TRANS MODS TO HANDLE DIFFERENT POWER LEVELS
Street and race transmission mods
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 400500 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 horse-
power, 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.
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 $4,500 crated and shipped and the Full Billet Race build will set you back about $7,000.
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. UDBG
6 David Browning loads the first set of clutches and steels into the Allison case to begin the build.
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.
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...
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.
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 additional clutch in the pack.