Overachiever

With a supercharged engine, ultra-wide tires and downforce-producing bodywork, the 2015 Z06 sets a new standard for Corvette performance—while offering the convenience of an automatic transmission and the open-air fun of a lift-off top.

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March 14, 2014

When the 2014 Corvette Stingray was unveiled last year, nearly everyone’s attention was focused on the car’s somewhat radical styling; its new, more powerful engine, stiffer aluminum chassis and host of sophisticated, driver-oriented technologies played second fiddle. As soon as the media got behind the wheel of a C7, however, the focus shifted to the new car’s driving experience. In short order it became abundantly clear that the new Stingray delivered performance on par with the sixth-generation Corvette Z06. This was a crystal-clear omen that any higher-output edition of the C7 would utterly rock the sports-car world, which is exactly what happened when Chevy rolled out its 2015 Z06 at the Detroit Auto Show. This supercharged tour de force out-performs all previous Corvettes, including the C6 ZR1—which is exactly what Corvette Chief Engineer Tadge Juechter challenged the members of his team to achieve.

Engine: Blown Away

The heart of the Z06 is a supercharged 6.2-liter V8 that generates an estimated 625 horsepower. John Rydzewski, Chevy’s Assistant Chief Engineer for Small Blocks, explains how this powerplant, dubbed the LT4, ended up being supercharged—something that’s never been done to a Z06 engine before. “We were looking for a higher-output engine to meet the objectives for what Tadge Juechter wanted in the Z06, which was the performance of a ZR1 with an engine that’s as compact and efficient as possible,” says Rydzewski. “We looked at several different options for how to do this, from a naturally aspirated, large displacement, ‘spinner’ engine to various methods of boost, and we determined that the best solution is a compact supercharger design on the existing LT1. This was the most elegant solution because it was least disruptive to the existing vehicle.”

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In order to minimize changes to the C7, the Corvette team needed a supercharger compact enough to fit under the standard hood, yet potent enough to reach their power targets. Such a supercharger didn’t exist, so they worked with supplier Eaton to create one. The newly designed R1740 TVS is 10 mm shorter and 10 mm smaller in diameter than the LS9’s supercharger. As a result, the LT4 is only 25 mm taller than the LT1.

Though design changes such as different rotor spacing and higher speed—the R1740 TVS spins up to 20,000 rpm, 5,000 more than the supercharger on the LS9—yielded greater efficiency, the smaller supercharger does squeeze less air than the larger unit used on the LS9, 1.7 liters of air per revolution instead of 2.3 liters. Emblematic of how Chevy engineers tackled challenges throughout the new car, Rydzewski’s team stretched their thinking to find clever ways to extract more power out of a smaller package.

“We needed to minimize all of our losses to meet our power objectives with less boost,” reports Rydzewski. “We worked a lot on the airflow path coming into the engine, with computational flow dynamics, with a lot of iterations, and we came up with a very efficient flow path going into the supercharger. We also developed an improved discharge port that minimizes turbulence, reducing heat and speeding up airflow through a more efficient intercooler and into the engine.”

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While the LT4 employs the same cylinder case as the LT1, it uses a number of unique parts to handle higher cylinder pressures. Explains Rydzewski, “We added titanium intake valves expecting that the customers who use this would race it and spin it a lot at high speeds. We also added stainless-steel cast exhaust manifolds to handle the added heat; we modified the cam, adding a bit more duration on the exhaust side; and we went with improved, forged pistons.”

The LT4 cylinder-head design is essentially the same as the LT1’s, but the LT4 units are more robust. They are rotocast from A356T6 aluminum alloy, which yields castings that are stronger and better able to withstand high heat. Heat management is also aided by a larger-capacity oil cooler for the otherwise standard Z51 dry-sump oiling system.

Unlike the LS9, the LT4 does not have titanium connecting rods. Says Rydzewski, “We went with a high-strength powdered-metal connecting rod with a lot of machining to take the mass out where we didn’t need it and that way we kept the rotating mass down while at the same time maintaining the safety factors on this engine up where we want them to be.” This decision offers insight into how Chevy keeps adding performance to the Corvette from one generation to the next without commensurately increasing the price of the vehicle.

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Though Chevy has not completed fuel-economy testing and certification, and therefore isn’t sharing numbers or saying whether or not this new Z06 will avoid a gas-guzzler tax—which the naturally aspirated C5 and C6 Z06s were able to do, but not the supercharged ZR1—low fuel consumption was a high priority from the start. Getting to that goal with the LT4 will be helped in large measure by the fact it retains advanced technologies already found on the LT1, including direct injection, continuously variable valve timing and cylinder deactivation.

Transmission: Eight is the new Six

Every previous version of the Z06, going all the way back to 1963, has only been available with a manual gearbox. Owing to advancing technologies and changing consumer tastes, the Corvette team decided to offer the C7 Z06 with an automatic transmission, as well. But the existing 6-speed automatic used in the C6 would not suffice, so Juechter challenged engineer Bill Goodrich and his colleagues in the transmission-engineering group at GM Powertrain to design a new one.

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In essence, the same engineering goals that guided LT4 engine development also drove the creation of the new 8-speed automatic. Compared with the 6-speed automatic it was to replace, the new unit had to be lighter, strong enough to withstand the supercharged engine’s enormous 635 lb-ft of torque, be no larger (and preferably smaller) and better-performing—and it had to do all of this at approximately the same cost as the 6-speed.

Going back to the founding of its Hydra-Matic Division in the 1930s, GM has been a leader in automatic-transmission design, and Goodrich’s tight-knit engineering group drew upon the company’s decades of experience, as well as today’s most advanced computer tools, to meet the daunting goals that had been set for them. “We started with a clean sheet of paper,” Goodrich explains, “looking at various power-flow combinations and options, and what was best for fuel economy, what was best for the speed and torque and how we could achieve packaging size that the Z06 required. Meeting those three goals while also doing it efficiently for fuel economy was our greatest challenge. In trying to package and arrange the components, we made extensive use of finite element analysis throughout the development process to optimize for mass. Mass is obviously a key characteristic when you’re talking about a performance car like the Z06. We’ve made extensive use of aluminum and we have a few magnesium parts inside the transmission. We actually have an aluminum gear set and carrier, which is a first for GM.”

Besides using lighter materials, Goodrich’s team did a lot of mass optimization, putting what they call “metal savers” wherever they could. This means they removed material by drilling, machining or modifying molds and tooling to save metal, and thus mass, wherever it could be done without compromising safety and durability.

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By rethinking the age-old ways some of an automatic’s functions are performed, the team also reduced mass. “In the torque-converter area,” explains Goodrich, “we’ve gone to an optimized drive plate as opposed to specific lugs that have been typical in the past. We have a new oil pump that we’ve moved from the centerline axis of the transmission down into the valve body in the bottom pan, which helped reduce the size, the spin loss and the mass. Those are just examples of things we’ve put in place to try to get the mass down. Getting sprung mass out helps the overall performance of the car, so we want to be as light as possible.”

Even with an extreme performance car like the Z06, when Chevy engineers refer to “overall performance of the car” they are talking about fuel economy as much as anything else. The initial fuel-economy target for the Z06 was a 5-percent improvement over the C6 version. The addition of two more gears obviously helps by keeping the engine in a narrower operating range, where fuel consumption can be optimized. “With the 8-speed, you’re able to have smaller steps between the gear ratios,” says Goodrich, “which allows the engine to run more optimally at its most efficient area. Our first gear ratio is 4.56:1, which allows us to lower the numeric axle ratio. So we have great performance with the first gear ratio and improved economy at the same time. When highway cruising in eighth gear, you can cruise along at about 1,450 rpm at 70 mph.”

Besides benefitting from its extra gears, the new transmission also achieves improved fuel economy by virtue of some very clever innovations. “The basic power-flow design that we came up with, which is the configuration of clutches that are on and off in a particular gear, improves economy,” Goodrich points out. “This power flow is such that it has five clutches and four gear sets and in any particular gear you always have three clutches on and two clutches off, and the importance of that is, the fewer clutches you have off, the less parasitic drag you have. So we devised a power flow that allowed us a two-clutch-open configuration in any particular range or conversely a three-clutch engagement for any particular gear, and this was a key enabler for better fuel economy. And we’ll also be using a synthetic transmission oil with this transmission, which again allows us to operate more efficiently with lower losses. Less parasitic loss means better fuel economy.”

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A strong, light, fuel-efficient automatic that fits the given space would still not have been suitable for the new Z06 if its shifting performance didn’t set new standards for speed and accuracy. “One of the objectives was to get lightning-quick shifts, particularly when you’re using the paddles and commanding the shifts,” says Goodrich. “They need to be responsive, quick, crisp and clean, and we’ve been able to achieve that with our second-generation transmission controls that GM is employing. We’re at the point where we are getting shift time performance equal to and in some cases better than the DCT that’s in the current Porsche 911.”

DCTs (dual-clutch transmissions) are typically used in super-high-performance cars, and are often looked at as the benchmark for short shift times. Because this type of transmission is manual-based, it does not have a torque converter, and with the resulting very low rotating inertia, it is able to shift very quickly. On the other hand, DCTs are not known for the smoothness of their shifts, especially at low speeds. “Our goal was to get an automatic transmission to shift as quickly, and we’re there,” says Goodrich. “So we have the very quick shift with the convenience and comfort of a true automatic. We’re very proud of what we’re able to achieve right now.”

In addition to the new Hydra-Matic 8L80 automatic, which will be manufactured in GM’s Toledo Transmission plant, the 2015 Z06 will also be available with a 7-speed manual gearbox. Chevrolet feels that it is important to offer this choice, especially since its competitors in the 500-plus-horsepower supercar class are increasingly offering just a single transmission.

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Both of these rear-mounted gearboxes are connected to the LT4 engine by a carbon-fiber torque tube. As a result of the harmonic imbalances created by the C7’s cylinder-deactivation system, Chevrolet had to go to a stronger steel torque tube on the Stingray, which added mass compared to the C6’s aluminum unit. The 2015 Z06’s carbon-fiber torque tube has been engineered to handle the LT4’s tremendous amount of twist while being even lighter than than the previous alloy version.

As with the base Stingray, manual-transmission Z06s will come equipped with Active Rev Match that can be turned off via steering-wheel paddles if the driver wishes. Speaking of the steering wheel, it receives a flat bottom on the Z06; other than badging, this is the main difference between Z06 and Stingray interiors. (By the way, the Stingray moniker is indeed dropped from the Z06’s model designation.) However, the C7 Z06’s cockpit offers something new when you compare it to those of its C5 and C6 predecessors: a removable roof panel. Because the new Corvette’s aluminum frame was designed with this feature from the outset, it made sense to include it on the Z06. Even with the roof panel removed, the C7’s structure is 20-percent stiffer than the C6 Z06’s fixed-roof alloy frame; with it in place, it is a full 60-percent stiffer.

Exterior Design:

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Downforce Producer

Not surprisingly, virtually all of the Z06 exterior design changes were driven by functional considerations. According to Corvette Exterior Design Manager Kirk Bennion, one goal the design team began with in transforming a Stingray into a Z06 was to “widen its stance to improve its stability and accommodate wider tires for better grip.” The Z06 comes with Michelin Pilot Super Sports sized 285/30ZR19 up front and 335/25ZR20 at the rear, compared to the Stingray’s standard 245/40ZR18s and 285/35ZR19s. The Z06’s body is correspondingly wider, 2.2 inches in front and 3.2 inches in back.

Added cooling capacity for the engine, transmission, differential and brakes was another important consideration in the track-oriented car’s exterior design. Very large front fender vents exhaust more air from the engine compartment, while unique air blades over the inlets on top of the rear quarter panels channel about 50-percent more air into the transmission and differential cooling ducts than on the Stingray. The Z06 rear fascia, which is wider than the Stingray’s, has larger vents adjacent to the taillights to help expel the added air volume. Large ducts in the corners of the front grille bring cooling air to the front brakes. As has become a Z06 hallmark, scoops in front of the rear wheel openings direct air to the rear stoppers.

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Perhaps the most interesting cooling feature, however, is the new front grille. According to Bennion, the beautifully crafted egg-crate design flows more air than the opening would with no grille at all. “It actually enhances the airflow,” says Bennion, “and that’s due to its construction, and how we used minimum draft angle and how we oriented the shape. You start off with minimum material thickness and you minimally draft that back, so if you were to look at those grille blades, they’re only about 1.5 mm thick but they’re drafted back about 3 degrees—and for a mass-produced car that’s about the outer extreme. What that does over that length is streamline and actually increase airflow.”

Part of the air ingested through the grille goes through a forward-tilted radiator and the supercharger’s intercooler before exiting out the louvered hood vent. This airflow path helps reduces lift. The Stingray utilizes a similar layout, but the Z06’s hood vent is larger, thus enhancing this aerodynamic benefit.

Taking their cue from the Corvette Racing team, Z06 designers went beyond just reducing lift. “The goals for the aerodynamics on this car were much more extreme than what we’ve ever done before,” explains Bennion. “This is an actual downforce car.” Exactly how much downforce a Z06 produces depends on which of the three available aero packages it has.

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In its standard state, the Z06 includes a plastic front splitter, the aforementioned hood vent and the same rear spoiler found in the C7 Stingray’s Z51 Performance Package. A Z51 coupe, according to Bennion, is just about neutral, meaning it has close to zero lift. The base Z06 aero elements generate marginal downforce up front; this is balanced by the Z51 rear spoiler, which actually generates downforce on the Z06 because of the car’s slightly different pitch moment—that is, the car is pitched forward more.

A higher level of downforce is generated by an optional carbon-fiber aero package. It consists of a carbon-fiber front splitter with end plates, carbon-fiber rocker-panel extensions and a larger rear spoiler with fixed wickerbills. The customer will be able to choose between a black-painted finish or visible carbon fiber.

The third and most extreme aero configuration comes as part of the Z07 Performance Package. It includes two different-sized carbon-fiber end plates for the splitter, allowing the customer to adjust the amount of front-end downforce. At the rear, the Z07 package adds a tall, center wickerbill with about one inch of vertical adjustability. It is clear so that it doesn’t interfere with rearward vision.

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Bennion explains the rationale for offering different aero setups: “There’s a diverse customer base. There are people who enjoy driving the Z06 [on the road] and then there are people who really enjoy driving the Z06 on the track. And for those people who want to maximize the car’s potential on the track, we offer the Z07 package.”

The Z07 Performance Package also includes super-sticky Michelin Pilot Sport Cup 2 tires (sized the same as the standard rubber) and larger, ceramic-ceramic rotors. Measuring 15.5 inches in front and 15.3 inches in the rear, these brakes nevertheless weigh 23 pounds less than the standard two-piece steel rotors, which measure 14.6 inches in the front and 14.4 inches in the rear. In both cases, fixed aluminum calipers are used at all corners, with six pistons in front and four pistons in the rear.

While a Z07-equipped Z06 is squarely aimed at buyers who will push the car on a racetrack, there’s nothing inherent in the package that detracts from its every-day drivability. One example of this is the intense development that went into the rocker-panel extensions. “We did a lot of pushing and pulling on the rocker in the wind tunnel, and found that the forward half of the rocker is the most functional part and the back half plays a more minor role,” explains Bennion. “This enabled us to put in a contour there so the customer can get in and out of the car easier. We got the aero benefit we wanted from the rocker without taking anything away from the ease of entry and egress.”

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Final Words: Waiting Game

Sadly, C7 Z06 production is still a long way off; deliveries aren’t likely to begin until early 2015. The only reason it was unveiled so early was because of Chevrolet’s desire to introduce the Z06 and C7.R race car simultaneously. (The latter debuted at the Rolex 24 at Daytona in January, which you can read about on page 82.) The new model is still undergoing final testing, tweaking and certification work. However, the Corvette development team has already achieved one of its principal goals: On its very first outing at GM’s Milford Road Course, the 2015 Z06 was faster than a C6 ZR1. That alone is truly amazing, and a sure sign that we can justifiably expect great things from this new Z06.

While it’s too soon for Chevy to provide pricing details, Corvette Product Manager Harlan Charles did provide these encouraging words: “If you could afford the previous Z06s, you’ll be able to afford the new one.”

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Also from Issue 89

  • 1,000-hp Twin-Turbo C6
  • Buyer’s Guide: C4
  • 1988 Coupe
  • Tech: Performance Data Recorder
  • Harley Earl’s C2
  • 1967 Small-Block Coupe
  • 1969 L89 coupe automatic
  • Racing: C7.R
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