First Look: 2015 Corvette 8-Speed Automatic and Supercharged LT4 V8
The upcoming C7 Z06 will undoubtedly be the quickest and fastest Corvette produced so far, with 650 supercharged horsepower and world-class handling. But one thing about the Z06 that confused and upset a lot of purists was the announced availability of an 8-speed automatic, and a traditional torque-converter-equipped one at that. In a market space where dual clutch transmissions have become de rigueur for sports cars, a “slushbox” automatic seemed like an atavistic throwback to the days when Corvettes were “secretary’s cars,” not finely-focused performance vehicles.
Then, Chevy leaned into it even more and announced that all 2015 Corvettes would be available with the new 8-speed, not just the Z06. We had a chance earlier this year to spend a week driving a Z51 coupe equipped with the current 6L80E automatic, and while we enjoyed the car (and apparently America does too, since the majority of 2014 C7 production was Z51 automatics) we wouldn’t have necessarily picked that car over a three-pedal Stingray if we were spending our own money.
To convince us, and by extension you, of the righteousness of the new 8L90 transmission, General Motors invited us out for a whirlwind visit to their Milford Proving Ground to experience the 8-speed in person, and also get some additional education on the inner workings of the Z06’s LT4 powerplant.
8 Speeds, No Waiting
For those who remember the Powerglide, the Turbo 350, and even the TH700R4, an 8-speed transmission seems almost like overkill, but there are good reasons for the extra ratios. While the top two cogs are overdrive gears on both the 8L90 and the 6L80, 8th is a 0.65 ratio in the new transmission, compared to 0.67 for the 6L80, and Chevy has paired it with 2.41 differential gears in non-Z51 2015 Corvettes (compared to a 2.56 ratio for the 6-speed auto) to reduce engine rpm by 8 percent at 70 mph.
First gear in the 8L90 is 4.56 as opposed to 4.03, and the closer steps between ratios mean improved performance as well as better fuel economy – Chevy is claiming 29 mpg highway, 0-60 in 3.7 seconds, and an 11.9 second quarter mile for the 8-speed LT1. Overall, there is a 7.0 ratio stretch between first and eighth gear.
Dimensionally, the new transmission fits in the same “box” as the 6L80, and is actually 8 pounds lighter thanks to extensive use of magnesium and aluminum where appropriate. Per Bill Goodrich, Assistant Chief Engineer, 8-Speed Transmissions, the 8L90 incorporates more than 24 new patents, and has a lot of features not seen before in GM automatic transmissions.
One important change is an off-axis pump – rather than the typical pump layout that’s concentric to the input shaft, the 8L90’s pump is mounted in the valve body, and driven by a chain. The pump is also a “binary” design, meaning that during times of low pressure/volume demand, part of the pump can be shut down, resulting in 60 percent less torque required by the pump. The low mount also means that the pump primes better in cold weather, and the transmission has been extensively tested on a “tilt rig” transmission dyno to ensure that even during high-g cornering, the 8L90 maintains a full fluid supply.
In another departure from the outgoing 6L80, the new 8-speed uses an external T87 controller instead of internal electronics, allowing for more processing power and faster computational speed. The 8L90 also picks up a third speed sensor, so that in addition to keeping track of input and output shaft speeds, the new transmission can also monitor internal rpm as well.
But It’s Still An Automatic…
Of course, all of that doesn’t sound too exciting to the person who feels that there’s no substitute for a clutch and an H-pattern shifter. While the new C7 has the highest “take rate” for manual transmissions of any GM vehicle, that still only means that 37% of them are leaving Bowling Green with three pedals – most Corvette buyers simply prefer the automatic.
To win over manual transmission purists, GM put a lot of effort into making sure that the 6L90-equipped Stingray doesn’t give up any performance to its MT counterpart. One of the benchmarks used by the Corvette team was the dual-clutch transmission found in current rear-engine Porsches, and per Kavoos Kaveh, Global Chief Engineer 8-Speed Transmissions, the 8L90 shifts .08 seconds faster than the ZF-sourced Porsche DCT.
As with the previous automatic, the 2015 Stingray is equipped with paddle shifters, or can be left in “drive” for fully automatic operation, with different shift strategies determined by the mode selected by the driver.
So what’s it like to drive? To find out, GM let us lap a set of 8L90-equipped 2015 Stingrays at their Milford test center in Michigan. The Milford track, which we have visited before for the Z/28 technical introduction, is a course that combines features from some of the world’s most challenging road courses into one amazing roller coaster ride.
Inside the LT4
Jordan Lee, Global Chief Engineer Small Block Engines, and John Rydzewski, Assistant Chief Engineer Small Block Engines took us through some comparisons to the C6 ZR1’s LS9 powerplant:
- The LT4 is approximately 20 pounds lighter than the LS9, and 75mm lower overall thanks to an all new intercooler and plenum layout
- The LT4 is only one inch taller than the naturally-aspirated LT1
- The LT4 uses a new-generation R1740 TVS rotor pack with a 160 degree twist, with a 1.7-liter displacement compared to the LS9’s 2.3-liter supercharger
- While the LS9’s supercharger turned at a maximum of 15,000 rpm, the new LT4 supercharger maxes out at 20,150 rpm
- Peak boost for the LT4 is actually lower than the LS9, at 9.4 psi instead of 9.7
The LT4 and LT1 share the same basic cylinder head design – per Rydzewski, “The LT1 ports ‘scale’ well,” though the heads themselves are Rotocast A356-T6 aluminum for added strength, and feature larger 65.47cc combustion chambers, compared to the LT1’s 59.02cc chambers.
Combined with a new piston design in place of the LT1’s domed slugs, compression ratio is lowered to 10.0:1 to accommodate forced induction. The fuel system has been upgraded, with 25cc/second injectors replacing the LT1’s 20cc units, and a higher flow direct injection fuel pump running at 2,900 psi compared to the LT1’s 2,175 psi.
We asked Rydzewski about the possibility of using the LT4 fuel system in conjunction with aftermarket forced induction on the LT1, and he indicated that as long as there were no clearance issues with the manifold, such an arrangement should work.
Other differences include a camshaft with longer exhaust duration (189 degrees intake, 223 exhaust, compared to 200/207) and a revised cam timing map optimized for boost. The combined dual intercooler cores are 23 percent smaller in total volume than the single “brick” used in the LS9, but thanks to an improved design they have 10 percent greater heat rejection.
The net result is an engine that produces 12 more peak horsepower and 7.5 percent more peak torque than the LS9, and a whopping 208 pound-feet more torque than the LS9 at 3,500 rpm. With the smaller rotor pack, higher step-up drive ratio, and a more direct discharge port, the new LT4 supercharger is optimized for “area under the curve” on the horsepower and torque chart.
When combined with the 8-speed automatic to keep it centered firmly in its broad “sweet spot,” we’re expecting explosive acceleration at any speed from the Z06.
With that said, we tried to keep a 7/10ths pace on our two hot laps of the track, and managed to only get a bare handful of wildflowers stuck in the Corvette’s grille in the process. How does the automatic perform when it’s “out of its element” on a race track?
The best way to sum it up is that the 8L90 was never in the “wrong gear” at Milford. It was a lot like driving in automatic mode in Grand Turismo – it felt almost like a cheat to be able to just concentrate on the throttle, brake, and steering and let the car keep the 1LT in its sweet spot by handling gear changes.
In the Real World, the automatic felt more like a traditional setup. Instead of instant up- and downshifts we experienced on the track, there was the familiar calibrated-for-comfort part throttle hesitation between gears, no doubt intended to stave off complaints from non-enthusiast Corvette owners about “harsh” shifting. Jump on the go pedal at 70 on the highway, though, and the 8L90 dropped down a ratio or four without hesitation.
Our road loop driving partner, Vette magazine’s Steve Rupp, did manage to finally confuse the 8-speed with a combination of quick application of full throttle followed by backing off immediately as soon as the transmission began to shift.
To the 8L90’s credit, though, it held the lower gear rather than jumping back up, almost as if to say, “OK, figure out what you want to do, but in the mean time I’m gonna stay here in case you want to accelerate hard.”
Basically, it takes determination, stupidity, or a combination of the two on the part of the driver to get this transmission into the wrong ratio with the lever in D.
The Bottom Line
When we drove the C7 for a week earlier this year, we were impressed with the car “even though” it was the 6L80 automatic rather than the clutch car we had wished for. It’s an easy car to drive, an absolute assassin at the dragstrip with launch control engaged, and a pretty decent corner carver when paddle-shifted.
But if you asked us what we’d buy for ourselves, we’d all have told you that we’d take the stick. The 8L90 is a game changer for this car, though – it’s good enough that we’d take it instead of the manual without remorse.
It’s refined to a level we’ve never experienced in a GM automatic before, and we’d be surprised if there was even a tick of difference in lap times between the 2015 automatic and manual Stingrays, unless piloted by someone better than 99.9% of anyone who buys one.
What about the Z06, though? Despite how good this transmission is, there will undoubtedly be a chorus of indignation from manual fanatics (none of whom have actually driven an 8L90 Stingray) when the car starts to hit the streets.
Though we will have to wait until January to see for ourselves, our initial experience with the 8-speed backing the LT1 makes us feel like it would be a winning combination when paired with the LT4 as well.