Magically Transforming the 5-Speed Allison into a 6-Speed Allison
To most general automotive enthusiasts, the automatic transmission is somewhat of a mystery, with gears, clutches, planetary sets and maze-like valve bodies somehow working together to get the power from our diesel engines to the ground. To many, an automatic transmission is quite simply a magic box that somehow moves the power from the engine to the wheels and as long as it’s working properly we don’t generally worry about it. Of course, most diesel enthusiasts know that a stock transmission in any of our trucks won’t handle much more power than stock and we’ll need to turn to one of the many premier transmission builders to upgrade the internals to handle the power our modified diesels can dish out. We know that the clutches, shafts and torque converters are the key areas that need improvement, but most of us couldn’t disassemble and rebuild an automatic transmission on our own.
The Allison transmission is known to be a smooth-shifting transmission that is reliable at stock power levels and robust when fully built to handle tons of power. The Allison 1000 transmissions used in LB7 and LLY trucks from 2001-2005 were 5-speed units that offered a .71 ratio for the overdrive 5th gear while LBZ versions of the truck received a 6-speed version of the same Allison 1000 that delivered a double overdrive with .71 in 5th and .61 overdrive in 6th gear. The extra overdrive provided a cruising rpm at 75 mph about 325 rpm lower than the 5th gear overdrive of the 5-speed Allison. In normal daily driving situations lower rpm generally means less fuel being burned and higher mpg, along with less noise and reduced wear on the engine.
Obviously, many early-model Chevrolet and GMC Duramax truck owners want the new 6-speed version of the Allison in their trucks but they don’t want to buy a new truck or new transmission to get it. That is where the magic of the Allison transmission and the team at SunCoast Performance comes into play. In some magical way, the internal clutches, shafts and planetary gearsets in the new 6-speed Allison were still the same as the 5-speed—only the TCM and the valve body were different to apply the clutches differently and come up with a double-overdriven 6th gear. But a snag that prevented people from simply swapping the TCM and valve body out of a newer truck into their transmission was that the newer trucks used a different communication protocol so the new parts would not talk to the old truck.
SunCoast and their engineers came to the rescue and developed a modified valve body and TCM that would literally bolt and plug into place of the existing TCM and valve body, talk to the truck and properly to operate the Allison in a 6-speed mode. Somehow these transmission wizards have magically transformed the 5-speed Allison into a 6-speed without removing the transmission from the truck or touching any of the clutches or gearsets. With the SunCoast TCM and valve body installed, the truck will operate the same as it always has in gears 1-5, but then it will shift into the double-overdrive .61 6th gear.
While the installation is pretty simple, it does require you to remove and replace the TCM under the hood as well as drain the transmission and remove and replace the valve body, which can be a messy task that may not be well suited to the faint-of-heart. Most gearheads will be capable of performing the upgrade on their own if they choose to do so. Just be careful and practice safe shop techniques while you are working on and under your truck.
We took our 2001 Chevrolet Silverado desert race project truck up to RLC Motorsports in Cookeville, Tenn., where shop owner Michael Dalton performed the installation with a little help from Drew Richards on a few occasions where an extra set of hands was needed. To make it easier for us to shoot the photos and document the process, the installation was done on one of the shop’s two-post lifts so that we had full access to the transmission pan and valve body once the pan was removed. The installation really is pretty simple and started with disconnecting the truck’s batteries before the old TCM was removed and replaced with the SunCoast replacement TCM. Then the truck was lifted and the fluid was drained from the PPE cast aluminum pan before the pan was removed to access the valve body. A trick to know which valve body bolts need to be removed to replace the valve body is to look at the new one and see which ones are missing, then remove those from the transmission.
After the old valve body was removed from the transmission, Dalton transferred the manual selector valve and pin over to the new valve body and installed it in the Allison transmission, being sure to torque the bolts to 100 inch-pounds as specified. Then he installed a new filter along with a SunCoast Filter Loc and reinstalled the pan. We also opted to install a new Allison spin-on filter and Merchant Automotive filter guard, which had to be modified slightly to work with the PPE cast aluminum pan. Once everything below was buttoned up, he lowered the truck and filled the transmission with about 2 gallons of SunCoast full synthetic D-Type ATF. Then he started the truck, checked for leaks, and topped off the level before taking it out on a test drive. Be sure to return your old valve body and TCM to SunCoast for the core charge refund. Despite our typical photography slowdowns, Dalton completed the installation in about three hours.
Driving the truck felt completely normal until you got out onto the open highway and it literally shifted into another gear! We have 37-inch Goodyear Wrangler MT/R tires on the truck. After we swapped the original 3.73 gears for 4.56 gears we noticed that the rpm climbed significantly while cruising along on the freeway and our fuel mileage dropped by almost 2 mpg down to about 15.5. At 65 mph the Duramax was spinning along at around 1,900 rpm, while 70 mph was about 2,000 rpm and 75 mph was about 2,200. With the truck in 6th gear, 65 mph saw the Duramax spinning at about 1,600 rpm, 70 mph was about 1,750 and 75 mph was about 1,850 rpm for a reduction of around 300 rpm across the board. When we measured fuel consumption we also found that we gained back the 2 mpg we lost with the gear change, so we now have better acceleration while turning about the same revs as we did before our gear change. With the same gearing we would have had similar results with a similar decrease in rpm and a similar increase in mpg.
Coming in at just over $2,000, the SunCoast Allison 5-6 Speed Conversion is not the cheapest upgrade, but cruising along the highway at about 300 rpm less is somewhat magical. It may not be for everyone, but we were happy that we made the swap in our truck and can now grab an extra gear on the highway. If you are looking for more mpg, lower rpm and would like another gear in your Allison, the magicians at SunCoast have just the setup for you.