With all the prep work complete, the 4L85E was raised into place. Needless to say, it fits beautifully beneath a fourth-gen and looks like a factory install. The custom fabricated low-profile Moroso trans pan features a rear-mounted pickup tube to avoid starvation at the dragstrip, and provides improved ground clearance over a stock 4L60E.
Gearstar prefers factory rubber transmission mounts in most street strip applications, but went with a custom urethane piece due to the SS’s 8-second potential. If the added vibrations are too harsh for your tastes, a factory GM 1LE rubber mount (PN 22146231) is a good alternative.
Measuring slightly longer than a 4L60E, swapping in a 4L85E requires shortening the driveshaft by ¼ inch. Consequently, Gearstar installed a custom 3-inch-diamter heavy-duty steel shaft with 1350 U-joints and a 32-spline slip yoke. It’s affixed to the Moser 9-inch rearend with custom billet steel U-joint straps.
The final order of business underneath the car was securing the shift cable to the gear selector on the trans case using a lock washer, then attaching the cable bracket to the transmission pan rail.
Prior to shipment, or installation in this instance, Gearstar runs all of its transmissions on a specialized dyno. Not only does this allow properly setting shift timing and firmness, it also breaks in the fresh clutches and steels, after which the old fluid and debris is drained. Furthermore, the dyno can simulate engine and driveline load—which enables precisely dialing in converter stall speed—and replicate road speeds up to 130 mph. In essence, the dyno catches any potential problems before a transmission in bolted into a car.
Mounted on the passenger side of the firewall where the heater box used to be, a GM Performance Parts transmission control computer (PN 12497316) sends electronic signals to the 4L85E. Since it’s a standalone unit, the plug-and-play harness attaches directly into the transmission and doesn’t require splicing the factory wiring.
The system includes the computer, a wiring harness, a software disk, and an interface cable that hooks up to a laptop. The software allows programming shift points, part- and WOT shift firmness, and the final drive ratio. Hutter Racing (www.hutterracing.com) dialed in the final tune. Gearstar can also set up the 4L85E with a full-manual valvebody, which eliminates the need for a separate computer. Note the location of the cowl-mounted dipstick.
TH400 vs. 4L80E vs. 4L85E
The alphabet soup that is GM transmissions can be a bit confusing, and there were some key evolutionary tweaks that accompanied each change in designation of the General’s stalwart automatic. First introduced in 1964, the TH400 has always been GM’s heavy-duty transmission of choice. During its day, the factory has bolted it behind everything from 500ci Cadillacs to big-block Chevelles, to full-size vans and school buses.
The first major revision to the TH400 came in 1991, when GM added a 0.75:1 overdrive and electronic controls, and renamed it the 4L80E. It soldiered on mostly unchanged for over a decade until it was updated again in 2002 and dubbed the 4L85E. Upgrading the 4L80E’s four-pinion front and rear planetary gear sets significantly increased torque capacity and durability. Other changes included higher-capacity clutches and drums, and a more sophisticated valvebody. Since the 4L85E was designed specifically for use behind LS-series motors, it features an additional boss at the top of the bellhousing that aligns with the extra dowel present on LS blocks.