The big news from GM is that it’s all-new, Hydra-Matic 8L90 eight-speed transmission is now matched with the available 6.2L EcoTec3 V8 engine in the 2015 Chevrolet Silverado 1500.
“The new 8L90 eight-speed is designed tougher to handle the greater power of the Silverado’s powertrains, with great improvements in spin loss to complement that capability with greater efficiency,” said Kavoos Kaveh, global chief engineer for eight-speed automatic transmissions. “It is a marvel of packaging efficiency and driving efficiency that has garnered more than two dozen patents for General Motors.”
With four simple gear sets for better efficiency and five clutches (two brake clutches and three rotating clutches), more efficient packaging enables the new eight-speed automatic to fit the same space as the previous six-speed automatic, and it weighs less. A wider 7.0 overall gear ratio spread provides a numerically higher 4.56 first gear ratio, helping drivers start off more confidently with a heavy load or when trailering.
Smaller steps between gears, compared to the 6L80, keep the engine within the sweet spot of the rpm band, making the most of its horsepower and torque to optimize performance and efficiency. They also enable numerically lower rear axle ratios, which reduce engine rpm on the highway for reduced fuel consumption.
Additionally, friction-reducing design features – including all-new synthetic fluid – and lower spin losses via a powerflow that uses only two open clutches contribute further to efficiency. A new torque converter design with turbine damper enhances refinement, particularly during low-speed gear changes, for smoother performance when the truck is loaded or pulling a trailer.
The new design features a one-piece case with an integral bell housing for enhanced powertrain stiffness. Similar to GM’s six-speed automatic design, the eight-speed locates the grounding clutches rearward of the middle of the structure, outside the gear sets. However, the grounding clutches are splined to the case, eliminating the center support, reducing weight.
It 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 very fast shifts.
Clutch compensators are fed by lubrication oil rather than the dedicated and regulated feed design of the 6L80. 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-percent 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.
As well, a new torque converter clutch friction lining and new control strategy, allow the 8L90’s squashed torque converter to use 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 a new Gen II controls system. VFS solenoid technology and three internal speed sensors give the 8L90 the capability for world-class shift performance. The new transmission controller is mounted external to the transmission and has processor that executes hundreds of calculations and commands every 6.25 milliseconds.