Our 5,400-pound, 400hp 4x4 has chewed through no less than four 4L60E four-speed automatic transmissions in less than a decade. The failures have included: a smoked torque converter clutch, which contaminated the oil and rewarded us with a fried transmission; two broken rear output shafts; and overheated transmission clutches after the installation of more aggressive tires. We were ready for an upgrade.
High-performance electronic overdrive automatic transmissions have become increasingly commonplace, and good rebuildable cores for the venerable yet antiquated GM TH350 and TH400 three-speeds are drying up. Also, we wanted to maintain highway streetability with this truck, so a swap to a three-speed, heavy-duty TH400 was out of the question.
The only logical transmission for our 4x4 was the 4L80E four-speed automatic transmission commonly found in 3/4- and 1-ton GM trucks and SUVs. It is essentially a four-speed TH400. In fact, the rear half of the 4L80E is nearly identical to the TH400, and many internal components are interchangeable between the two transmissions. The only downfall of this swap is that the 4L80E is an electronic transmission that generally requires an additional computer and wiring harness to operate properly. However, the 4L80E can be easily converted to a manually shifted valvebody, which eliminates the need for the computer and much of the electronics and wiring. This conversion turns the 4L80E into a great heavy-duty analog overdrive transmission that can be easily swapped into almost any high-horsepower 4x4.
With our minds shifted into heavy-duty overdrive, we made a call to Monster Transmission to get our hands on a complete 4L80E Monster in a Box rebuild and shift kit. We also sourced Monster Transmission for a 300M input shaft, a steel forward hub, and an engine-matched lockup torque converter with a 2,800-3,200 stall speed.
Parts in hand, we made our way to Well Built Transmissions in Oceanside, California. The company is run by Cliff Whynaught and his son, Mark, who are second- and third-generation transmission rebuilders. The Whynaughts reaffirmed our transmission selection by telling us that the 4L80E is an incredibly durable transmission that you can beat on off-road all day long. The only real disadvantage is the weight, which clearly is not a concern for our 4x4. Here is how our transmission came together.
When looking for a core transmission to start with, you’ll want to find a 1997-up version of the 4L80E. This newer version has many oiling and other internal improvements that make it superior to the older version. The easiest way to spot a 1997-up 4L80E is by the oil cooler ports. The early version has both threaded ports next to each other at the front of the transmission. The 1997-up version has an oil port to the rear and one in the front of the transmission, as shown here.
Our Monster Transmission 4L80E Monster in a Box kit came with all the necessary parts to rebuild our transmission. The included shift kit bumps up the line pressure for firmer shifts and improved oiling throughout the transmission.
Cliff Whynaught of Well Built Transmissions in Oceanside, California, says that the stock forward hub (right) is fine for stock applications, but he recommends upgrading to a billet steel forward hub (left) for anything else. The stock unit can split in even mildly adverse conditions.
The factory 4L80E input shaft (right) is good for up to 400 hp. Whynaught recommends a 300M input shaft like our Monster Transmission shaft (left) for most applications up to 800 hp. For extremely heavy-duty applications up to 1,100 hp, Whynaught recommends a Vasco input shaft.
Our Monster in a Box rebuild kit includes red waffle-type clutches. Whynaught prefers these for off-road vehicles where the throttle is applied on and off. The grooves let the transmission fluid escape when the clutches are quickly disengaged and engaged during throttle manipulation.
Whynaught recommends a Kevlar band with tack-welded ends like our Monster Transmission part for any off-road application (middle). The stock band (right) is fine for stock applications. The heavy-duty wide band (left) can be used in any 4L80E, but Whynaught says it is best suited for applications over 800 hp.
The stock 4L80E four-pinion planetaries are fine for most applications, and the stock intermediate shaft is good for up to 800 hp. For extremely heavy-duty applications up to 1,000 hp you can swap in the five-pinion planetaries from a 4L85E and install a 300M intermediate shaft.
Contrary to what you might think, the smaller 1997-up sprag (left) is stronger and more desirable than the larger earlier sprag (right). They are not interchangeable, so this is yet another reason why the 1997-up 4L80E is preferred.
Whynaught recommends installing a stepped case bushing (left) instead of the stock bushing (right) in all off-road applications. The step keeps the bushing from walking out of position under load. Our Monster in a Box kit came with the stepped case bushing.
The Monster in a Box shift kit fits into the factory 4L80E pump. It improves oiling and helps firm up the shifts between gears. Whynaught recommends a heavy-duty intermediate snap ring on any application that utilizes an aftermarket shift kit. The weaker stock snap ring can allow the intermediate assembly to walk in the case when combined with the increased shift kit line pressures.
All of the rebuilt components are dropped into the case and the pump is cinched into place. If you are looking for decreased component wear and just a bit more strength than stock, Whynaught says you can cryogenically treat the factory parts for about an 18 percent increase in strength.
This is where all the magic happens. Whynaught sent our stock automatic valvebody out to Culhane Racing Transmissions to be converted into a manually shifted valvebody. About half of the wiring in the stock 4L80E is no longer needed. However, the solenoid to actuate the lockup torque converter is retained. We will tap into this wire and put it on a toggle switch instead of connecting it to a computer.
Monster Transmission matched up the proper lockup torque converter with our 400hp V-8 engine. The 2,800-3,200 stall is a bit looser than a stock converter to keep our engine in the sweet spot. We filled our torque converter and transmission with Maxima Synthetic Racing ATF.