transmission case sections are painted in DTS blue before reassembly begins. Browning feels that painting the case sections before assembly gives his transmission rebuilds a more professional look rather than painting over everything including the hardware as some shops do.

The photos included here highlight the components that go into an Allison performance upgrade. If you’re cranking up the power on your D-Max, a performance transmission will put the power to the ground safely and reliably. DW

Diesel Performance Converters
Dept. DW
2216 Commerce Place
McDonough, GA 30253

Diesel Technology Source
Dept. DW
171 HD Atha Road
Monroe, GA 30655