PASADENA, CA - It's difficult to think of Ford Motor Co.'s 1999 Mustang Cobra as a mere refresh, even if its all-new skin does sit atop a 21-year-old chassis. After all, when's the last time a mere engineering tweak has shifted the performance direction of an entire segment?

With the twist of eight simple bolts, Ford does just that, sending the ponycar screaming down a wholly different road than it has traveled since April 1964.

The big news: Its haunches now squat confidently on an IRS. No, not the IRS. An IRS, as in independent rear suspension.

Taken together, the two independently slung rear wheels - hung in a short and long arm, or SLA, configuration - arguably mark the most significant technological advancement Ford's cardinal performance icon has seen since the first foal pranced off the line at its Dearborn Assembly Plant 35 years ago.

And with feet like these, the Cobra should now be able to play with the big boys. "You have to go up to true sports cars, like the Corvette, the 300ZX or the Supra, to find the same combination of straight-line performance and handling," says Thomas Scarpello, Ford Special Vehicle Team (SVT) marketing manager. "With the new Cobra, you get 90% of a sports car at 60% of the price."

The IRS is compact and complex - yet elegant. Its design impresses immediately, so much so that when seeing it extracted and placed side-by-side with the older rear, it's difficult to picture how it all fits.

At its heart, an isolated steel tube subframe supports the entire structure, including cast aluminum lower control arms and steering knuckles, iron upper control arms, tie rods, slightly bigger stabilizer bars and the 3.27:1-geared, aluminum-cased differential.

And it's a totally new design. While many guessed Ford would merely re-engineer the rear setup found on the former Thunderbird to withstand the increased loads cranked out by the Cobra, SVT engineers borrowed only two bits from its parts bin in designing the Cobra's IRS, lifting the aluminum differential case and the wheel bearings from the now-defunct Lincoln Mark VIII.

It's a true self-contained, bolt-in rear end, too. To fit it on any other Fox-chassised vehicle simply requires the addition of two extra bolt holes and weld nuts where the former Quad-shocks sat. In theory, the IRS can even bolt on to any old Fairmont - the original Fox-body car - a few of which still happen to be alive and kicking.

Ford likens the design process of the IRS to reverse engineering, taking the space and layout of the former solid rear axle and directly replacing it with the significantly bigger and more complex independent unit. As a result, all of the loads go to the same places as with the solid rear, making the vehicle's performance fairly predictable.

But then that was the intent. At the outset, Ford proscribed SVT engineers from altering the Mustang's chassis, body or other major systems to accept the IRS. Doing so would've added millions in additional costs to alter the Fox skeleton, something Ford has obviously and successfully resisted since its inception in 1978.

Even so, the IRS' net shape comes surprisingly close to what Eric Zinkosky, Ford's suspension systems/development engineer for Team Mustang, penned in his initial sketches.

The bolt-in design brings with it some inherent manufacturing benefits, as well. Assembly workers install it side-by-side with the solid axle still residing in GT and V-6 models. And it arrives at the plant as a fully assembled system - including the differential - put together by Benteler Automotive Corp. of Galesburg, MI.

But the question remains: What does the IRS ultimately offer the Cobra that the solid rear axle could not? In three words: superior road manners.

The IRS reduces rear lift under braking, making driving the Cobra hard a much more forgiving task. It improves the vehicle's attitude and control, with a full 0.75 ins. (1.9 cm) more suspension travel available. And it reduces the unsprung weight of the Cobra by 125 lbs. (57 kg).

As expected, the IRS carries with it a fairly stiff weight penalty. To help compensate for the system's extra 70-lb. (32-kg) heft, SVT engineers extracted close to 50 lbs. (23 kg) from the car. The result: a weight distribution that moves from a 57%/43% front-to-rear split in the 1998 model to 55%/45% with the 1999, a significant enough amount to equal the effect of moving 80 lbs. (36kg) off the front wheels.

Ford didn't limit its tweaks to the rear wheels, however. Besides the addition of the IRS, Ford also addressed other aspects of the chassis to improve the car's handling characteristics, like widening the rear track by 1.2 ins. (3 cm) to match the space between the front wheels and lowering it overall by 0.25 ins. (0.6 cm).

Power is up for 1999 as well, to a peak 320 hp available at 6,000 rpm, an increase of 15 horses. Torque improves by 17 ft.-lbs. (22 Nm), topping out at 317 ft.-lbs. (428 Nm) at 4,750 rpm.

Thank new tumble-port cylinder head and intake manifold designs and reshaped combustion chambers for the power jumps - and for some of the car's shifted weight distribution. The car lost almost 20 lbs. (9 kg) due directly to the tumble-port head design and coil-on-plug arrangement.

All this translates into a 0-to-60 mph (97 km/h) time of 5.4 seconds, an improvement of 0.5 seconds over the 1998 Cobra.

Special Vehicle Operations, Ford's own in-house aftermarket performance parts supplier, plans on offering the IRS as a kit in the near future. When that day comes, there'll surely be a lot of very happy late-model Mustang - and Fairmont - buffs out there.