t has been two decades since General Motors introduced the 6L80E six-speed automatic transmission. Developed for trucks and higher-performance cars, including the Corvette, it was a clean-sheet design—unlike almost all previous GM electronically controlled four-speed automatics, which were based on earlier versions of hydraulically controlled gearboxes.

Compared to a 4L80E, the 6L80E (and later 6L90E) is approximately the same length, but a bit wider and about 20 pounds heavier. The gearbox is shifted via performance algorithms in the internal, adaptive, 32-bit Transmission Control Module. They’re designed to minimize upshifts during closed-throttle driving to prevent unnecessary shifting. Three-way solenoids and variable-bleed solenoids are employed to optimize shift quality.

After debuting in the 2006 model year, the 6L80E proved itself reasonably strong, but as with just about every other modern gearbox, it inevitably developed its share of common issues that generally resulted in poor shift performance, burned-up friction elements, and more. Overheated and burnt fluid is typically the culprit in most instances, along with some programming issues with the internal controller and torque converter shudder.

Additionally, the boundaries of the 6L80E’s capabilities reveal themselves pretty quickly when significantly more power is shoved through it. Mostly, the clutch packs struggle to keep up under hard acceleration and can start slipping, while the input shafts are vulnerable to breaking. There can be valvebody and solenoid issues, too.

“The 6L80E is a strong performer in stock trim but when you’re pairing it with a higher-output engine, you really need to support it with a few targeted upgrades,” Ryan Steele, the marketing manager at Gearstar, says. “When you beef up the known weak spots, you’ll have a transmission with the capability to match the full potential of your powertrain.”

In a nutshell, that means replacing and upgrading all the friction materials and strengthening the hubs. At Gearstar, that means they’re welded and sent for cryogenic treatment and hardening. Additionally, the intermediate shafts are replaced with hardened billet components and there are critical pump upgrades designed to alleviate one of the 6L80E’s most common sources of wear and weakened performance.

Gearstar’s 6L80E offerings include Level 3 (650 hp/600 lb-ft) and Level 4 (750 hp/700 lb-ft) for 2WD and 4WD drivetrains. A new Yank torque converter is included with each build, too.

“Along with standing up to more power than the stock assembly, the common issues are addressed in ways intended to substantially improve durability and longevity,” Steele says.

To learn more about what goes into the build of a higher-capacity six-speed that will support power-adder and high-output naturally aspirated combinations, we visited Gearstar’s facility in Ohio to follow the construction of a Level 4 6L80E. We’ve highlighted the details in the accompanying photos. Of course, this isn’t a step-by-step assembly guide, but a look at the key elements of the upgrades.

Two decades after its debut, the 6L80E continues to find its way into more and more performance builds—and aftermarket upgrades, such as those incorporated by Gearstar, are helping ensure the six-speed gearbox stands up to all the power enthusiasts can push through it.