When news arrived earlier this year that the Chevrolet Corvette’s touted eight-speed automatic transmission would migrate to GM’s full-size trucks, the C/D office responded with a resounding, “Well, that makes sense.” Don’t get us wrong, we love trucks almost as much—more, in rare instances—as we do Corvettes, but it can be hard to make headlines in the wake of the C7 Corvette’s brouhaha. Well, now we’ve sampled the General’s eight-speed pickups and SUVs.
Although GM kept our drives to a short route of nonchallenging terrain, we did get seat time in both a 2015 Chevrolet Silverado with the Z71 package and a GMC Yukon Denali, each equipped with the 6.2-liter Ecotec3/eight-speed combo. First impressions? Low-speed drivability improves by comparison to the outgoing six-speed. Under light throttle, the shifts click off in rapid-fire succession, occurring somewhere between 2000 and 2500 rpm. Leaning harder on the accelerator elevates the shift points to around 3500 rpm, all the while remaining smooth, precise, and never busy; were we not watching the action on the tachometer, gearchanges could have gone unnoticed. Floor the pedal and the eight-speed lets the 6.2-liter V-8 spread its wings, holding each gear to about 5750 rpm, just short of the 6000 fuel cutoff.
According to fueleconomy.gov, the 2015 Chevy Silverado/GMC Sierra equipped with the 6.2-liter V-8 and the eight-speed automatic are rated at 15 mpg in the city and 21 mpg on the highway. That’s an improvement of 1 mpg in both measures for the four-by-four; the two-wheel-drive trucks retain their 15/21-mpg rating from 2014 to ’15. Combined ratings remain at 17. The EPA site says the 6.2-liter GMC Yukon 4WD gains a significant 2/3 mpg city/highway compared to the same engine with the six-speed; that means it matches last year’s 5.3-liter, six-speed SUV (which picks up 1/1 mpg for 2015).
GM says the eight-speed’s 7.0:1 spread of gear ratios is wider than that of its six-speed units and allows a numerically higher first-gear ratio. While its primary benefit is providing smoother takeoffs with less strain on the engine when carrying heavy loads or towing, it also contributes a sense of low-speed lightness and agility in tight parking-lot maneuvers requiring multiple-point turns. The eight-speed also enables taller rear-axle ratios, reducing engine rpm on the highway—which, incidentally, wasn’t part of our drive. We did manage to find a short stretch of roadway suitable for a 50-mph steady cruise that had the transmission acting as the engine’s silent partner. Stepping on the throttle prompted an instant response, downshifting to put the engine in the meat of its torque curve. Based on our limited exposure, the eight-speed automatic makes for a much-improved dance partner for the 6.2-liter V-8.
We already know that GM’s eight-speed in Corvette trim is lighter, quicker-shifting, and more efficient than the six-speed it replaces, but to get the lowdown on how the 8L90 automatic’s hardware was prepared for truck duty, we checked in with Kavoos Kaveh, GM’s global chief engineer for six- and eight-speed automatic transmissions. According to Kaveh, the power flow and gear architecture remain identical, but a single clutch disc was added and the torque converter was tuned specifically for this application with some truck-specific internal parts.
Trucks needed their own bellhousing to mount the unit to the engine (the Corvette has a rear transaxle, remember), and a different case extension accommodates variables between the rear- and four-wheel-drive configurations. Although the 8L90 actually reduces weight in the Corvette, GM notes that the eight-speed in truck guise “is roughly the same size and weight as the 6L80 six-speed automatic” it replaces—chalk up at least some of the reason to the truck-specific bellhousing.
- Chevrolet Silverado Research: Reviews, Specs, Prices, Photos, and More
- Chevrolet Silverado 6.2L 4×4 Tested: A Smooth Operator with a Heavyweight Punch
- GMC Yukon 4WD Tested: Go Big or Stay Home
Although 2015 model-year production started with the six-speed, now that the eight-speed 8L90 automatic transmission is ready it becomes standard on 2015 Chevrolet Silverado, GMC Sierra, and GMC Yukon models equipped with the 6.2-liter V-8. Shoppers should check which transmission is in any truck on the dealer lot before making an offer. You might negotiate a bigger discount on a six-speed, but the new hardware is preferable.
from Car and Driver Blog http://ift.tt/1sRc2fJ
via Agya
