Muscle Mustangs & Fast FordsHow To Chassis Suspension
Cobra IRS Under Fox-body Mustang - Declaration of Independence
Realspeed Automotive retrofits a Cobra IRS under a Fox-body Mustang.
In 1999, Ford Motor Company rolled out its special edition SVT Cobra to the usual fanfare and excitement of a new-car launch. The company had just updated the sheetmetal of the SN-95 platform and the New Edge appearance garnered rave reviews as the Mustang began to separate itself—in terms of sales—from the competition. Lots of attention was focused on the New Edge GT and Cobra bodywork, but not to be overlooked was the Four-Valve 4.6L engine underhood. Ford jumped the power rating to 320 hp, up from the 305 hp of the '96-'98 SVT Cobra models.
More power and better looks are always a good thing—but there was more. For the first time in Mustang history, the engineers added an independent rear suspension (IRS) system to the '99 Cobra and stuck with it until the Special Edition's demise in 2004. There are several attributes to an IRS, with the highlights being better handling and ride quality than a live-axle arrangement. The better ride and handling are due to each axle/side absorbing the bumps/depressions independently of the other side. An IRS features axles that move through their respective range of travel independently of the other side. In a live-axle combination, the axles are housed in a solid rearend, so when one axle goes up, the other dips down, thus leaving less tire contacting the road. Additionally, with an IRS you can adjust camber, while it is fixed with a solid axle. Many feel an IRS is the superior combination for road racing, but not so much in drag racing applications.
"Number one for me is ride quality," said Dan Carlson of Realspeed Automotive. "I can get pretty aggressive with spring rates and still have a nice ride [with an IRS]. On a smooth track, there isn't that much advantage, but on a bumpy track or road, the IRS will shine," he added.
The strength of the Mustang IRS for drag racing is questionable, due to the addition of half-shafts that are prone to failure (especially in stock trim), and so the application of power is often limited. But the world isn't just about drag racing. Kevin Hamel is living proof of it.
"I used to be a drag race guy, but the local dragstrip was shut down, so I ended up getting an SVO and auto-crossing," comments the New York resident. The SVO was his gateway drug into the turning and twisting courses, but it was eventually sold when he picked up this '91 Mustang LX convertible-turned-Saleen clone.
The engine is warmed over with a heads, cam, and intake package to go with the usual exhaust and header upgrades. But it is the suspension system that garners a majority of attention from his local Mustang speed shop—Realspeed Automotive. The shop added the usual repertoire of modifications to the rear axle, like upper and lower control arms, a Panhard bar, and even a torque arm—all sourced from Maximum Motorsports. The front end was also upgraded with a tubular K-member, tubular A-arms, and coilover front struts.
Hamel relied on Realspeed Automotive's extensive road racing background for the parts selection and chassis tuning. The combination worked well, but Hamel has his sites set on open-track days at Lime Rock Park (Lakeville, Connecticut), New Jersey Motorsports Park (Millville, New Jersey), and Pocono Raceway (Long Pond, Pennsylvania). He wants a little extra bite for his open-track aspirations, which led him to investigate the IRS conversion from the New Edge generation of SVT Cobra models.
The swap is possible because the Fox-body Mustang ('79-'93 models) and the Fox-4 platform ('95-'04) are very similar, save for a few changes here and there for the wider body, with similar framerails and floors. In some pre-build research, Hamel and the guys at Realspeed Automotive determined that everything bolts in except for rear IRS subframe mount. That would require some fabrication and welding. Some Internet postings have shown the rear mount being bolted to the factory framerail, but Realspeed's Carlson and Rob DeMartinis decided to add through-bolts top to bottom and side to side to prevent crushing the framerail.
A cruise around eBay and other classified message boards showed a lot of IRS suspension systems for sale. Hamel looked at a few '99 Cobra IRS suspension setups, but decided against it and narrowed his search to the '03-'04 Cobra setup. "I found a Terminator setup on eBay for $1,500, plus shipping. It was built with a 3.55 rear gear, Billetflow IRS support, and Maximum Motorsports bushings. Typically a stock Terminator IRS goes for around $1,000," notes Hamel. He paid a little more for this one because of the mods and just 10,000 miles of road use.
The addition of the IRS added 48 pounds, which isn't significant in a road-racing application since the trade-off is significantly better handling. The swap was performed during the dead of winter in New York, so before and after testing wasn't possible. However, Hamel did share his thoughts on how it drove on the street. "There is a big controversy over the IRS swap, whether it's worth the money and work. I'd like to add my two cents and say yes, it's worth it!"
Hamel tells us the car feels totally different, from the way it reacts in turns, around town, and even over bumps on the New York roadways.
"We installed Maximum Motorsports coilovers in the rear and we plan to install a bumpsteer kit after he takes it to the track. There are a lot of tuning with spring rates and shock valve adjustment based on how the car handles," stated Carlson.
If drag racing isn't your thing and the open track days are calling your name, then definitely look at retrofitting an IRS into your Fox-body—then go Porsche hunting.