5.0 Mustang & Super Fords
2013 Shelby GT500 Chassis Development - Total Package
The 2013 Shelby GT500 is much more than a 662hp 5.8
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.
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.