2004 Z06 Corvette Turbo - Overkill: Twin Turbo

Could J.R. Granatelli's twin-turbo terror be the most powerful C5 on the road?

Drew Hardin Oct 11, 2006 0 Comment(s)
Vemp_0611_01_z 2004_z06_corvette_turbo Front_view 1/10

The 345/30ZR-19 Michelin Pilot Sports on the rear of J.R. Granatelli's '04 Z06 are massive-so wide that Lingenfelter mini-tubs were needed to tuck them inside the body. And yet, with the right amount of throttle, those tires become little more than clouds of white smoke and two strips of semi-molten rubber on the pavement. Well, or the wrong amount of throttle, depending on who's springing for the $400-plus Michelins.

This Z06 has some major power mods wrapped around its LS6: a Granatelli Motor Sports (GMS) twin-turbo kit featuring two Turbonetics 76mm ball-bearing turbochargers. With the hair dryers pushing 12 pounds of boost, the engine put 972 hp and 1,057 lb-ft of torque through Granatelli's chassis dynamometer on the day we photographed the car.

Vemp_0611_04_z 2004_z06_corvette_turbo Interior 2/10

That pull was made burning nothing more exotic than premium pump fuel, and the engine was fitted with a full exhaust system, including cats. The exhaust is actually an area of the car J.R. still wants to improve. He figures if he could go from 211/42- to 3-inch pipes behind the headers, he could pick up another 100 horses.

Like the car, or the Michelins, needs another 100 horses.

But overkill is exactly what this car is all about. "I built it for two reasons," he explains. "I did it to get a rise out of my dad [J.R. is a scion of the famous Granatelli family, with authentic Indy 500 memorabilia in his office lobby], and to prove our company's potential to our customers." So consider the Z06 as the ultimate demonstrator model for GMS's twin-turbo kit and mass airflow sensors.

Vemp_0611_07_z 2004_z06_corvette_turbo Hood 3/10

All those ponies aren't coming out of a stock LS6; if that were the case, the turbo kit's output would be closer to 550 hp and a like amount of peak torque from six pounds of boost. Instead, the stock block was bored to 4.25 inches (with Darton sleeves in the holes) and stroked to 3.76, resulting in a displacement of 427 cubes. Knowing the turbos would stress the engine's bottom end, J.R. built it with a Lunati forged crank and Ross forged pistons swinging on Lunati rods.

Houston's Motorsport Technologies performed a Stage 5 port job on the stock heads and dropped the compression ratio down to 8.8:1 to accommodate the rush of air from the turbos. We'd like to share the Cam Motion camshaft specs with you, but J.R. is keeping those a closely guarded secret. We can tell you that the car idles almost like a stocker, with just a hint of a lope, but that there must be some serious lift and duration going on to shove 1,000 horses' worth of air into the chambers.

Vemp_0611_turbo_02_z 2004_corvette_Z06_twin_turbo Dyno_hood_up 4/10

Speaking of which, the compressed charge from each turbo routes through a massive TTI intercooler (with dedicated fans to keep a chill in the air), then up through a modified GMS LS2 mass air sensor and into a FAST intake manifold.

Getting enough fuel to burn with all that air was a challenge. J.R. modified the car's stock fuel lines to feed a 5-gallon fuel cell located inside the driver-side front fender. From there he created a fuel-injection subsystem with a return line, so that the fuel-pressure regulator had a means to reference boost pressure and increase fuel pressure as needed. "When you're making big boost, the [stock] returnless system offers no way to boost-reference the regulator," he explained. Two Bosch 930 high-pressure pumps are tasked with sending the increased amounts of fuel to ACCEL 55-lb/hr injectors.

Vemp_0611_05_z 2004_z06_corvette_turbo Dyno_run 5/10

The standard GMS twin-turbo kit comes with a mapping tool to tune the engine's computers. For this application, J.R. leaned on mapping software from EFI Live to optimize the air, fuel, and spark.

J.R. figures that this engine, in naturally aspirated trim, would produce around 480 hp. "The turbos basically double the psi over atmospheric pressure, and double the horsepower and torque." The addition of an adjustable wastegate and an in-dash boost controller means the car can be set anywhere from "cruise" to "kill" at the turn of a knob.

Vemp_0611_09_z 2004_z06_corvette_turbo Burnout 6/10

Power routes through an Exedy carbon-fiber clutch to a G-Force T56 six-speed transmission. The stock 3.31 gears are still in the car, though Mark Williams halfshafts direct power to the Pilot Sports. Each corner is fitted with Baer Extreme-Plus 6R six-pot brakes, a good idea when you have this much power under your right foot.

The GMS Z06 doesn't exactly look tame, thanks to its flamed paint, Classic Design Concepts Tiger Shark front and rear fascias, RKSport carbon-fiber hood, and Catz HID conversion. Yet it can be surprisingly docile when you resist the urge to stand on it. In putt-putt mode, about the only tip-off to the car's potential is the whine from the T56's straight-cut gears.

As we idled down the freeway on our way to the photo location, J.R. was happy to show us, via the Corvette's onboard computer, that it'll return respectable mpg if you behave. (Yes, it felt odd to talk about fuel economy in the context of a street Vette that'll turn the quarter-mile in less than 10 seconds, but it's a hard topic to avoid these days.) He was well on his way to achieving 20-plus mpg until we asked for burnouts for the camera. "Those burnouts just did us in," he said later, pointing at the 14.1-mpg reading above the steering wheel.

That ain't bad, especially since our burnouts weren't entirely to blame for the dip in fuel economy. On the way to our shoot, J.R. couldn't resist giving us a taste of just what this Vette can do. Stomping the accelerator is a traction-avoidance maneuver that'll just vaporize the tires. But when J.R. leaned into the pedal, the surge to speed felt like a meaty hand shoving us back into the seat, while the needles in the dash tracked not just engine and road speed but our own adrenaline spike. Just as quickly, the big Baers hauled us down to match the flow of traffic, and we had a chance to catch our breath.

So, do customers ever ask for more than this? we asked.

J.R. smiled. "No."

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