In our March '12 issue we reported in-depth on the '13 Shelby GT500's thundering 662hp V-8. Now, along with Editor Turner's drive report on page 40 of this issue, we're able to detail the GT500's powertrain and chassis.
Like its engine, the rest of the Shelby GT500 is the work of Ford's Special Vehicle Team. A relatively small group by Ford Motor Company standards--just under 50 people, almost all engineers--SVT develops everything from the concept of what the '13 GT500 should be to the finished product. It's a big job, and when necessary, SVT interacts with mainstream Ford engineering and Tier One contractors such as Roush Industries to get the job done.
Internal goals for the new GT500 were set by SVT using a combination of existing sales data, customer surveys, and rigorous market analysis. Like seemingly everything at Ford, the proposed GT500 attributes were weighed in statistical computer simulations to arrive at goals we enthusiasts would respond to. What emerged as notable GT500 goals were 650 horsepower and a 200-plus-mph top speed.
Jamal Hameedi, chief engineer at SVT, emphasizes the GT500's power goals were set relative to the Boss 302 and well before Chevy's ZL1 Camaro numbers were known, but clearly Ford's go-big-or-go-home attitude paid off nicely.
Frankly, we were a little surprised at the 200-mph goal. Top speed hasn't traditionally been a Mustang selling point, but Ford is apparently thinking otherwise these days. Our guess is the double century simply serves notice that the GT500 is a no-excuses car, a new sort of Mustang exotic. Plus, with global sales planned in places such as Germany and especially the Middle East, ultra-velocity bragging rights are a must. Actually, "due to aerodynamic considerations," the convertible GT500 is speed-limited to 155 mph, which ought to be sufficient to thoroughly muss your hair. The coupe remains unfettered by a speed limiter and is drag-limited.
Understandably SVT isn't eager to pin a definite top speed to the 2013 Shelby other than 200-plus mph. For sure it has gone 202 mph while testing at Nardo, Italy, but weather (air density) is the final arbiter of precisely how fast a GT500 will go. It's all academic in North America anyway as there are precious few places to go that fast, even on a track.
A bit surprising is the lack of an automatic transmission. We think SVT wanted the option, but there is no existing automatic that can handle the 5.8's torque, and the GT500 development budget couldn't fund an all-new transmission, so it's three pedals all the way. You got to love it.
We're tickled the new GT500...
We're tickled the new GT500 continues with a live rear axle. It's both the right technical and marketing solution. The money saved not developing an IRS can be seen in the $54,495 base price and the car's minimal weight gain. Better yet, it can be felt in the amazing stability when powering out of tight turns.
Improving the GT500's dual-disc...
Improving the GT500's dual-disc clutch to 5.8 standards was challenging, not only because of the 5.8's extra torque, but also due to its higher 7,000-rpm capability. New friction material forms a two-layer sandwich, with one layer optimized for wear and the other for burst strength. Furthermore, the material is less dense to keep inertial loads manageable so the clutch will function at high rpm. The four marcel springs in the center of each of the two discs were also redesigned for quicker response during clutch release. Here the front disc is propped up for a better view, while the rear disc's center is visible in the pressure plate assembly.
Ford's powertrain engineers...
Ford's powertrain engineers speak in nearreverent
tones when referring to the 5.8's "tremendous
increase in torque output"--with an extra 100
lb-ft of torque to manage, we don't blame them.
While not visible, the clutch's diaphragm spring
fingers feature detail improvements, and the spring
load has been upped to provided the necessary
clamping force.
Thanks both to the higher torque of the 5.8 engine and the higher speed it propels the car to, every major component of the '13 GT500 driveline is new or substantially upgraded. In turn, the new power and speed required retuning the suspension, plus notable upgrades in supporting systems such as fuel delivery and electronic controls. All of which underscores the point that simply putting more boost to a '12 GT500 will not deliver a '13 GT500--the new car is a cohesive mix of upgraded mechanicals, electronics, aerodynamics and support systems.
With the new 5.8 engine thoroughly detailed in our March '12 issue, we'll start off where we left off--at the clutch. Beginning with the dual-disc '12 clutch, SVT found that the metal portions of the clutch--the flywheel, pressure plate and pressure plate "hat"--could withstand the additional 100 lb-ft of torque from the 5.8-liter engine, but the friction portions of the system and clamping load needed upgrading. Furthermore, SVT wasn't pleased with the speed of clutch disengagement on the '12, so that was also on their to-do list.
Thankfully the flywheel and pressure plate were originally designed to accept up to 260mm-diameter friction materials, and so the clutch disc diameters grew from 250 mm to 260 mm, but the nodular-iron flywheel and pressure plate are carryover items. To get the desired faster clutch travel--especially the release--the marcel springs, friction material, and hydraulics were all adjusted.
Hydraulically, the need was to increase flow volume, so SVT increased the master cylinder bore diameter. This reduced the hydraulic leverage in the system at the same time the clutch spring pressure had been increased, so clutch pedal effort rose considerably. An over-center spring was therefore added to the pedal arm to drop pedal effort back to something similar to the '12 level, but ultimately the end result is that the '13 clutch is a little heavier to the driver.
SVT's goals for the Tremec TR-6060 six-speed manual transmission were to bring its torque capacity in line with what the 5.8 was dishing out, along with adjusting its gear ratios for increased driveability. The existing gear ratios were too deep--provided too much torque multiplication--which resulted in hair-trigger clutch engagements, clunky herk-and-jerk throttle response, and generous tire smoke instead of acceleration.
For starters, SVT numerically lowered the final drive ratio in the rear axle from 3.55 to 3.31; in the gearbox, First gear was lowered from 2.97 to 2.66. Second, Third, Fifth, and Sixth gears were altered only slightly, mainly as required to accommodate different tooth pitches in the new, mechanically-stronger gears. Fourth gear is 1:1 and remains unchanged.
For higher torque capacity, the entire gearbox was reviewed, with practically all bearings and shafts made larger or changing styles. A new pocket bearing between the input and output shafts was developed, because this bearing is encased by other mechanicals and doesn't receive much lubrication. So the old tapered roller bearing was replaced by a two-bearing arrangement using one thrust and one needle bearing.
Finite Element Analysis showed the loads from the greatly increased road speed required more ribbing to the transmission's tailshaft housing casting and the clutch housing. Even with the reduced final drive, the ability to hit 200 mph makes us recall the loads going into the transmission and driveshaft are mainly a function of the shaft speeds squared, so the high-speed '13 car is transmitting significantly higher inputs into the driveline. Slight retuning of the transmission and engine mount cushions was done strictly for reduced NVH.
It used to be that an annular...
It used to be that an annular throwout bearing was made by Tilton and found in Trans Am cars. Now they're called Concentric Slave Cylinders and standard fare on Mustangs--and have been for a while. For '13 only a minor change to the TO bearing's retainer clip was needed, and the new TO assembly serves as the new service part for '10-and-later GT500s. What's truly new in this '13 GT500 bellhousing is the inverted V of oil passages cast into the bulkhead shared by the bellhousing and front of transmission. The passages converge at the transmission oil pump, with the passage at right forming the pump pickup, and the passage at left connecting to the cooler lines on cars with the optional Track Cooling package.
Mainly a carryover item for...
Mainly a carryover item for '13, the pedal assembly uses a lightweight composite bracket to mount the brake and clutch pedals. The brake pedal and master cylinder are unchanged, but the clutch is rigged with a new master cylinder and an effort-reducing over-center spring. The new spring is easy enough to see here; the coilspring is mounted vertically, parallel to the clutch pedal arm. Grease points and nylon bushings keep it quiet.
A close-up of the pedal assembly...
A close-up of the pedal assembly shows the new clutch master cylinder. It features a larger-diameter bore, plus a considerably more rigid forged- aluminum construction to withstand the system's higher hydraulic pressures. The '12 clutch master cylinder used a plastic housing with an aluminum sleeve in the bore. It flexed excessively when trying to operate the '13 clutch and so was replaced.
The '13 one-piece carbon-fiber...
The '13 one-piece carbon-fiber driveshaft was the only way to accommodate the new GT500's higher torque and road speed. This saves nearly 15 pounds, can transmit 36 percent more torque, and eliminates the friction of the center support bearing used in earlier GT500s. Constant velocity joints are fitted to each end of the new shaft. Closest to the camera is the plunging (telescoping) joint at the shaft's transmission end. The plunging CV joint looks unusual because it eliminates the traditional slip yoke.
Visually there is nothing...
Visually there is nothing new in the GT500 shifter.
It still bolts to the transmission tunnel at the shifter
end closest to the camera, while two reaction rods
reach forward to the transmission. The shift rod
runs between the two reaction rods. For '13, the
shift lever ratio was slowed to reduce effort and
keep "their hand from beating their foot," as Jamal
Hameedi put it.
Rough on the outside, all...
Rough on the outside, all the precision is on the inside of the carbon-fiber driveshaft. Tier 2 supplier Toray lays up the shaft with a precise inside diameter--the non-critical outside diameter is low tolerance--so that Tier 1 supplier Neapco can precisely press-fit the shaft's steel ends. The carbon shaft is built from 12 carbon-plies laid in combination of longitudinal and angled biases. A second, thicker layer of fabric at each end of the shaft helps during crashes. The outer layer expands and shatters the shaft under high axial loads, greatly reducing the threat from the driveshaft as the car compacts in a major crash.
The non-plunging CV joint...
The non-plunging CV joint at the differential end of the new driveshaft fits the same flange as the '12, but just forward of it inside the shaft, now incorporates a torsional damper. Built like an engine's crankshaft harmonic damper, the shaft damper uses an elasometric ring and steel weight to dampen annoying frequencies.
No slip yoke on the driveshaft...
No slip yoke on the driveshaft means the transmission employs a large-diameter, bell-shaped flange on its output shaft. Furthermore, the forward end of the output shaft uses a higher spline count than the '12 (40 versus 28 splines), so it is not possible to backdate the '13 driveshaft and transmission output shaft to a '12 or earlier GT500. SVT figures anyone needing the strength of the '13 driveshaft would also need the stronger '13 transmission and clutch, so they recommend buying the new clutch, transmission and driveshaft as a package. Or better yet, just swap payments for a '13 GT500 and be done with it.
Some 5,000 dragstrip launches...
Some 5,000 dragstrip launches during development showed a need to beef the GT500's axle tube-to-carrier junction torsional stiffness. That's a fancy way of saying the car was trying to twist its axle tubes out of the differential. The fix? Increase the axle tube diameter slightly for a tighter interference fit and add puddle (rosette) welds where photographer Dale is pointing. Inside, the axle shafts are carryover from '12.