Circle D 6L80E Transmission Upgrades - Safe Six - Tech

The row-your-own-gears crowd will always think that they're better drivers, but at least stab-and-steer boys needn't suffer from gear envy anymore. In Camaros and Corvettes of just one generation ago, the term "six-speed" distinguished a manually shifted machine from an automatic, but that's not the case anymore. With the introduction of the 6L80E in the '06 Corvette, the gear tally is all tied up at six for both manual- and automatic-equipped GM performance machines alike. That means automatics can now run just as quickly as their clutched stablemates down the dragstrip in stock trim, and outright destroy them once a higher-stall torque converter is thrown into the mix. However, as with any piece of hardware that's brand new from the ground up, hot rodders were destined to blow them to bits with prodigious doses of LS horsepower before the aftermarket had a chance to catch up with stronger internals. Fortunately, thanks to companies like Circle D Transmission, the wait for near-bulletproof 6L80E automatics is over. Circle D has been beefing up GM's new six-speed since they first hit the streets, and we recently visited its shop to find out what it takes to prep a 6L80E for battle.

Compared to the 4L60E GM bolted into C5 Corvettes and fourth-gen F-bodies, the 6L80E is one brute of a transmission. In GM speak, the "6" in 6L80E denotes the number of forward speeds, while the "80" is an arbitrary figure that represents its strength. Consequently, the 6L80E is essentially the replacement for the venerable, TH400-based 4L80E rather than a successor to the 700R4-based 4L60E. Not surprisingly, GM's new six-speed is reserved for only the most demanding applications such as fifth-gen Camaros, C6 Corvettes, and heavy-duty trucks and SUVs. With fifth-gens weighing in at a portly 4,000 pounds, an extra two speeds make it easier to get all that mass moving off the line while still keeping the motor in the fat part of the powerband at WOT. "Stock versus stock, the 4L80E and 6L80E are very similar in terms of strength," explains Kyle House of Circle D. "The main difference is that the 6L80E has much more versatile gear ratios. With a ridiculously low 4.03:1 First gear, it's a stump puller. The 4L80E is more proven because it has been around much longer and more parts are available for it, but the 6L80E has come a long way and they can handle tons of power with a few basic upgrades."

According to Circle D, GM rates the 6L80E's capacity at 375 rear-wheel hp and it often burns up clutches once exceeding the 400 hp mark. It certainly doesn't help that this transmission tends to run very hot, which Circle D rectifies on its shop car with two in-line trans coolers. "The mechanical workings of these transmissions are pretty stout, but the weakest links are the 3-5-Reverse clutches and the 4-5-6 clutches. The next component most prone to failure is the spline area of the 4-5-6 hub where it engages into the output carrier," says Kyle. "To address these needs, our standard rebuild includes heavy-duty 3-5-Reverse, 4-5-6, and 2-6 clutch packs. They feature custom nitrided steels to prevent warping, and more durable clutch linings that are more resistant to heat. With these modifications, a 6L80E can handle 500 to 600 rear-wheel horsepower in the typical street/strip application, although we have had customers push them all to the way to 1,000 hp."