The eight speed ratios are generated using four simple planetary gearsets; two brake clutches; and three rotating clutches. The resultant on-axis transmission architecture utilizes a squashed torque converter; an off-axis pump; and four close-coupled gearsets. The three rotating clutches have been located forward of the gearsets to minimize the length of oil feeds which provides for enhanced shift response and simplicity of turbine shaft manufacturing.

In order to deliver an eight speed transmission in the same package as the current six speed, the kinematic content must be kept to an absolute minimum. Total kinematic content can be described as the number of torque transmitting devices plus the number of planetary gearsets. Based on this definition, the incumbent GM six speed has a kinematic content of eight. That being the sum of three planetary gearsets and five clutches (note that the 1-2 Freewheeler in the GM six speed is a redundant device).

Although it is theoretically possible to achieve eight forward speeds and one reverse using this content, no competitive solutions have been found in this space. An additional clutch or gearset is, therefore, required. GM first set about inventing powerflows with three gearsets and six clutches. This could be executed by adding a clutch and making some modifications to the existing six speed rear wheel drive. Completely new three gearset six clutch options were also considered. All such three gearset powerflows were found to be suboptimal in terms of gearmesh efficiency and ratio flexibility. None of these solutions were found to meet the program targets. A search for a four gearset powerflow commenced.

Many effective powerflows were invented by GM in this rich search space. After an intensive search and trade-off study, GM engineers selected a powerflow lever which has four gearsets, three rotating clutches, and two grounding clutches.

—Hart et al.

The selected powerflow offers a unique combination of four major benefits, GM says:

1. It can be effectively executed using all simple gearsets (unlike the incumbent six speed powerflow) which provides for improved gearmesh efficiency, low pinion count, and increased flexibility in pinion count.

2. All three rotating clutches attach to a single node of the transmission which provides for simple and effective rotating clutch oil feed options.

3. It exhibits relatively low carrier speed ratios which keep pinion bearing g-loads to a minimum.

4. It offers a high reverse-to- first gear ratio unlike some of the other powerflows considered.

The five clutches are used three at a time, leaving only two open clutches in each gear state. All sequential shifts are performed in single transition while most multi-step shifts can also be executed in the same manner.

Although this powerflow features very low clutch slip speeds, GM notes, it does generate some relatively high absolute speeds. As a result, some of the clutch housings must be designed to tolerate high rotational speeds. To mitigate the resultant imbalance effects and to minimize mass, the rotating housings and pistons were designed to be made of aluminum. The extensive use of aluminum and magnesium also make it more than eight pounds (4 kg) lighter than the predecessor six-speed.

In the 2015 Corvette Stingray, the new 8-speed enables a class-leading 29-mpg (8.1 l/100 km) EPA highway estimate—a 3.5% increase in fuel economy over the previous six-speed automatic—and a quicker 0-60 time of 3.7 seconds, all while delivering wide-open-throttle upshifts quicker than those of the dual-clutch transmission offered in the Porsche 911.

The 2015 Silverado 1500 equipped with the 6.2L engine and eight-speed automatic transmission will have a maximum available trailer rating of 12,000 pounds, based on SAE J2807 Recommended Practices.

The greater overall performance and efficiency enabled by the 8L90 is due primarily to a new, wider 7.0 overall gear ratio spread, which enhances off-the-line performance with a more aggressive first gear ratio—4.56 vs. 4.03 on the six-speed—helping achieve the quicker 0-60 time.

The top gear ratio is 0.65 vs. the six-speed’s 0.67. Combined with a new, numerically lower 2.41 rear axle ratio in the Stingray vs. the 2.56 gear used with previous six-speed automatic models, engine rpm is reduced by 8% (123 rpm) on the highway at 70 mph. The lower engine speed reduces fuel consumption, while a new torque converter design enhances refinement, particularly during low-speed gear changes.

Stingray models equipped with the Z51 Performance Package feature a more aggressive 2.73 axle ratio, while the Corvette Z06 features the 2.41 axle ratio.

For performance driving, the transmission offers full manual control via steering wheel paddles. A new transmission-controls system and unique algorithms deliver shift performance that rivals the dual-clutch/semi-automatic transmissions found in many supercars, but with the smoothness and refinement that comes with a conventional automatic fitted with a torque converter, GM says.

A new Gen II transmission controller analyzes and executes commands 160 times per second, and wide-open throttle upshifts are executed up to eight-hundredths of a second quicker than those of the dual-clutch transmission offered in the Porsche 911.

Smaller steps between gears, compared to the previous six-speed automatic keep the engine within the sweet spot of the rpm band, making the most of its horsepower and torque to optimize performance and efficiency. Additionally, a torque converter design with a turbine damper complements performance with excellent refinement at low engine speeds.

Unique clutch and torque converter specifications matched to the torque capacity of the Stingray’s LT1 6.2L naturally aspirated engine and the Z06’s LT4 supercharged engine distinguish the applications for the different Corvette models.

More than 550 computer-aided engineering analysis were made during the development of the 8L90 to ensure strength, durability, performance and refinement. The architecture features a one-piece case with an integral bell housing for enhanced powertrain stiffness, as well as a detachable extension for the Corvette’s rear transaxle arrangement.

Similar to GM’s six-speed automatic design, the eight-speed’s architecture locates the grounding clutches rearward of the middle of the structure, outside the gearsets. However, the grounding clutches are splined to the case, eliminating the center support, which reduces weight.

This architecture also features a turbine shaft node that reaches to the outside of the barrel, enabling easy execution of a high-resolution magnetic speed sensor. The turbine shaft itself is very short, keeping oil channel drill lengths to a minimum. The rotating clutches are located near the front of the transmission, with short oil feed channels, supporting the transmission’s very fast shifts.

Additional design and performance features include:

• Clutch compensators are fed by lubrication oil rather than the dedicated and regulated feed design of the 6L80 six-speed. There are two benefits of this new design feature: The reduction of one oil channel between the valve body and the rotating clutches, which reduces the number of rotating oil seals and oil channels within the turbine shaft; and, secondly, the capability for rapid discharge of oil in the compensators during clutch apply, for greater control.

• An industry-first off-axis, chain-driven binary vane-type pump—located within the valve body—effectively allows for two pumps in the packaging size of a single, which contributes to lower parasitic losses and optimal priming capability, as well as ideal oil routing to the controls system.

• The binary pump enables a 60% reduction in pump torque at points that represent large portions of the operating duty cycle, compared to the 6L80 six-speed, which is a significant contributor to the overall efficiency gains offered by the 8L90.

• The binary pump is located very low in the sump, for greater cold prime operation—as low as -40 ˚F (-40 ˚C) and excellent high-speed operation with stable line pressure.

• Thanks to a new torque converter clutch friction lining and a new control strategy, the 8L90’s squashed torque converter uses conventional dual-path converter feeds.

• New synthetic fluid with improved cold temperature performance and reduced friction characteristics.

Shift time quickness and improved responsiveness are accomplished with the new Gen II controls system. VFS solenoid technology and three internal speed sensors give the 8L90 its shift performance. The new transmission controller is mounted external to the transmission and has a processor that executes hundreds of calculations and commands every 6.25 milliseconds.

The 8L90 is built at GM’s Toledo, Ohio, transmission facility.

Resources

• James M. Hart, Tejinder Singh, William Goodrich, Larry Diemer “General Motors Rear Wheel Drive Eight Speed Automatic Transmission” CTI Symposium 2014