Which is better, supercharging or turbocharging? Sitting squarely on the fence, our answer to that question might come in the form of another question: If the opportunity presents itself, why not piggyback the two?
Of course, that idea is hardly original. The combined whammy of compound turbocharging and supercharging is nothing new. It trickles down from the world of aviation, where a number of WWII-vintage fighter and bomber aircraft engines were so equipped, to great effect. Case in point-the 28-cylinder Pratt & Whitney Wasp Major R-4360 radial engine produced as much as 4,300 hp from harnessing a pair of turbos and a gear-driven supercharger. Mind you, that sucker displaced 71.5 liters and handily outweighed a modern F-350 dually. By comparison, Tony Alm's innocent-looking '08 GT500 produces nearly 1,200 hp at the rear wheels from a mere 5.4 liters.
While at first it may sound a little, well, excessive to foist two distinct sources of boost upon an unsuspecting engine, the theory is sound, because positive-displacement superchargers generate immediate low-rpm power, while the more mechanically efficient turbos spool up to maximize top end thrust. Thus big fun is had all the way across the tach. Besides, one man's excessive is another man's almost enough.
We suppose that was the reasoning behind Hellion Power Systems' Shelby-specific twin-turbo kit, engineered to work in concert with a GT500's stock supercharger, aftermarket supercharger, or no supercharger at all. In Tony's case, this pair of hairdryers is plumbed into the stock Eaton M122.
The combination of two 64mm Turbonetics turbos and the Shelby's factory supercharger are good for more than 39 pounds of boost, helping this 4,160-pound (with driver) coupe rip down the quarter-mile in 9.49 seconds at 155.97 mph on drag radials, while being giddily throttle-responsive at any rpm. Boost is overseen by a sophisticated AMS-1000 boost controller from NLR Control Products.
Tony says the Hellion kit's base 61mm turbos were tried first, but they maxed out at about a 1,000 rwhp (funny, many of us might have been happy with ... oh, never mind). Anyway, about that time, Turbonetics introduced these ball-bearing 64mm Hurricane units which have obviously had the desired effect.
Now, the Shelby's stock iron short-block is known to be stout, but no one will suggest it was designed for 1,200 hp, so Tony's was torn down by Thunder Autosports and reassembled with upgraded Oliver rods, CP pistons, and Manley valve-springs. But, according to Tony, everything else remains just as the Romeo niche line produced it, including stock unported heads and cams. Its tyrannical power comes from oodles of boost, careful tuning, and an E85-optimized fuel system that could quite likely feed a Boeing 747.
And let's be clear-Tony had no desire to end up with power at the price of driveability. He says he drives the Shelby "three or four times a week." He lives down south in Boynton Beach, Florida, so retaining air conditioning and all factory amenities was essential. Truthfully, it wasn't his original plan to get quite so radical with the power ("It'll blow the rear tires off at 100-mph-plus"), but as Tony explains it, "We had a little local rivalry going on and I kind of overdid it ... "
Amazingly, despite his Snake's brutal 1,100 lb-ft of torque at the wheels, the stock tranny is still in place (though it has been overhauled and now sits behind a SPEC P-trim twin-disc clutch). Tranny longevity is probably helped by Tony's use of a WOT box, which momentarily cuts ignition during speed shifts, thus going a bit easier on the input shaft. Out back, the factory rearend is gone, replaced by a 9-inch, packing 35-spline axles and a spool.
Regardless of this Shelby's intended street focus, the 9-second timeslips generated on its regular dragstrip outings required a rollcage that was up to the task-and complied with track safety regulations. Tony turned to Tig-Vision Welding (West Palm Beach, Florida) owner Dave Dodge for a 10-point chrome-moly cage that, despite being 8.50-certified, conforms amazingly well to the contours of the GT500's interior, and retains its street practicality. It envelops a cabin that is otherwise nearly stock, meaning it's not missing any of its factory baubles.
Saying he built the Shelby "to prove how far technology has come," Tony sums up the project as "the most fun car I've ever had ... It makes 1,200 rwhp, has ice-cold A/C, and still drives like stock."
For our part, we look at its innocuous, stripe-less exterior and wonder just how many rivals have been shocked and awed by this earthbound stealth fighter. Kind of fitting, given that the compound turbo/supercharging thing got its start in combat aviation.
5.0 Tech Specs
Engine and Drivetrain Block Stock
Pistons CP, stock compression
Power Adder Stock Eaton M122 plus Hellion twin turbos (64mm)
Cylinder Heads Stock
Intake Manifold Stock
Fuel System MagnaFuel 750 pump, custom rails, and Injector Dynamics 212-lb/hr injectors
Exhaust DynoMax (aft of turbo kit components), with electronic cut-outs
Transmission Stock, with SPEC P-trim twin-disc clutch
Rearend 9-inch, 35-spline axles, 3.70 gears, and spool
Engine Management Stock, with Thunder Autosports tune
Gauges Stock, plus TurboSmart boost gauge
Chassis and Suspension
Wheels Bogart, 17-inch
Tires M&H Racemaster skinnies, 17x4.5-in
Control Arms Racecraft
Wheels Bogart, 15x10-in
Tires Mickey Thompson, 325/50-15