The race to higher gear counts is in full swing, and automatic gearboxes
are leading the way. Driven partly by marketing one-upmanship and
partly by the need to meet ever-tougher fuel-economy mandates,
eight-speed autoboxes are everywhere, Chrysler’s running a nine-cogger in select models, and 10-speeds—from Ford, GM, Volkswagen, and others—are just around the corner. The General’s own in-house-developed eight-speed units
(for its trucks and rear-drive V-8 cars) are just making an appearance
after work on them had all but stopped during the company’s plunge into
bankruptcy in 2009.
Now the Corvette
gets updated with its rear-axle-mounted version of this new eight-speed
automatic. Why is this news? While we’re firm believers that manual
transmissions still provide a higher and more nuanced level of organic
driver involvement and control, it’s no secret that high-performance
cars are often faster and quicker during limit testing when equipped
with the automatic gearboxes. Ferrari and Lamborghini have gone
exclusively to automatics, and the new Porsche 911 GT3 and the 911 Turbo are automatic-only. One might say that Chevrolet is taking a page from the Porsche playbook by even offering an automatic in the forthcoming Z06—a
model that had been limited to manual-transmission offerings in the
past. Its supercharged LT4 small-block V-8, with 650 horsepower and
650-lb-ft of torque, will be the most powerful engine General Motors has
ever produced. So don’t think of an automatic, even if it is a more
common planetary-gear type rather than the more exotic dual-clutch
variety, as a handicap the Z06 must suffer.
Gears With Benefits
How can we be so sure? Chevy hasn’t shown us a running Z06 in the flesh,
but the company did let us sample the new eight-speed automatic in the
2015 Stingray and Chevy claims that the new ’box is quicker in a
straight line than the manual. We discovered that the old six-speed auto was quicker than the manual version
with individual tests of Stingray convertibles, but knocking another
tenth off of the zero-to-60-mph and quarter-mile times puts the
estimated times of the eight-cog Vette perilously close to those
recorded for the C6 ZR1 in the first metric and nearly into the 11-second range in the second.
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The new GM 8L90 eight-speed automatic, which will also go to work in the
2015 full-size trucks and SUVs equipped with the 6.2-liter V-8, doesn’t
hamper the fun one bit. On a road course, taking hot laps in an
automatic that performs ignition-spark-cutting upshifts and
positive-torque downshifts (engineer jargon for blipping the throttle to
match revs) quicker than humanly possible allows the driver to
concentrate on the steering and braking, potentially turning quicker
laps in the process. Plus, from our seat-of-the-pants perspective, the
GM 8L90 shifts as quickly as any dual-clutch unit available. Corvette
engineers benchmarked the ZF’s dual-clutch automatic that goes into the
Porsche 911 and claim the new GM ’box actually shifts quicker in some
cases.
Goin’ With the Flow
Several laps of GM’s Milford Road Course revealed that the transmission
programing in good-old “D” with the car’s Performance Traction
Management switched on is pretty good for track duty. It’ll hold gears
up to redline, downshift by itself just as one might do while tapping
the steering-wheel paddles, and automatically upshift when it should. A
two-three midcorner upshift doesn’t upset the chassis as it might with a
manual transmission because the torque flow at gearchanges is smoother.
We’re not saying the automatic is more fun to drive than the
seven-speed manual, but don’t be surprised if the next Corvette we put
on a racetrack has an automatic.
GM wouldn’t confirm whether the automatic Corvette is quicker on the
track, but the Z51-equipped cars are certainly built for it; they will
lap continuously for a full tank of fuel without overheating. When the
8L90 goes into the Z06, it’ll have two transmission coolers—one in the
hindquarters, as with regular Stingrays, and another farther up the
chassis laying flat on the underbelly.
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Through ample use of aluminum and even some magnesium (a first for GM
automatics), the new 8L90 weighs eight pounds less than the six-speed
6L80 it replaces, so curb weights should not increase for 2015. The new
transmission gets its eight ratios, with a 7.02:1 spread, from four
simple planetary gearsets and five clutches, three of which are always
closed for increased efficiency. And speaking of efficiency, Chevy says
it missed its goal of achieving a 30-mpg EPA highway rating by the
narrowest of margins—one hundredth of a mpg. Corvette chief engineer
Tadge Juechter says GM will retest the car next year with an extra 2 psi
in the tires in an attempt to attain the 30-mpg label. That’ll require
Chevy to change the recommended pressure on the door-frame placard and
then print in the owner’s manual that it is okay to run at a lower
pressure for “comfort.” That’s a trick Porsche’s been doing for years.
Watch this space for a full test of the eight-speed-automatic 2015
Stingray as soon as we can hook up our gear to one. All that talk about
another mile per gallon is nice, but we’ll let you know if our
prediction of a 0.1-second quicker zero-to-60 time compared with that of
our last test of a six-speed automatic Vette—down to 3.6 seconds—pans out.
BY: K.C. COLWELL
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