5.0 Mustang & Super FordsCar Reviews
2013 Shelby GT500 Chassis Development - Total Package
The 2013 Shelby GT500 is much more than a 662hp 5.8
Another aspect of the '12 car SVT was keen to improve was the harmony between clutch and shifter efforts, especially the short-throw quick shifter. Therefore the shifter ratio decreased, and as we've already seen, the clutch action sped up so that the lever ratio, throw, and clutch effort would be better matched.
What was not changed is the shifter design, with one end attached to the body and the other to the transmission. This is a different shifter than the Mustang GT's, and SVT is adamant the '13 shifter will not bind even when the car is torquing out of a tight corner.
Behind the transmission is an all-new one-piece, carbon-fiber driveshaft, a first for a standard-production Ford. It is lighter, stiffer, and transmits torque better than the two-piece aluminum shaft it replaces. The carbon fiber is not so much for lightness as stiffness. It was the only way to get the bending frequency of the shaft above the rotational speed driven by the 200-mph vehicle speed, according to its lead engineer, Brian Zorman.
There is some tricky machining at each end of the carbon-fiber tube, where case hardened steel inserts provide the interface with constant velocity joints. The steel ends are precision-machined with small spline-like serrations that mate into the carefully controlled ID of the tube. They rely strictly on the controlled interference fit between steel and carbon fiber to hold together as there is no glue or other bonding agent. If the interference fit is not exact, the steel inserts will either spin in or split the tube, so there are some tight tolerances involved.
At their outer ends, the steel inserts carry rolled splines to mate with the CV joints, and at the driveshaft end there is a plunging (telescoping) section to absorb rear axle movement as a traditional splined-yoke would. All told the steel inserts cost more than the tube, leading Jamal to joke, "We managed to design steel that's more expensive than carbon fiber!"
Aside from installing taller 3:31 gears, the main job SVT engineer Scott Beiring had with the rear axle was to keep it in one piece. Trapped between the supercharged 5.8 and the sticky Goodyear F1 tires, during testing the axle tubes tried twisting out of the center carrier housing, and the pinion wasn't looking overly robust either. So the first improvement was to install the F-150 pickup's larger diameter pinion and pinion head bearing for a 15-percent increase in stiffness and a corresponding improvement in gear deflection, wear, and long-term durability. That way the rear axle wouldn't start whining after only a thousand drag launches or a mere 150,000 miles.
Lubrication improvements included detail changes to the lube flow path, along with a lighter weight 75/85W synthetic gear lube, which includes an effective high-pressure additive. SVT says it's just the thing for the GT500's 631 lb-ft of torque.
There are no open differentials on GT500s. Base cars continue with the existing Traction-Lok; that's a spring pre-loaded clutch pack with six sets of carbon fiber friction discs and a mechanical pre-set pre-load. It's fine for normal street driving and light track duty, but SVT now offers an optional Torsen limited-slip differential. The same part has proven transformational in the Boss 302 and should work at least as well with the GT500's extra power. A stout piece, no changes were needed to the Torsen for GT500 duty. For the record, both GT500 differentials are purely mechanical, with no electronic management.
What's difficult to pinpoint when reviewing the driveline mechanicals is how efficient they've become. Some of this is the quality of the parts--the ring and pinion gears are premium, low-tolerance pieces, for example. Much of it is the cumulative benefit of eliminating the driveshaft's center bearing, the stiffer driveshaft, more rigid rear axle assembly, more efficient numerically lower final-drive gears, and so on. The result is that the GT500 is said to have amazingly low driveline losses, which helps deliver the power to the tires, improves fuel economy, and shows up in chassis dyno tests as higher-than-expected readings.
Time will tell what tuning shops will accept as typical GT500 losses between the flywheel and tires, but listening between the lines at Ford suggests 8 percent might be more accurate than the rule-of-thumb 15 percent.
The first clue of the upgraded GT500 fuel system was the change from 46.7-lb/hr to 54.8-lb/hr injectors listed in our engine story. Feeding those lawn sprinklers was clearly going to take more than last year's system could flow.
As the SVT team brainstormed its options, the staff realized replacing the GT500 steel gas tank with the Mustang GT's plastic version would save 7 pounds, and that two Mustang GT fuel pumps could supply the required 300 liters per hour. A suitable fuel pump driver module holds sway over the twin pumps.
Because the Mustang GT fuel tank is a saddle-type slung over the driveshaft, there is a jet-style transfer pump inside it. Its job is to shuttle fuel from the passenger saddle to the driver-side saddle, where the main fuel pump resides. And with the 5.8-liter's rapacious appetite for fuel--it can about empty the tank in 25 minutes of track driving--the jet pump tubing was enlarged, too.
Considering Ford Racing has sold more GT500 fuel systems in recent years than SVT has in new cars, everyone is expecting this new GT500 fuel system to be a hot aftermarket item as well. SVT says fitting a Mustang GT with the '13 GT500 tank, pumps, filters, and electronic drivers is a cost-effective way of gaining a no-excuses, bolt-in fuel system. But while the parts bolt in, they don't plug in; some electronic magic is required. SVT figures anyone who's merged a previous GT500 fuel system into a Mustang GT will use the same approach with the '13 version.