What happens when you apply military-spec engineering to an automotive project? A nuclear-tipped, road-hugging cruise missile, that's what. Of course, creating a high-caliber, 760hp Corvette takes some serious brainpower-just throwing on some wings and things won't give a Vette a ballistic leap in performance. No, you need a company like Pratt & Miller Engineering, a full-service concern that specializes in design, development, and manufacturing for the motorsports, OEM automotive, aerospace, and defense sectors-a potentially explosive synergy, to be sure.
To light the fuse on the weapons payload of its latest Z06-based C6RS Corvette, P&M sought assistance from Brian Thomson of Thomson Automotive, who does a lot of development work on engine blocks for General Motors. Rather than trying to custom fabricate a new supercharger mount for a 6.2-liter LS9 crate engine, Thomson felt P&M could achieve prodigious output more efficiently and cost-effectively by using the 7.0-liter LS7 block as a foundation, and then adding on the LS9 intake and heads. That way, the ZR1 Eaton supercharger would fit right on-no fuss, no bother.
Of course, bolting on a blower case was the easy part. Optimizing the air /fuel mixture was the real challenge. To increase airflow from the puffer to match the increased cylinder displacement, Thomson reduced the size of the pulley so it would spin at 18,000 revs-2,000 more than on a stock LS9. He also richened up the fuel ratio by throwing on a set of bigger, 65-pound injectors, which required increasing the flow from the pump. Lastly, he switched out the factory E40 engine computer for the LS9's E67 unit, which provides a fatter fuel map.
Given the 10 pounds of pressurization (the same as on the LS9, but feeding a bigger volume of 427 cubes), Thomson went to Diamond Racing for a set of Teflon-coated, forged-aluminum, ceramic-topped pistons, dished to lower the compression ratio from 11.6:1 to 9:1. Fitted with bigger rings to prevent blow-by and oil consumption, they spin through a bore and stroke of 4.125x4.000 inches. The LS9 heads didn't need any porting, considering the final dyno numbers, but they were equipped with 2.16-inch titanium intake and 1.59-inch Inconel exhaust valves.
The result? An explosive 760 horses, along with an outrageous 830 lb-ft of torque. Although turning the blower faster (Eaton admits it can spin as high as 20,000 rpm) or swapping cams would unleash even more power, Thomson feels that would be overkill. As it is, there's more than enough impact at ground zero to make everybody run for cover.
But we didn't duck into the bunker when this bomb landed, paint still drying on the blower case, at Mid America's Funfest in late September. We took aim with both camera and a deft foot on the throttle (well, stomping with our clodhoppers, actually), and started shooting at anything that moved. Turns out we were the ones really moving, as the traction control struggled to keep from being overwhelmed by an exquisite excess of power. We whipped around the placid farmlands of Effingham, Illinois, like we were impossibly late for a cornhusker's convention. Farmhands dropped pitchforks and cattle stopped in mid-chew as we whooshed by like a prairie tornado, a screaming yellow zonker scorching the local landscape.
The hot mill is only part of the C6RS story, though. Pratt & Miller incorporates other aspects of leading-edge R&D from its Corvid Technologies division. Based in the heart of stock-car country in Mooresville, North Carolina, this firm exemplifies P&M's scientific approach. Although initially conceived to bring state-of-the-art engineering and high-performance computing to competition cars (supporting a variety of GM Racing programs in the ALMS, SCCA, Grand-Am, and NASCAR racing series), Corvid also provides proprietary services to military and aerospace clients such as the U.S. Navy's Aegis Ballistic Missile Defense program.
Working smarter, not just harder, Corvid employs an in-house supercomputer and Computational Fluid Dynamics (CFD), a "virtual wind tunnel" that mathematically simulates airflow around a vehicle. (And we thought high school calculus was a challenge!) Indeed, the aerodynamics of the championship-winning Corvette C6.R race car were developed entirely on a computer screen using Corvid's CFD programs. The company also draws on its massive computing power to tackle finite-element analysis, shock physics, and structural mechanics. (We won't pretend to know what all that means, but it definitely gives them an edge on the track.)
No surprise, then, that when the C6RS was first conceived a few years ago, it cloaked the Corvette in the same material used on most military aircraft-carbon-fiber. In the interim, the aluminum-framed ZR1 came on the scene as well, using this same high-tech laminate on the fenders, hood, B-pillar, and roof, along with the front splitter, rocker extensions, and rear spoiler. But the C6RS goes several steps further in order to optimize aerodynamics.
The aggressively styled lightweight body measures 1.6 inches wider than a production Z06. The carbon-fiber front fascia has integrated brake ducts and ram-air induction. Aero enhancements include a functional front splitter, front fender louvers, a carbon-fiber "under wing" and rear diffuser, and enlarged front wheel vents and rear brake ducts. There's even a newly available active wing that's recessed into the rear lip and which deploys at higher speeds to increase downforce-and that's not just theory.
Drawing on computer simulations of running flat out at 200 mph, P&M's Mike Atkins says a stock ZR1 calculates out to 987.19 pounds of drag, while a C6RS with the wing down has only 864.4 pounds. In this setup, the rear lift is 121.62 pounds, and the front has a feathery -0.56 pounds of downforce. Engage the active wing, though, and the drag increases to 927 pounds. But more important, the downforce on the tail increases to 218.22 pounds, with 32.8 pounds of front lift. Bottom line: At speed, the C6RS hunkers down on the deck like a fighter jet on a strafing run.
"Every body modification has a purpose," sums up Gary Pratt, co-owner of Pratt & Miller Engineering. "The features were modeled on the C6.R race cars, and there are no nonfunctional styling elements."
Previous C6RS engines generated nearly the same output as the ZR1's 6.2-liter LS9 (600 versus 638 horses), but did so without the use of a supercharger. Instead, they relied on that time-honored technique of increasing displacement, in this case to a whopping 8.2 liters (500 cubes). Katech, the engine builder, initially wanted to use GM Performance Parts' LSX engine block, but an aluminum version wasn't available, so the company had Dart cast an exclusive billet block.
It featured a 4.5-inch stroke-about a half-inch more than stock-to increase the swept piston volume. That change in geometry required moving up the camshaft and switching to a priority main oiling system. In addition, the water jackets were welded on instead of bolted on. Katech chose not to use a blower in order to keep the package emissions legal and save on weight. So why did P&M move to a supercharger on the latest model?
"More bang for the buck," Atkins says. That, and Thomson says the emissions issue shouldn't be a problem on the supercharged LS7.
Other enhancements reside in the aesthetics, both inside and out. The hood is in the process of being revised with a bigger bulge to make room for the blower case, and it won't have an opening like the prototype unit shown here. Also, the intake manifold will be made of carbon-fiber, in keeping with the dozen-and-a-half or so P&M body panels (only the roof, doors, and rear deck are factory pieces). Note, too, that the carbon-fiber is not merely for decoration-several of the pieces are laid up with a structural-grade epoxy resin, then vacuum bagged or baked in an oven at 160 degrees to prevent post-cure.
Enhancing the vehicle dynamics of the C6RS is an ArvinMeritor computer-controlled adjustable suspension system. The Dynamic Height Control (DHC) provides three driver-adjustable settings: Low, Drive, and High. In the Low mode, the system drops the ride height to improve handling, while the High setting increases ground clearance to negotiate steep driveways and ramps. The suspension is upgraded with P&M-tuned dampers, along with heavy-duty wheelhubs and bearings.
The same manufacturers that supply tires, wheels, and brakes for the C6.R race cars also provide the rolling stock for the C6RS. The BBS forged-aluminum wheels (18x11-inch front, 19x13.6-inch rear) have racing-style center nuts and are wrapped in Michelin Pilot Sport tires (295/30-18 front and 345/30-19 rear). The Brembo Gran Turismo brake system delivers clenching power via six-piston monoblock front calipers with 14-inch rotors and four-piston monoblock rear calipers with 13.5-inch rotors.
Inside the cockpit, Dynamat sound-deadening materials give the C6RS the noise level of a bombproof vault. The steering wheel, upholstery, and bolstered seats all create an exclusive, limited-edition ambience. But as you might expect from a Department of Defense contractor, this hardware doesn't come cheap: $180,000, plus the cost of a Corvette (using '06 to current models, preferably with low miles on the odometer).
Even though the C6RS is now in full production, even more features are being evaluated at the company's skunkworks, such as a Mastercraft sequential paddle shift and a few other proprietary items. But they're military secrets for now, so we'll just have to wait and see Pratt & Miller's next plan of attack.