Displacing nearly 3.0 liters of air with every turn of its twin screws, this GT 500-specific, second-generation Whipple supercharger also compresses that air with outstanding thermal efficiency and low frictional losses. Think of it as a 255hp bonus in a matte-black box.
Horse Sense: At press time, the list price for the black W175ax kit was $5,549. A polished version was also offered at $5,899.
Isuppose if we were normal, we'd all be happy-ecstatic, really-with a stock GT 500's 500 flywheel horsepower and 480 lb-ft of torque. Thinking back even five years, those numbers from a factory Ford would've had us dancing in the streets like some chemically demented, flower-power hippies of the '60s-you know, the ones who are now running San Francisco. Then again, any and all claims of normalcy pretty much fall on deaf ears once you've become a regular reader of this monthly journal of Mustang excess.
So, in the way-too-much-power-is-just-right philosophy of 5.0&SF, may we present Whipple Superchargers' new twin-screw upgrade for the '07-'08 Shelby. To save you from risking paper cuts by flipping madly to the dyno results, let's just say right off the bat that this impressive piece of boost engineering upped the rear-wheel peak potency on an otherwise factory-stock '08 GT 500 by-are you ready?-255 hp and a mighty 264 lb-ft of torque.
As if that pot needed sweetening, Whipple's Shelby kit has also earned 50-state emissions legality (EO #D231-26) at up to 17 psi. It also constitutes one of the least time-consuming blower installs we've ever witnessed-at least as we watched the experienced crew at Paul's High Performance go about the task.
Here are the important contributors to the kit's success. The single-blade throttle body is said to be capable of flowing upward of 1,900 cfm, and the 120mm mass air housing is fed by a 9-inch S&B free-flow filter in a cold-air housing.
Probably the most knuckle-scraping part of the whole exercise is swapping in the kit's included fuel pump upgrade. This is a bolt-on-and-drive-away proposition since a seemingly flawless recalibration is an essential part of the deal. We're sure it had to be in order to get the California Air Resources Board's vaunted EO number.
The heart of the upgrade is Whipple's second-generation W175ax twin-screw supercharger, displacing 2.9 liters or 175 ci (the Shelby's factory hybrid-Roots blower displaces 122 ci). How does it make so much more power? First of all, the Whipple displaces more air per revolution, and its twin-screw design is known to be more thermally efficient than the Roots. For out purposes, that means less heat is generated per unit of boost so more spark advance can be retained. It also generates few horsepower-sucking parasitic losses in going about its business. Then there's that dead-bang calibration. The result is a generous 16 psi at low-discharge temperatures and vastly improved power right across the tach, which our dyno figures will reveal.
That's just what we need: one more reason to lust for a GT 500.
An upgraded fuel pump assembly is part of the deal. Similar to the factory GT 500 setup (shown), it has twin pumps, but these are capable of notably higher volume than stock. On the other hand, Whipple's testing concluded that the Shelby's stock fuel injectors are up to the task for the kit's rated 16 psi of boost. Though not shown, a 160-degree thermostat and a pregapped set of NGK TR7IX iridium plugs also come in the Whipple shipping carton. | 
After the Whipple swap, things won't look all that different than this shot of our stock GT 500 engine bay. The kit's cold-air inlet hardware will be the most obvious change, along with the absence of the factory strut-tower brace due to the size and placement of said inlet ducting. | 
What makes this swap a relatively quick operation is that underhood, only the blower unit itself-not the lower manifold/intercooler assembly-must be exchanged. To get things underway, PHP technician and chief philosopher Kodak Karl Roekle unbolts the coolant overflow bottle. Still connected to its plumbing, it's moved aside to get at the serpentine belt tensioner to slip the belt off the blower snout. The factory belt will be reused. |