When Tadge Juechter and the rest of the Corvette team were tasked with building the C7 Z06 Corvette, the best factory Corvette ever, they had to engineer a car that could be wielded on a track with surgical precision by the best of drivers, yet behave as a daily driver and sell in high enough volume to justify the expense of the vehicle’s development. One of the ways the team was able to create such a formidable car and help widen its appeal is the addition of the 8L90 eight-speed automatic. Before any purists cry foul, the Z06 is still offered with a seven-speed Tremec transmission, yet the sports-car market is definitely moving away from traditional manuals, so it was a matter of evolving the car to survive.
From day 1 of the eight-speed development, the goal was to design a competitive transmission for the Corvette that improved on the existing six-speed in every possible way, including performance, shift times, fuel economy, and weight. It had to have the torque capacity to handle the LT4 and yet fit in the same area as a 6L80 to minimize platform changes. The result is a transmission that offers no compromises. It’s lighter than the 6L80, it shifts as fast (or faster) than competitors’ dual-clutch transmissions (DCT) when asked to, yet its torque converter allows for smooth operation at low speeds and part throttle. It delivers on all those fronts and delivers a 5 percent increase in fuel economy, which is a bonus for Corvette, yet means a whole lot more for corporate average fuel economy when it will be built in volume for light trucks and SUVs.
Starting with a ratio spread of 7.00—up from 6.04 in the six-speed—the transmission development team used software to analyze power flow and help select the optimum gear ratios, which are closer than on the six-speed, to keep the engine closer to its peak power. The trans uses four gearsets and five clutches, the minimum number of components to get eight speeds. Three clutches are applied in every gear, so there are only two open clutches at any time. In contrast, the six-speed has three open clutches at all times, which requires fluid pressure, meaning there was more parasitic loss than in the 8L90.
Internally, every surface was scrutinized for peak performance. Bearings are used whenever possible instead of bushings to reduce friction. By tightening up tolerances and using lightening holes, there’s less fluid being flung around inside the case and therefore less fluid needs to be pumped. The 8L90 used a second generation of controls that use a variable force solenoid (VFS), so there’s more control of pressure with less fluid bleed as well. The fluid transfer efficiency enabled the pump size to be reduced from 1.75 ci on the six-speed to 1.00 ci on the eight-speed, and it can operate at a reduced output to reduce pumping losses even more. The VFS also allows for very precise movement of the clutch packs, keeping them open and free of drag to help with fuel economy, while allowing them to stage quickly for split-second shifts during performance driving.
Unlike most eight-speeds that use two speed sensors, the 8L90 uses three speed sensors to know what’s going on inside. The new external transmission control module (TCM) uses an external controller and is similar in stature to an ECM. The TCM processes signals 160 times per second to quickly respond to driver input. For example, to quickly accelerate to pass, the driver floors the accelerator while in Eighth gear. The trans makes two movements to synchronize speeds, moving from 8-4 and then 4-3. It’s two events to the trans, but the driver only feels one. The third speed sensor helps make that possible. Basically, there’s a lot more information to process, and going from the six-speed to eight-speed added 27 additional combinations of shifts to calibrate, but the transmission can use that information to make the correct shift and keep the engine at the optimal rpm.
Software allows for multiple driving modes with noticeable differences in feel. Modes include: normal, sport, track, paddle shift, and temporary tap that lets the driver use the paddles in normal mode. The vehicle monitors lateral and longitude acceleration, throttle, and brake input, and will automatically put the trans into sport or track mode. On WOT, upshifts are chosen to maximize power, and on braking it does rev-matching downshifts so it’s ready immediately to go back on the throttle. Also in sport mode is a performance algorithm that holds the gear when the driver lifts their foot off the throttle, rather than upshifting, anticipating the driver will want to get back on the throttle and resume accelerating. For quick upshifts, the engine cuts fuel so engine rpm can drop quickly to match the transmission. The engine also cuts fuel on deceleration, and the torque converter helps smooth the transition when fuel comes back on. In normal drive mode, shifts are more biased to smooth driving, fuel economy, and efficiency with the torque converter slipping in First through Fourth. In performance modes, it uses torque multiplication for launch, but locking up the converter even sooner allows the engine to rev higher in each gear and improve performance.
There’s a lot more to the 8L90’s performance:
- Mass Savings: The 8L90’s engineers used aluminum whenever possible, and even magnesium in the piston housing and valvebody. Magnesium was not used at all in the 6L80. More than 550 fluid-dynamics simulations were run to know where to add lightening holes. The tone wheel for the speed sensors had to be steel, but their size was minimized. Thin wall castings were used whenever possible.
- Power Handling: Box input torque capacity, which is after torque converter multiplication, is 1,000 newton-meters—or 738 lb-ft. LT1’s version of the 8L90 is nearly identical, the LT4 version has an additional clutch pack, and the torque converter clutch has higher rate springs.
- Fluid Control: Dexron High-Performance synthetic fluid was developed for the 8L90 for better cold performance. It is a lower viscosity than previous fluids, which allows for smaller passages, meaning everything can be scaled down, saving weight. Transmission oil is a complex fluid, it has lower change intervals than engine oil, it affects clutch friction, it’s a lubricant, and it’s a hydraulic fluid. The fluid moves to the bottom of the pan, but where? Fluid simulation helped find common intersections of oil flow. There’s a shield around the pump drive chain to keep it from whipping up the oil. Capacity is 11 quarts, including torque converter, and they’re shipped full—the vehicle assembly plant doesn’t add fluid. The added benefit of the computer controls and quick shifts is less heat. That means greater longevity for the trans and that smaller and more efficient clutch packs can be used.