5.0 Mustang & Super FordsCar Reviews
Shelby Cobra GT 500 Prototype Drive - Sneak Bite
Join Us Behind The Wheel Of The '07 Shelby Cobra GT 500 Prototype A Year Before It's Released
Horse Sense: When we asked if there might be other Shelbys, we were told we might see efforts along the same lines-probably other variations of the Mustang-but probably nothing like the recent show cars or other ultra-high-performance cars. So don't look for anything like a Cobra roadster from Ford; it sounds as if that'll remain a Shelby American exclusive.
We know you're anxious-but before we verbally spin the '07 Shelby Cobra GT 500's tires around a road-racing course, there are hardware and marketing details to steer straight. Foremost is this car could just as easily-and a lot less confusingly-be named the '07 Mustang Cobra because that's what it is, the next Mustang Cobra.
The Shelby prefix and GT 500 suffix are a happy confluence of heritage styling combined with Ford Motor Company and Carroll Shelby playing nice with each other again. Shelby was brought in to review the Special Vehicle Team's work and lend some input to the car's styling and dynamics, but otherwise the machine is the creation of SVT.
Which begs the question, what about SVT? Gone mainstream, we're told. SVT has gained marketing muscle and an engineering staff-an amazing 75 engineers were said to be working at SVT when we drove the GT 500, and the thinking was that up to 125 might be assigned there depending on the workload. If anything, SVT is tougher to erase from the Blue Oval flowchart than ever before, so that bodes well for the group's survival.
On the other hand, SVT has become a larger target on the corporate radar screen, so budgets are more closely watched and product development could suffer from focus groups and corporate hand wringing. Countering that threat is a tremendous depth of enthusiast talent throughout all levels of SVT, which certainly shows in the upcoming GT 500.
There's no danger of small thinking in the GT 500. The car delivers superb power in a near-perfect street chassis, but let's talk engine first, as it is the GT 500's defining characteristic.
Wanting Ford GT-like thrust in the GT 500, SVT replicated the GT's 5.4L supercharged Four-Valve engine using the iron 5.4 Triton truck block instead of the GT's exotic dry-sumped aluminum casting. Ford really didn't want a $20,000 engine in its newest Cobra, nor did any of us.
This means the GT 500 engine is wet-sumped, but because it's a slightly lower-rpm engine than the GT's-redline is marked at 6,000 rpm and the fuel cutoff is 6,250 rpm-it's said the dry-sumping would not be much of a power improvement over the GT 500's carefully designed oil-scraper design. While the Ford GT benefits from a lower center of gravity by dry sumping, the GT 500 wouldn't as the steering rack limits downward engine placement.
A reverse-rotation water pump was also fitted to the truck block in order to work with the blower drive.
Past the block, most of the engine is pure Ford GT excellence. That means the crank is a strong forging, while the connecting rods are Mahle forgings. The engineers said these were on par with the Manleys used in the '03-'04 Mustang Cobra, but more affordable. No one on hand could recall the piston source, but could say they were supercharger-friendly 8.4:1 forgings.
Best of all, the four-cam, Four-Valve cylinder heads are straight off the Ford GT, right down to the part number. That means the best cams, valves, springs, and ports. In fact, SVT said these heads surpass the vaunted '00 Mustang Cobra R castings for power production, and they are credited with the majority of the power gain over the previous supercharged Mustang Cobra engine, and not so much the displacement increase.
Originally, the GT 500 was to be boosted using a twin-screw supercharger from Lysholm, but the Swedish company couldn't come close to meeting Ford's 40-a-day production demands. Therefore, SVT reverted to the M1.22 Eaton Roots blower, which is supposed to be slightly more responsive in the midrange but lacking in top end. (We don't know about the midrange response, but we've definitely shown top-end limitations with the Eaton in '03 Cobra tests compared to twin-screws.) Water-to-air charge cooling is a given, as previously seen on the '03 Mustang Cobra and Ford GT. Manifolding is from the Ford GT or is close in function, and the exhaust is the expected 2 1/2-inch system.
SVT has yet to assign a definite power rating to this potent powerplant, referring to it as "450-plus horsepower" in preliminary literature. The engineers were fairly optimistic in conversation, as if they'd blown by 450 hp half-stepping it in the slow lane-so we'll estimate 465 hp and hope we're right.
No matter the ultimate power rating, it's high enough to warrant a double-disc clutch. Built by volume manufacturer Valeo, the clutch spans just 215 mm (8.44 inches) for low inertia, but with two discs it easily handles the big power. It isn't the Ford GT clutch, in case you're wondering; although a twin-disc as well, that one measures 240 mm and is built by AP.
As expected, Tremec got the transmission contract with its ubiquitous T56 six-speed manual. It's an improved version, boasting triple synchronizers in First and Second gears, as well as new design gears with wider teeth and reduced shift effort. First gear has been lowered from the '03 Cobra's 2.66:1 to 2.97:1, but the change in final drive gearing-which is 3.31-is supposed to even this out.
We were hoping for a super-tall Sixth-gear ratio for quiet, fuel-efficient freeway cruising, but at our drive time it turned out we were on the losing side of an internal argument over the issue. John Coletti, who oversaw the GT 500 program before his recent retirement (and $3/gallon gasoline), favored relatively low-ratio top gears to preserve responsiveness, while SVT Chief Engineer Jay O'Connell and Hau Tai-Tang, who directs SVT, favor tall cogs for increased mileage and reduced engine wear in exchange for the occasional downshift on the freeway. We prefer the latter, but so far the GT 500 prototype approximates the traditional Ford gear spread as illustrated by the '03-'04 Mustang Cobras.
It isn't news the GT 500 employs the same live axle and three-link rear suspension as the current Mustang instead of the previous Cobra's independent rear. Discussing why would take a small book, but factors in play include cost, weight, performance, and perception. Frankly, we're thrilled to have the live axle because it works better with a high-horsepower, relatively long-travel suspension such as the Mustang's. But it's also obvious from earlier Cobra experience that back dating an IRS to the otherwise live-axle Mustang adds to already considerable weight and, as there isn't an IRS available at this time, developing one would add substantial heft to the engineering budget.
That leaves perception, and our sense is potential GT 500 buyers have come to understand live axles can work well, and to many a live axle is a benefit, not a deficit. So, from a product-planning viewpoint, not having IRS is hardly a deal breaker for the GT 500; engineeringwise, IRS is more a hindrance than anything.
Of course, the basic Mustang GT chassis is being retuned for GT 500 duty, with bushing firmness, sway-bar sizes, wheel and tire sizes, tire construction, shock tuning, spring rates, and such on the table for revision. Many of these details were decided when we drove the GT 500 prototype, and the work remaining during our visit centered around fine-tuning, such as setting the T-bar stiffness in the steering rack for steering feel and working with the front lower-control-arm bushings to balance bushing response front axle to rear axle.
Rumors were the K-member had been changed for the GT 500, but not so said the engineers; it's stock Mustang GT. Every bushing in the car, however, has been firmed up without getting carried away, and the spring and shock rates have been likewise upped, of course. SVT has been considering adding some bracing, but had not decided if it was cost effective, as it adds weight and cost.
Braking has been significantly upgraded in the front by four-piston Brembo calipers working with 14-inch discs. The rear brakes are stock Mustang GT, but with more aggressive, track-friendly pads. Furthermore, an experimental brake fluid was being tried during our test day, which is supposed to aid ABS action during cold weather.
In the photos, you're seeing the GT 500 show car, not the engineering hack we drove. An important difference is the show car is wearing 19-inch wheels, while our prototype and the production cars are using 18-inch rolling stock. Just before our drive, SVT had finalized massive 285/40ZR18 Goodyear Eagle supercar tires on the rear; the fronts remain 255/45/ZR18s. These Goodyears are the same model fitted to the Ford GT and are true premium rubber for maximum grip and responsiveness along with good durability on road courses. That's definitely not to say they're "high-milers" on the street, but rather have the internal construction to hang in there on track day.
Like nearly every car today, weight is the great evil. SVT admits the GT 500 currently scales a sumo-like 3,850 pounds (almost exactly 300 more pounds than the civilian Mustang GT). It's the price one pays for all those horsepower gizmos under the hood, the big wheels, tires, and so on. Weight distribution worked out to 57 percent front and 43 percent rear, and with the additional weight mainly just behind the front axle, SVT says the car works better than a 57/43 car should. Still, add in 10 gallons of fuel and our editorial bulk and that meant a test weight of 4,155 pounds! That's SUV territory.
Behind The Wheel
All that weight makes our primary driving impression incongruous-on the sinuous mini-Nurburgring road course called Grattan Raceway Park. The GT 500 feels somewhat like a more powerful, less tank-like '03 Cobra.
The first thing you notice is the big, smooth power. As we've come to expect from SVT, the 5.4 delivers electric motor urge. From idle to redline the power curve is seamless and linear, and when you let it all out, it's manly. Torque is obviously plentiful, and it's nothing to swing the rear end around slow corners with the throttle. At the same time, the top-end charge is exhilarating, and this is with the engineers warning us the tune in the test mule was definitely flat at high rpm that day due to a rich fuel mixture and scaled-back ignition timing. Even SVT doesn't want to pay for blown engines when all they're testing is suspension. For us enthusiasts, it was perfectly clear a blown 5.4 is that much better than a blown 4.6, and the premium cylinder heads on this engine are not going to waste. It's 12 seconds quick and easy to drive.
As we started to push, the GT 500 took on a massive handling feel-not sloppy like a '68 Galaxie, but weighty without a rock-hard suspension to back it up. We soon became confident the chassis doesn't have a mean bone in its suspension; it goes right where pointed and answers the helm with complete predictability. Happily, the best way around the road course is to boot it around the slow turns with the throttle-it doesn't want to be tip-toed around-and accurately line it up on the fast corner entries, squeeze the trigger, and ignore the small wiggles and waggles because the tires grip.
Chassis balance is magnificent for a powerhouse Mustang. Sure, it lightly understeers eventually, but considering the front axle weight, hardly at all. Combine the great fore-and-aft balance with the wonderfully tight chassis structure and enthusiast tuning, and the GT 500 is easily faster and considerably more predictable left-right-left than the current Mustang Cobra.
And that live rear axle is nigh perfect. There's no drama (wheelhop) with standing starts, and it works great on the road course-definitely better over the bumps than the IRS in the Ford GT we also drove that day.
In fact, like a good street suspension put to work on a road course, the GT 500 suspension felt a hair soft or plush, if anything. It does a wonderful job soaking up bumps and undulations, and our impression was the sharp-edge potholes in the real world would be absorbed nicely indeed, again better than in the '03 Mustang Cobra. But combined with the huge weight, the comfy suspension needs a calm hand in the 90-mph sweepers.
Steering feel-which the engineers were working on that day-was good but everyone on hand agreed it could use a little more personality. Our weak development-driving skills would not allow us to pinpoint a steering change we'd make and, truthfully, if the rack-and-pinion goes to market just the way we drove it, we'd be pleased.
Braking was a mixed bag. The basic package was powerful and modulated well with a firm, reasonably short-travel pedal. But fade was significant when trying hard, which the engineers immediately and disgustedly attributed to the experimental brake fluid. Our bet is we won't see that fluid in production, and the GT 500 will stop powerfully with its modest nosedive and complete composure every time.
Shifting was '05-Mustang rubbery, probably as good as it gets with some isolation in the shift mechanism and given the half-tranny, half-body mounting of the new Mustang shifter. Clutch action was unremarkable-a good thing as it felt perfectly normal despite its twin-disc composition.
In short, the GT 500 is a musclecar with sports car intentions. The huge power and comfortable ride give it the musclecar aura, while the unexpectedly well-balanced and rigid chassis allows remarkable finesse for something so heavy and powerful. Like everything about the new Mustang, the GT 500 is a big improvement, in this case upon the earlier Mustang Cobras. When it arrives next summer, the Shelby Cobra GT 500 will definitely be worth the wait.
Say "Eaton Roots blower" and fond memories of being g-forced back into the seats of Terminator Cobras will immediately spring to mind, followed shortly thereafter by similar reminiscence of force-fed Lightning and Harley pickups. Farther back in the memory banks are the blown V-6 T-birds and Cougars of the late '80s and early '90s.
Each one of these Ford factory applications of Eaton-manufactured intercooled, positive-displacement supercharging has been a hit with enthusiasts, and we can safely predict the M122H sitting proudly atop the GT 500's fearsome 5.4 will continue this torquey tradition. In our initial look at the new Cobra (July '05, p. 54) we mistakenly said the GT 500 would have a screw blower. Well, it won't; but it will have the M122H, or what Eaton refers to as a hybrid-thus explaining the "H" in its designation. It's a hybrid of the fifth-generation Roots design as used on the Terminator and a patented, all-new sixth-generation that won't debut until 2008. It's still a Roots-unlike a screw blower, all air compression will take place in the manifold and not within the blower itself.
So why all the excitement? Efficiency. Both mechanically and thermally, the hybrid is substantially more efficient than its Gen V predecessors-thanks to reengineered rotor and port designs-and the GT 500 is the first production application of this hybrid design. The full Gen VI Roots, when it eventually comes along, will be even more efficient as it will have an optimized center-to-center distance between rotors, also known as "pitch spacing." In the meantime, the hybrid essentially combines Gen VI rotor and port designs with existing Gen V pitch diameter, or rotor spacing. In even more basic terms, think of it as Gen VI rotors in a Gen V housing.
According to Eaton, thermal efficiency is up by about 15 percent over the Gen V, meaning substantially lower blower-discharge temps. Lower air-charge temps offer less chance of detonation, permitting more spark advance to be dialed in. More advance equates to more power. And because the hybrid is also more mechanically efficient with fewer parasitic losses than its older Gen V siblings, it consumes less horsepower to generate boost. Are you detecting a pattern here?
Eaton's engineering and product strategy manager for Air Induction and Cylinder Head Systems, Craig Sell, nicely sums up the hybrid in fewer words than it takes to describe his title: "Turbo efficiency from a Roots blower." Sounds good to us. - Dale Amy