Shifting Into Higher Gear: Next-generation 8-speed Automatic Transmissions Rolling Out More Extensively with 2015 GM Model Year

| March 9, 2015

2015 Hydra-Matic 8L90 (M5U) Eight Speed RWD Automatic Transaxle

The 8-speed automatic transmission era has arrived at GM.

A decade after it helped usher in the 6-speed era with the 6L80 transmission, GM is rolling out its successor, the first in-house-developed 8-speed, the Hydra-Matic 8L90.



The rear-wheel drive transmission is available for the first time in select 2014 model-year Chevrolet Corvettes, Silverados and Expresses; GMC Sierras and Yukons; and the Cadillac Escalade. More vehicles now equipped with 8speeds will be added in the 2015 model year, notably the Camaro and the Cadillac ATS and CTS.


2015 Cadillac CTS-V CoupeThe new-generation transmission delivers an optimal blend of efficiency, power, performance, durability and NVH suppression that’s never been offered, one uniquely suited to the engines and vehicles with which it will be paired and the demands of 21st-century drivers and advanced automotive engineering.

The 8L90’s engineering essence is a wide, 7.0 overall gear ratio spread, exceeding that of the 6L80 it replaces. With smaller steps between gears, the transmission is able to optimize engine horsepower and torque for improved efficiency and overall performance. A higher, 4:56 first gear ratio, for instance, aids with heavy loads and trailering in the 6.2L EcoTec 3 V-8 Silverado application, and boosts 0-60 time to 3.7 seconds in the Corvette Stingray 6.2L LT1.

Lighter, More Efficient

GM engineers also delivered on the goal of keeping the 8L90 sized to fit in the same space as the 6L80, and reducing its weight to improve efficiency. It employs four simple gearsets and five clutches (two brake and three rotating) to generate the eight speed ratios, as well as a more compact torque converter. It all fits into a die-cast aluminum case that has an integral bell housing for increased stiffness and reduced NVH.

While the same approximate size as the 6-speed, making it easy to incorporate into existing vehicle body designs, the transmission comes in at an eye-popping eight pounds lighter. That’s achieved through the use of aluminum and magnesium components, such as castings for the clutch piston and valve body.

The reduced weight, combined with gear ratios that reduce engine rpm by up to 8 percent in top gear, promise to deliver better fuel mileage in some applications,  notably the Stingray. With the existing 6L80E, the Stingray has an EPA city/highway rating of 16/28 mpg, and 20 mpg combined. Engineers were eyeing a 29 mpg overall rating for the 8-speed version, a boost of 3 ½ percent, without sacrificing anything in performance.

“GM’s new 8L90 represents a rare win-win-win scenario for customers,” says Kavoos Kaveh, global chief engineer for 8-speed automatic transmissions for GM. “It offers greater performance and efficiency, while weighing less than the transmission it replaces. That’s a rare accomplishment in the industry, and one for which GM has been awarded more than two-dozen patents.”


Tags: , , , , , , ,

Category: The Technical Side