2013 Shelby GT500 Chassis Development - Total Package

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.

Powertrain

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.

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.

Fuel System

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.

Cooling

The 662hp GT500 has hellish heat rejection to battle. This is especially true at high speeds, when the engine is working hard, and why SVT took a two-tier approach to cooling the new Shelby. That is, GT500s in standard duty cool just fine in base form, while the cars that are run at "... high speeds for a long time" are the reason for Track Cooling.

Erin Gibb, the SVT engineer tasked with working out the cooling details, started with the supercharger's charge air cooler (intercooler). This took some doing. Apparently three systems were tried before everyone was happy, but the result is a massively larger heat exchanger and a coolant pump with twice the flow capacity.

The larger exchanger and pump yield a huge 45-percent increase in heat rejection, which fully manifests itself at the road course. Whereas early GT500s would heat-soak after a few laps, then pull ignition timing for detonation protection--positively murdering horsepower in the process--the '13 GT500 laps continuously with only a minor reduction in power. We don't think you'll notice it. The car is consistent at the dragstrip, too.

By comparison, relatively little was done to the engine-cooling hardware. The radiator is a carryover (save for removing the trans cooler lines), as is the water pump, but after computer analysis and wind tunnel work, the electric radiator fan has gone from six to seven blades and the fan motor from 400 to 450 watts of power. Also, the gaps around the radiator were closed to conduct air through the radiator instead of the many pathways around it.

Where the big cooling push came was getting air to and through the GT500's up to six heat exchangers. What SVT saw in its computational flow dynamics and wind tunnel testing was they needed more air--and high-quality air--flowing through the heat exchangers. By high-quality air, they cite "first air," the direct ram air not pre-heated by some other radiator.

Thus, as much as possible, the various heat exchangers were given their own air paths, and considerable work was done to the front fascia and grille. In the end, the grille was mainly eliminated--all those little eggcrate shapes kill airflow--and various ducts were added to direct ram air to the engine oil and differential Track Cooler radiators.

Cooling at high road speeds was augmented by four air flaps in the lower corners of the fan shroud. They are not spring-loaded--gravity holds these simple flaps shut because they pivot fairly close to their upper edge, but at around 70-mph ram air is sufficient to open them, increasing ram air flow to the radiator. This bypasses the radiator fan, but at high road speeds the fan is more of an obstruction to airflow than a help, so the flaps are a definite gain.

These cooling changes seem straightforward now, but developing them took considerable CFD and tunnel time. The engineers report the computer analysis can only get a design so close before real-world development or wind-tunnel work is required to vet the final three or so designs. Also, effort was put into looking for useful air paths for exiting all this ram air out of the engine compartment, but in the end, the traditional waypath out the bottom rear of the compartment proved the best.

Suspension

Several years of Mustang and GT500 development meant the basic unibody needed nothing as the foundation for a 200-mph car, the suspension was well understood and wanted only a greater performance personality. But at the same time, SVT knew they needed a wider performance span, something they could get with cockpit-adjustable shocks.

Fortuitously, Bilstein was introducing a new, affordable DampTronic dual-mode shock, and it was soon incorporated into the '13 GT500's optional suspension tune, the Performance Package. Between the standard and Performance Pack suspensions, the GT500 services the broad mandates of SVT customer goals of street cruising, track lapping, and dragstrip action. The base suspension uses Tokico twin-tube shocks, while the Performance Pack utlizes the higher-rate springs, sway bars, and adjustable Bilstein DampTronic shocks (re-styled wheels and the excellent Torsen torque biasing differential are also included).

To get started building grip, SVT's chassis dynamics team, headed by Eric Zinkosky, canvassed industry partners, including Ford Racing and NASCAR teams, plus they looked far outside the box when it came to test and development tools. With a car of the GT500's capability, most development drives are done on race tracks. Eventually the GT500 test mules ran on seven road courses and four dragstrips, including the famed Nurburgring in Germany, and the Nardo road course and high-speed tracks in Italy.

Some of the test-track data was recovered, and in conjunction with Ford Racing, was re-run for four days on the multi-million-dollar, seven-post chassis rig at Roush Fenway Racing in Concord, North Carolina. The ability to instrument and quantify pitch and roll information on the shake rig is its primary contribution.

From a ride and handling standpoint, the GT500 coupe and convertible are somewhat different cars. In general, the more rigid coupe was tuned by Gene Martindale for maximum performance tempered by reasonable comfort, while David Harmison ensured the convertible was comfortable but with SVT-appropriate handling.

For an enthusiast, daydreams of running the Nurburgring and Nardo tracks are compelling, and as Gene, who did much of that driving, happily admits, much of it is a blast. But it's also serious work, as he points out when he says initially he thought the 'Ring was going to be little more than a marketing ploy, but once there he found the famed German track offered pretty much all the corners, hills, and straights found across North American tracks but in one lap. Plus it has heaves and jumps not available here. The 'Ring turned out to be a challenging, effective development tool, and it was visited twice by the GT500 team.

Other times development driving isn't the romance we envision. SVT guidelines call for "significant" steering inputs at all speeds the vehicle is capable of to validate chassis behavior. Well, do you want to be the guy tasked with making "significant" steering inputs at 200 mph? Gene had to do just that at Nardo.

There is far more to the GT500 technical development than we're able to present here, but we hope to at least hint at the effort SVT has made to make the GT500 a machine designed to please discriminating drivers no matter how adventurous they get. For a car aimed at both the street and track, that's a lofty order, but the '13 Shelby GT500 is up to the task. 5.0

Horse Sense: Ford truly bends over backward to help us get our stories. Our only complaint is there is never enough space in the magazine to present all the fascinating tidbits that come from a visit to Ford. This is especially true of the Special Vehicle Team--a motivated, skilled and enthusiastic group any 5.0 Mustangs & Super Fords reader would love to spend the day with, especially at the track.

The RevLIne

We now have the full scoop on the 5.8-liter V-8's rev limiter. The engine makes maximum power and redlines at 6,250 rpm, so normally there is no need to exceed that speed.

Given the 5.8's frenetic piston speed, staying at redline or below is a good thing. Occasionally, however, it's beneficial to over-rev a gear for a brief period, say to avoid shifting in the middle of a corner during an open-track, or for SVT to set a 0-60 time without shifting into Third...

For such occasions SVT has provided for up to 7,000-rpm over-rev bursts. As soon as 6,250 rpm is exceeded, a clock begins in Copperhead, and it allows up to eight seconds at 7,000 rpm as long as the rev limiter (fuel shut-off) is not reached. After eight seconds at 7,000 rpm, the PCM will drag rpm down to 6,250 and hold it there indefinitely.

What if you want to immediately return to 7,000 rpm? As John Pfeiffer, an SVT electronics specialist, explains, "It's a weighted timer, so it counts up if you are above (redline) and it counts back down if you are below (redline)."

"If you don't exceed the limit, you are good, but you've consumed some of your buffer. If you're in for three (seconds) and you are out for three, you have your full eight back. If you are in for three and you are out for two, then you only have seven left."

And what if you do bang off the fuel shut off?

"It will slowly bring you down to 6,250 if you stay in it, and it won't exceed 6,250 until you hit the reset conditions, which are 15 seconds below 6,000 rpm and at least one excursion below 5,000. It's set up around a shift, right? We don't want a guy to sit there; you're not doing yourself any favors."

Electronic Assist

As electronic aids proliferate, it is difficult to compartmentalize the handling, braking, steering, and other systems because they are now conjoined by electronic aids.

As a quick example, selecting the GT500's Sport shock setting does more than crank up the compression and rebound. It also changes the brake bias a little rearward to help turn-in, retunes the ABS, mutes the Advance Track stability control for less intervention, turns off nibble control in the EPAS electric steering assist, automatically selects the Sport steering setting--and texts the highway patrol for all we know.

Just kidding with that last one, but electronics are more intertwined than ever in the new GT500. And they help, especially for those new to 662hp cars. SVT recommends drivers new to a track do so with all aids in place (the so-called Key-On mode).

As confidence is gained, the Sport mode allows generous amounts of smooth aggression while maintaining a vigilant safety net in the background--good drivers never invoke it. Full-Off remains an option for elite drivers, although we find they rarely switch off intelligently tuned stability control systems such as the GT00's.

Track Cooling

Chevrolet has been keen to point out its Camaro ZL1 comes with engine, transmission, and differential coolers as standard and that the GT500 Ford makes them optional. SVT answers with a desire to keep GT500 pricing lower, and with the stock oil-to-water engine oil cooler the '13 GT500 has all the powertrain cooling it would ever need for all street and dragstrip driving, plus much of the typical open-track road course work. So why force perhaps 98 percent of GT500 customers to pay for and lug around weighty, excess cooling capacity?

For the other two percent, SVT offers the Track Cooling package. It puts air-to-oil coolers on the engine, transmission, and differential, and removes $2,995 from your automotive budget.

In our 5.8 engine article in the March '12 issue, we showed how engine oil cooling was swapped from water-to-oil to air-to-oil with Track Cooling. In this article, we're detailing the transmission and differential coolers.

All GT500 transmissions castings have a provision for oil cooling, but only the Track Cooling cars get the machining to accept the necessary hose fittings or the oil pump. The pump lives in the bottom front of the gearbox and is driven full-time off a tab on the countershaft. Furthermore the system has no thermostat, so the system is active whenever the engine is running and the clutch pedal is off the floor. Thin synthetic trans fluid means there are no issues with congealing oil on cold days.

The differential cooler plumbs its inlet and outlet through the back cover, and adds an electric pump mounted on the bodywork immediately above the rear axle, and an air-to-oil cooler mounted behind the right corner of the front fascia. The pump is purposely located close to the axle for the shortest possible suction line, which helps avoid cavitation.

A small temperature sender embedded in the differential cover senses the axle lubes temperature; the main ECU determines when to activate the pump based mainly on lube temperature.

SVT notes that the typical driver will never activate the differential cooler--it simply can't be done at legal highway speeds. Only hard road-course or sustained beyond-legal high-speed driving can get things hot enough.