Gearstar Transmission Upgrade - Automatic Improvement

6 Gearstar performs multiple modifications to improve shift quality and durability, including cryogenically stress-relieving all of the internal parts and expanding the capacity of the second, third, and fourth gear clutch packs.

7 The Chrysler NAG1 is identical to a Mercedes transmission with the designation 722.6, just with the proper bellhousing to fit the Mopar engine, in our case the 5.7 Hemi. While they wouldn’t tell us all the tricks they employed, we do know Gearstar upgrades the input to output bearing support, and internal pressure is increased for improved lubrication.

8 Each transmission’s valvebody is disassembled and reprogrammed and new electronics are installed. The NAG1 is a modern transmission, completely controlled by the car’s computer, wiring, and a series of solenoids on the valvebody. Offering better gear-reduction and overdrive, the NAG1 is light-years ahead of older three-speed models.

9 The torque converter is a full-billet unit from Gearstar Performance, set up specifically for our application with a custom stall speed. The lockup clutch is also upgraded for increased load capacity, a necessity in high-torque applications.

10 Once completed, our NAG1 was run on Gearstar’s transmission dyno, ensuring all of the parameters...

...including shift firmness and the stall speed of the converter are correct. It’s nice to know our unit will work properly and be leak free prior to installing it.

11 Our transmission was shipped complete, with the converter already installed, so all we had to do was swap it into the Challenger. Since the lift was occupied in our shop, we took advantage of Inline Performance Specialists’ offer to lend us one of their lifts.

12 Removing a late-model transmission is basically like removing any automatic. Once the linkage and wiring harness are disconnected, the converter bolts can be loosened and the crossmember and bellhousing bolts are removed last. It definitely helps to have a lift and transmission jack to accomplish this swap.

13 The driveshaft doesn’t have universal joints, but couplers at the front and rear to connect the transmission to the differential. The front coupler needs to be removed and installed on the new transmission prior to installing the transmission.

14 Since the upgraded transmission is externally the same as the one we removed, installation is straightforward. Be sure to disconnect the battery as the starter has to be removed and the wires could contact a ground and cause a fire hazard.

15 The Gearstar transmission comes with fluid in it, so we just need to top it off with fluid. The problem is that like most modern cars, the Challenger has no dipstick to use. ...

Fortunately, Inline has the kit which includes this dipstick and chart, telling how much fluid to install for certain ambient temperatures.

16 We took our Challenger on the road with the new transmission and immediately noticed how much crisper the gear engagement and shifts were. After warming everything up, we accelerated under part throttle and were impressed with the stall speed of the converter which seems to be nicely matched to the power range of the Hemi.

17 We’ve already performed baseline drag testing and will need to break our new transmission in before bolting on some stickier tires and going to the dragstrip. With the current tires, traction would limit the benefits of our higher stall speed and firmer shifts.

18 After a couple of weeks driving the car, we’re impressed with Gearstar’s improvements to the NAG1 automatic. The transmission shifts firmly, but not violently, and the heavy car responds nicely to the looser stall converter and modifications. Be sure to follow this project in future issues of Mopar Muscle as we further modify our Challenger and take it to the track for testing. mm