98-02 Camaro/Firebird 4L80E/4L85E Transmission Upgrade
If You’re Tired Of Blowing Up Your Fourth-Gen’s Slushbox, Then It’s Time To Upgrade To A 4L85E.
Muscle car guys can call it character if they please, but there’s nothing fashionable about single-digit gas mileage, subjecting your ears to a NVH beat down, or sweating like an Illinois governor. Late-models are all about knocking down 25 mpg on the way to the track, effortlessly clicking off 10-second passes while you’re there, then cruising back home in a quiet, climate-controlled cabin while the muscle car guys watch in disbelief.
Yeah, baby. The problem is that if your 4L60-equipped fourth-gen actually runs that fast, it won’t be long until the trans goes kaput. Not only is it ridiculously easy to eclipse the 500 hp mark with either an LT1 or LS1 Camaro, the fact that fourth-gens are already anywhere from 7- to 16-years-old—and usually have tons of miles on the clock—only compounds matters. Likewise, even the greatest of transmission builders have a tough time getting 4L60s to survive much past the mid-10-second mark. Perhaps the ideal solution is swapping in a heavy-duty 4L85E, and fortunately, Gearstar has it all figured out.
History has proven that the swap in question is an endeavor that’s very common, yet rarely executed. Instead, hard-core enthusiasts often opt for track-proven three-speeds such as the TH350 and TH400. Some simply adjust to the ignominy of life without overdrive, while others bolt on a Gear Vendors assembly to the tailshaft. Either option works just fine, but the 4L85E combines the best of both worlds. Essentially a TH400 with overdrive and more rugged internals, the GM 4L85E is the ultimate automatic.
When built right, they can handle over 1,000 hp reliably, and best of all, transplanting one into a fourth-gen is much easier than Internet mythology suggests. “People are very intimidated by the 4L85E. They think that swapping it into a fourth-gen requires a lot of fab work, but that’s just not true,” says Zack Farah of Gearstar. “The trans tunnel does not need to be massaged anywhere, and there’s very little cutting involved. It’s not the cheapest option out there, but it’s a long-term solution to a long-term problem, and it’s something you’ll only have to do once. Getting the electronics to work isn’t difficult at all, and you can even set it up with a full-manual valvebody which eliminates the need for a trans computer entirely.”
Made from flexible steel-braided hose, Gearstar’s custom trans dipstick eliminated the need to bend and wrestle with rigid tube when routing it through the chassis. Longer and larger in diameter than the stock piece, it doubles as a filler tube as well. An adjustable bracket enabled Gearstar to mount it on the cowl, although other locations will work as well.
The only cutting required was a 4-inch section of metal on the driver side of the trans tunnel just ahead of the crossmember. This provides clearance for the wiring harness bolt head. With the trans mocked into place, Gearstar technicians marked the area with a Sharpie, then cut and removed the metal section with a cutoff wheel. Since the metal flap isn’t a structural member, it has no adverse affects on chassis stiffness.
True to the car’s sleeker appearance, Gearstar removed the stock shift lever and mounted a B&M Pro Stick shifter inside the factory center console.
The B&M shift cable and trans brake line were routed through a factory hole located on the trans tunnel right above the crossmember mount. To get rid of some slack, the cables were angled rearward by attaching them to the fuel line with zip ties, then routed back forward on a gradual bend.
Should a transmission ever overheat, it’s designed to puke fluid out of the breather tube to relieve internal pressure. The factory tube is only three- to four inches long, and dumps fluid right on the case. To avoid a potential mess, Gearstar extended the tube all the way to the tailshaft housing.
Not only is the slip yoke used on the 4L85E significantly larger than that of the 4L60E, it features 32 splines instead of 27. Gearstar matched the yoke up with beefy 1350 U-joints. The upgraded yoke assembly can be welded onto any driveshaft during the shortening process.
With the popularity of the TH400 swap, many manufacturers offer torque converters that fit the LS1’s bolt pattern. This isn’t necessary with the 4L85E, as it was designed for LS-series motors from the get go. From this view, it’s clear where the dust cover area of the trans case will smack into the passenger-side header if it isn’t ground down. While it’s possible to dimple the Number 7 primary, the potential for impeding exhaust flow isn’t worth the risk.
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.