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Published: 12/13/2006 - by Josh Jacquot, Senior Editor
By now you know plenty about Ford's hottest Mustang, the 2007 Shelby GT500. You know that under the hood of this high-performance muscle car lies a 5.4-liter supercharged V8 that churns out 500 horsepower and 480 pound-feet of torque. You know that it comes with a six-speed manual transmission. And you no doubt know that many cars have run 12.7- or 12.8-second quarter-mile times at more than 112 mph, which is faster than any Shelby Mustang built in the muscle-car era of the 1960s.
This story isn't about what you know.
What you don't know is that the Shelby GT500s tested by Road & Track, Car and Driver, Motor Trend and Automobile were all preproduction units. This test represents the first evaluation of an actual production car by any media outlet. And it has left us with our eyes rolling and our heads shaking.
After months of waiting, Ford delivered a GT500 the second week of November. We went straight to MD Automotive in Westminster California, to confirm the Shelby's power figure before we hit the track. By the third pull on the Dynojet chassis dyno the reading had stabilized at 432 peak rear-wheel hp -- a perfectly believable figure for a car rated at 500 hp.
Then we headed to the track with the beast, expecting it to run the promised high-12-second quarter-mile times. It didn't. Regardless of launch technique, the Shelby refused to go quicker. Its best run was a 13.4-second pass at 108 mph. Clearly, something was amiss.
We called Ford PR and let them do some digging. The GT500 went back to its keepers. The following week Ford Brand Communications Manager Alan Hall called back. Apparently 3,500 GT500s have the potential to be afflicted with delaminating air filters, which was likely the source of our problem according to Hall. He said he'd have the car serviced and returned to us for a retest as soon as possible.
But this explanation didn't completely add up. We had checked the air filter at the track when we tested the car. Assuming some joker had filled the GT500's airbox with desert sand, we pulled it out and held it up to the sun just to be certain air could actually penetrate its element. It looked like a normal air filter.
The dyno numbers were also puzzling. Ford engineers had confirmed that the 432 wheel hp figure was right given the dyno type (Dynojet 248) and conditions. So if the car was making the right power, why was it so slow? Was it the surface? Or the driving? We didn't think so.
Round two. Ding.
Two weeks later Ford returned the car with a new and presumably correct air filter in place. We headed straight for the track. On its third pass, with the same driver behind the wheel as the first testing of the car, the Shelby ran a 12.82-second quarter-mile at 113 mph. That's 0.6 seconds and 5 mph quicker than before, a massive jump in performance.
Dumbfounded, we again headed for the chassis dyno at MD Automotive for answers. Instead we found more questions. This time the Mustang produced an additional 16 peak wheel hp (448 vs. 432) -- an undeniably significant difference. And a quick look at the two most representative dyno graphs shows that all the additional power was produced above 5,000 rpm -- the most critical point for gain since that's where the engine spends almost of its time during a quarter-mile run. Torque was marginally lower across the rev range.
With the car on the dyno we yanked out the air filter for a good, close look. It looked the same. Is it possible we had missed the delamination of the previous filter? Conceivable, yes, likely no -- or so we thought. But this second air filter also looked good. Its rubber seals were intact and the filter material was clean and unremarkable. So what's going on here?
At first we thought the Ford crew was trying to pull a fast one. After all, they did take two weeks to change an air filter. Skeptical, we checked the vehicle identification numbers and confirmed that Ford hadn't switched cars on us; then we measured the supercharger pulley and found it to be stock.
Turns out, we should be more trusting. You see, the air filter we inspected the first time looked fine because the delamination problem isn't visible. SVT Engineering Manager Bill Woebkenberg later explained that the problem isn't obvious because it doesn't occur until there's significant flow through the filter, which causes its pleats to flutter. According to Woebkenberg, the affected filters create an unequal distribution of air through the engine's mass airflow meter. This causes the meter to tell the engine control computer that too much air is reaching the engine, which causes conservative fuel and spark delivery and reduces power.
But what about the 16-hp difference? Isn't that enough to raise an eyebrow? Not according to Woebkenberg. "The average power under the curve is enough to account for the 113-mph trap speed," he said. He also provided us with a sufficiently nerdy formula for calculating wheel hp based on weight and trap speed. We plugged our test numbers into it and it spit out 449 hp -- only 1 hp more than the dyno. The owners we found who had dynoed their cars had wildly erratic peak power figures ranging between 421 and 460.
The rest of the story
Short of the air-filter fiasco, the GT500 is an impressive machine. It offers a raw driving experience that's found in fewer and fewer production cars today. Flip off the traction control and enjoy the ability of 500 hp to turn rubber into smoke until you're bored or run out of rubber, whichever comes first. Powerslides in 2nd gear are easily controlled and the car's overall balance is impressive considering its as-tested weight of 3,903 pounds.
Ford has made an art out of tuning the strut/axle combination that resides under the GT500, and it's been done well here. SVT engineers made it work admirably on smooth roads, but there's no hiding the axle's unsprung weight. Hammer the throttle over a rough stretch and the axle pounds the body like you've got Jenny McCarthy locked in the trunk. Still we found peace with the Shelby on most roads.
It was also a blast in the slalom, where it was easy to balance through the transitions at 68.1 mph -- 2.9 mph faster than our long-term 2005 Mustang GT. Around the skid pad it made more grip than most pony cars, turning in a 0.89g performance thanks to its Goodyear Eagle F1 Supercar rubber.
We weren't able to fade the brakes in our one-stop tests, and it halted from 60 mph in a relatively short 116 feet, 11 feet shorter than the Mustang Shelby GT-H.
In case the engine specs escaped you the first 50 times you read about the GT500, here are a few of the most critical. The 5.4-liter block is made of cast iron, topped with aluminum dual-cam, four-valve heads from the Ford GT and filled with a forged crank, rods and pistons. A roots-type supercharger blows through an air-to-water intercooler. Compression is 8.4:1 and boost tops out at about 8.5 psi.
With the exception of the brakes, the supporting hardware isn't quite as extravagant. Fourteen-inch front rotors are stopped by four-piston Brembo calipers. In the rear there are standard 11.8-inch Mustang rotors and two-piston calipers.
The rubber meets the road on 18-by-9.5-inch wheels. Staggered tire sizing is used to put down power: Tire sizes are 255/45R18 front and 285/40ZR18 rear.
Engineers didn't change the standard Mustang's 15.7:1 steering ratio on the GT500 but did add a brace to the lower control arm bushings to improve stiffness. There's also a proprietary steering pump and torsion arm to bump up steering feedback.
Other functional details include extractor vents in the hood that ensure proper airflow over the aluminum radiator. Look carefully at the GT500's nose and you'll notice far more void area than on a standard Mustang to allow air access to the new radiator.
Still a Mustang
Climb inside and you're not going to forget you're driving a Mustang. Our test car, which carried an as-tested price of $43,765, was fitted with the $595 Premier Trim Package, which adds stitching to the dash and center console, an electrochromatic rearview mirror and aluminum pedal covers. The shift boot and handbrake are leather. The only other added expenses on our test car's window sticker were a $1,300 gas-guzzler tax and a $195 bite for Sirius Satellite Radio. Base price is $41,675.
Although the seats offer more lateral support than the ones fitted to a standard Mustang, there still isn't enough for real cornering. Still, the seats are fairly comfortable and feature a cool embossed Cobra emblem in the backrest.
The instrument panel has a boost gauge and there's a nifty SVT logo in the tachometer that illuminates any time you're in the meat of the power band. The illumination on the tachometer and speedometer can also be switched between a blue/red combination and all-white.
The final word
Assuming the worst-case scenario for a GT500 owner is a short dealer visit to swap air filters, then 12-second quarter-mile times are a reality. And we can vouch for the GT500's potency at full crack. It is the embodiment of rubber-laying, gear-slamming, powersliding madness. And it's almost enough fun to justify the $20,000 premium dealers are collecting for these cars right now.
Bottom line? There's a hell of a lot of fun to be had behind the wheel of a GT500. Just be sure you've got the right air filter.
The manufacturer provided Edmunds this vehicle for the purposes of evaluation.
Specifications and Performance
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