Unless you've been living in something akin to tropical-island isolation for the past few months, you've already committed certain information about the recently announced ZR1 to memory. You know the skinny on its 620-odd-horsepower engine, probably because you read about it here ("LS9 Lives!," April 2008). You've likely also heard other anecdotal tidbits from outside sources, like the origins of the "Blue Devil" nickname, how the car was almost called "SS," and how the name "ZR-1" (as originally used on the LT5-powered cars of 1990-1995) will now do without its hyphen.
While whispers like who approved what in hush-hush board meetings may be interesting yarns of some historical significance, they have little to do with what die-hard GM muscle enthusiasts are really concerned with: the hardcore details on the 2009 Corvette ZR1's features. Below you'll find none of the former and all of the latter, so get ready for the most up-to-date info we've been able to uncover on GM's supercar.
Body / Chassis
The wonders start with the structure of the car. Not surprisingly, the ZR1 ditches the base Corvette steel frame for the Z06's Dana-built aluminum one--the reason of course being for weight savings. Even so, an optioned-out ZR1 will tip in somewhere close to 200 pounds heftier than a Z06's 3,162-pound curb weight. Why? Primarily, it's the powerplant. GMHTP readers are surely aware that all aluminum-block LS engines weigh almost exactly the same. The problem comes in with the LS9's supercharger system and its related plumbing. Plus, horsepower makes heat, so some of the additional mass comes from thermal insulation having been added to the car to ensure the passenger compartment stays cool.
Extensive efforts by engineers helped keep the car's total mass increase to a minimum, with greater use of exotic materials that wouldn't have made the cut in the ZR1's budget-minded siblings. Carbon fiber is now used on the front fenders (which are now wider to help accommodate the ZR1's meatier rubber), as well as on the hood, roof panel and bow, front splitter, and rocker extensions, but actually not on other parts like the rear fenders. Jim Danahy, chief engineer for C6 Vehicle Programs, told GMHTP: "We tried to take weight off the front and top of the car, so that we could keep a good balance of the weight distribution and keep the center of gravity low as well." Understandably, Chevy isn't bashful about its carbon fiber, and through a high-dollar protective clear coating, shows off as much of the stuff as aesthetically possible.All told, the ZR1's projected 3,350-or-so pounds of max curb weight will even be about 100 pounds portlier than a Corvette convertible--and it's weight distribution is less optimal than the superbly-biased Z06, too, with a bit of a forward tilt (around 52/48). But though it may not be the lightest amongst the bunch, the ZR1's power-to-weight ratio will still trump that of competing cars made by the likes of Porsche and Ferrari (whose 620hp, V-12-powered 599 tips the scales at over 3,700 pounds). But power-to-weight ratio is just one of myriad factors that affect a vehicle's actual on-track performance, so while superiority in this area is a great thing to have going into a fight, only time will tell how the ZR1 performs against the competition in a balls-out racetrack romp. Exactly how this will play out will probably have much to do with the ZR1's suspension.
Exceeding 1 g of lateral grip is no small feat, but Chevrolet says the ZR1 will do it. How? The role of fat rubber in achieving this peak number can't be ignored, but a sophisticated suspension system allows the ZR1 to shine no matter what the road condition. Though it uses the same basic front/rear SLA setup of other Corvettes (albeit with tweaks like larger anti-roll bars), the big news is the ZR1's use of a retuned version of the Magnetic Selective Ride Control that is optional on base Vettes. Many readers are probably already familiar with this system: by using special fluid exposed to magnetic fields, the damping properties of its shock absorbers can be altered in only thousandths of a second. This allows the system to adapt to driver inputs and changing conditions of the road virtually instantaneously.
Recently upgraded to Gen II spec for the 2008 model year, the MR system (as the engineers call it) is considered by most a near-$2,000 luxury alternative to the Z51 performance package on base Vettes. But for the ZR1, it's been retuned to allow for a top-notch suspension good enough for a supercar--with a ride quality that, although not soft per se, is said to be significantly more compliant than that of a Z06 when set in "touring" mode. A good part of the latter has to do with the MR system enabling the engineers to soften the rates of the composite leaf springs: the front spring is that used in Z51 cars, and the rear uses a modified Z06 spring with a bit less material. "In the past, we engineers had to compromise between ride quality and the way the car handles on a racetrack," says Danahy. "But with the MR system, you get the best of both worlds, as its variable damping allows the system to be tuned both for the track and for the street. [Corvette Chief Engineer] Tom Wallace likes to say this is a good car for going to get bread and milk--and it is. It's really a nice car to drive on the street!"
Delivering the best possible amount of grip to the tires along with a smooth ride is great, but the ZR1 also features a new, previously untapped side benefit to the MR system that is probably of additional interest to GMHTP readers (many of whom are, shall we say, "drag-happy"). Anyone who has driven a stock C6 Z06 surely knows that car's amazing propensity for traction, even from a slow-speed throttle stab. The situation can quickly change on hopped-up cars with aftermarket turbo kits and the like, so it's no stretch to imagine the additional 130 lb-ft (give or take) of blower-induced twist that will be provided by the LS9 would roast meats of any width in similar fashion. The MR system provides one nice solution by automatically detecting a high rate of lift on the front of the car and kicking the damping of the shocks down to around 33 percent, allowing maximum weight transfer to the back tires. Damping rates can then be modulated up to around 90 percent to hold it there as the car begins to accelerate. "So basically it allows the car to sit, squat, and then go. It's like an electronic traction bar," says Danahy. "It works great and we didn't have to engineer any new hardware. We just changed some 0s and 1s!"
Lots of horsepower and an adept chassis with twisty-capable suspension are great, but they're nothing without the ability to slow the car in a hurry. That's why the ZR1 gets special six-piston front and four-piston rear Brembo calipers--so special, in fact, that the front pads have double the total surface area of those on the Z06! Interestingly, a more conventional single-pad design is used, unlike the Z06's one-pad-per-piston setup. We're told this is just the way Brembo does it. Perhaps more significant are the rotors, which are made from carbon-fiber-reinforced ceramic silicon carbide. This material has only recently made its way from racing into high-end production vehicles. In addition to the advantage of being light weight, it is so resistant to wear, heat, and corrosion that the average owner will never need to replace his or her rotors. They're also vented and cross-drilled, measuring 15.5 inches in diameter up front and a half-inch less in the rear.
The biggest thing that went through our minds when we saw the press release on the LS9's 600-odd lb-ft of torque was, "How the heck is the rest of this Vette's drivetrain going to hold up to such a mind-numbing amount of factory twist--especially for the tens or hundreds of thousands of miles a factory car will need to go?" To get an answer, we decided to get on the horn with Scott Kline, assistant chief engineer for Manual Transmissions and Clutch Systems at GM Powertrain. "As you can imagine, the torque envelope continues to be pushed as the Corvette keeps gaining power," says Kline. "My group is always trying to stay a little ahead of the engine guys--as they keep getting more and more power out of the engines, we keep trying to figure out how to get it to the ground. With the ZR1, we've managed to do that with what I think is a very pleasing package--at least from the driveline back, the customer won't really notice anything different from the base and Z06 Corvettes. It just feels like you are in any of these very refined cars." Kline was more than happy to give us the lowdown on what's going to keep the ZR1's driveline in one piece--and how his team's innovations might benefit owners of lesser Corvettes.
GMHTP readers are surely familiar with dual-disc type clutches. Heck, we even installed one in our own 2006 Corvette project car ("Preemptive Strike," August 2007). They can be a great way to provide superb clamping power along with low-pedal effort, and that's exactly why the GM Powertrain team worked with ZF Sachs to come up with the ZR1's 260mm twin-disc clutch. "The current clutch in the Z06 and base Corvette is a 290mm single-disc," says Kline. "To work with the LS9's added torque, we needed additional clutch surface area, so we had started down the path a couple years ago of developing a single-disc clutch for the ZR1, which I think at the time was 310 mm. The problem was, we had a rather small packaging space to work with and we also wanted to make sure we kept a comfortable pedal load and low inertia. This way, the customer would not feel the high inertia of the clutch assembly both in shifting and in just trying to accelerate the engine."
The switch to a dual-disc clutch proved just the ticket to meet all of these goals, and when coupled with refinements to the clutch pedal over center spring, it has resulted in pedal effort nearly identical to that of the Z06. Though the clutch/flywheel assembly of the ZR1 carries about a 3kg mass penalty over that in the Z06 and base cars, its double-disc design means the mass is concentrated closer to its axis of rotation--so the actual increase in inertia is small (about 4.1 kg-m2 vs. 3.8 kg-m2 for the base/Z06). It's a great system, but, for you existing owners out there, don't get too excited: this clutch won't play with other Corvettes due to its unique flywheel that only fits the 9-bolt flange of the LS9's crankshaft.
Upgraded Torque Tube
The so-called torque tube connecting the engine to the rear-mounted transmission receives larger internal driveshaft couplings to help handle the monstrous output of the LS9. For those who want it, this item will be a direct swap into base and Z06 Corvettes, except that the area where the slave cylinder bolts up is slightly different (thanks to the increased stack height of the dual-disc clutch)--though Kline reckons the aftermarket can easily remedy this. Incidentally, bolting to the top of the torque tube is the same shifter found elsewhere in the Corvette line. The improvements made for MY 2008 are carried over for, as Kline puts it, "the same very pleasing shiftability of the base car in this much higher-performance vehicle."
Enhanced TR6060 Transmission
Another factor in the performance and durability of the ZR1 drivetrain is of course the transmission, which is an evolved version of the T56 we know and love. "The TR6060 launched with the 2008 model year, and that was just one more step in taking a transmission that 10 years ago was capable of 350 lb-ft and getting it up into the torque ranges of today," says Kline. Features like increased gear widths and a new synchronizer package all carry over into the 2009 ZR1, but the new car also gets high-strength materials, like an SAE 9310 input shaft and SAE 4615 mainshaft.
Additionally, fifth and sixth gear now sport higher-strength SAE 8822 material. Surely one of the reasons for this is that they'll be used to their full ability. Thanks to its application-specific close-ratio gearset, the ZR1 will make better use of its overdrive ratios and will actually top out in top gear--a first for six-speed Corvettes. Rpm drop during gear changes will be minimized--and first gear (by carrying a taller 2.29 ratio) also allows better traction at low speed, including during a launch. Other slight changes, like alterations to the gear helix angles were made for the ZR1, but despite all of its strength-enhancing features, GM says there's no driver-perceptible difference between this car's tranny and the other TR6060s in the lineup. "It's a very nice performance package, with the same type of no-noise operation that you expect out of the Tremec products--but with the strength to handle the torque of the LS9," says Kline.
Beefed-up Getrag 626 Axle Assembly
During development, the ZR1's rear axle assembly underwent a whole host of improvements over the Z06 unit--and with the exception of a new Dexron LS gear oil (which features better performance at high temperatures), none of them trickle down to other Corvettes. This is primarily due to expense concerns, but it's also because GM considers the axle assemblies in the base and Z06 cars adequately strong for their applications. (We'll bet that some hard-charging enthusiasts with modded Vettes might disagree with this latter assertion!)
Most significant is the Getrag 626's main housing, which is now made from a high-grade 357-T6 sand-cast aluminum. Ditto for the right side diff cover. The left side cover, forged on the Z06, is now machined from SAE 4140 steel billet, and it's "a very pretty piece," according to Kline. The ring-and-pinion is still a 3.42 ratio, but it's made from tougher material and is single shot peened. The differential output shafts are now high-grade 4340 steel. Other material changes, including to the differential itself, exist throughout the axle assembly. The great news is that Kline says many of these items should swap into non-ZR1 Getrags without much trouble.
Specially Engineered Axles
The axle halfshafts themselves were not developed by GM Powertrain, but rather by Corvette platform engineers. Aside from being high-strength, Jim Danahy told us that a 33mm halfshaft is used on the passenger side of the car, while a 40mm unit is used on the driver side. Why the drastically different diameters? "It helps to decouple the resonant frequencies of the individual sides of the suspension," he says. In other words, it's to combat wheelhop. It's a nifty solution that we'd like to see more widely implemented on other GM performance cars.
A brief glance at the ZR1's rolling stock can quickly become an hour-long stare. Even disregarding the impressive look of the carbon-ceramic brake rotors, the 20-spoke wheels (which can be had optionally in chrome) exude badass-ness. Tires are application-specific Michelin Pilot Sport 2--they're friggin' huge: P285/30ZR19 in front, P335/25ZR20 out back (the wheel/tire package does account for a bit of the car's weight increase over a Z06). In case you're wondering, the ZR1-specific dual vents on the front fenders are open to the engine compartment, acting as an additional outlet for heat to escape.
There you have it: the Z06 will officially be dethroned as King of Corvettes in summer 2008, when the ZR1 will be unleashed to the couple thousand U.S. buyers lucky enough to get their hands on one each year. The Z06 certainly has held its own against the competition (and is easily a hands-down winner in terms of value), but what remains to unfold is whether this reborn King of the Hill cannot just take on, but lay waste to the latest Dodge Viper and mutual European exotic competition. We're as anxious to see this pan out as you are. With the field's variety of engine layouts and power-to-weight ratios, it should be interesting indeed.
No matter what happens, it seems the cost-no-object Corvette ZR1 will still do its job for less money than anyone else. To the few fortunate readers who will buy ZR1s (or even just get the chance to drive one), all we can say is: Enjoy the ride, 'cause it's gonna be awesome!
GMHTP would like to thank Jim Danahy and Scott Kline, as well as Bob Tripolsky of GM Product Development Communications and Tom Read of GM Powertrain, for their assistance with this article. C