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.
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.