MAGIC ALLY TRANSFORMING THE 5- SPEED ALLISON INTO A6-SPEED ALLISON
Magically Converting the Allison 5-speed to a 6-speed
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. For many of us, 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 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. Lower revs generally means less fuel being burned and higher mpg, along with less noise and reduced wear on the engine.
Obviously, early-model Chevrolet and GMC Duramax truck owners would like the new 6-speed version of the Allison transmission in their trucks but they didn’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 in-
ternal 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 newer TCM and valve body 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 in place of the existing TCM and valve body and talk to the truck properly to operate the Allison in 6-speed mode. Somehow these transmission wizards 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 unit. 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 is to look at the new one and see which bolts are missing, then remove those bolts 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 the valve body 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 fired the truck up, 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 the truck literally shifted into another gear! We have 37-inch Goodyear Wrangler MT/R tires installed, and after we swapped the original 3.73 gears for 4.56 gears we noticed that the rpm climbed pretty significantly while cruising along on the freeway. 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, 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 rpm 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 Performance 5- to 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. Just be sure to tell them that your friends at Ultimate Diesel Builder’s Guide magazine sent you. UDBG
The Suncoast 5- to 6-speed Allison conversion kit simply comes with the new valve body and TCM. We opted to replace both the internal and spin-on Allison filters and add a Suncoast Filter Loc to hold the internal filter securely in place. We also choose to use Suncoast’s fully synthetic D-type ATF for our built Allison transmission.
1/2 After disconnecting the batteries, Michael Dalton loosens both bolts securing the TCM to the fan shroud and then lifts it out of the slots securing the bottom of the TCM cover.
6 Dalton lubricates the connectors, seals and contacts with dielectric grease to ensure a good connection that'll last for years—especially considering our predilection to get the truck dirty.
4 Then he unplugs each of the harnesses to remove the TCM from the truck.
5 Looking at the TCMS side by side, they are virtually identical—except of course for the mud and dirt stuck to our old one.
3 Laying the cover over, he unclips the tabs holding the TCM in the cover.
10 Next, Dalton drains the transmission before removing the pan to access the valve body inside.
11 When Dalton lowered the pan the filter dropped down with it, making us happy that we chose to use the SunCoast Filter Loc on the new filter when we put it back together.
12 Notice that the filter seal (see arrow) is still lodged in the transmission case. Be sure to remove it before installing the new filter and seal.
8/9 After seating the new TCM in the cover and securing the clips the cover is slid into position, making sure that the tabs at the bottom engage the slots in the fan shroud. Then the cover is bolted into position to complete this part of the installation.
7 The connectors in the truck plug right into the new Suncoast Performance TCM just like factory, because it is a factory TCM that they recoded to work with the truck and control the Allison as a 6-speed.
16/17 The manual selector valve and pin must be removed from the old valve body and installed in the new valve body; be sure not to lose the pin while handling the valve body or transferring the parts.
15 After it is loose, he carefully removes the valve body from the transmission.
14 Using the new valve body as a guide for which bolts to remove, Dalton removes the bolts securing the valve body to the transmission.
13 With the pan and filter out of the way, Dalton disconnects the solenoids and transmission connectors from the harness and removes the clips securing the harness to the valve body, then moves the harness out of the way.