Tom Wilson
March 17, 2010

OK, this family of IRS brake kits has been out for a year, but this is our first look at it. Developed on Maximum's American Iron race car (as driven by Dave Royce in 2008), the kit represents the culmination of many iterations to arrive at a rear IRS brake with the rigidity, thermal capacity and hydraulic sizing required. The -15 version uses a fixed rotor at $1,297; the -16 substitutes a floating brake rotor at $1,597. The advantage of the floating rotor is reduced pad kick-back and acceptance of minor misalignments in the installation. Wilwood supplies the four-piston caliper and Maximum the stuff to get it on the IRS spindle. It's a rigid racing caliper with improved thermal capacity and no parking brake. Made from aluminum, it's not any lighter than the stock, cast-iron piece, which gives an idea to its beef. The stock caliper is a sliding bridge design, too, so the fixed Wilwood is inherently stiffer as well. The same massive better-cooling, 12.75-inch, vented Wilwood rotor is used in both kits, the difference being if Maximum modifies the hat to float or not. This rotor easily exceeds the stock Cobra part in cooling/thermal capacity and is about an inch larger in diameter. Maximum says any wheel that clears a Brembo, StopTech or Baer kit will clear this caliper. Piston sizes have been selected to work with the big aftermarket front calipers and not the stock Ford stuff. "And as with any racing brake system, an adjustable brake proportioning valve is recommended," according to Maximum staff.

Cobra IRS differentials are suspended in their subframe by three bushings, and there are benefits from changing these rubber parts with urethane replacements. The trick is the stock rubber bushing needs to have its center rubber removed while leaving the steel outer bushing, a difficult trick. The new MMT-5 tool easily does the job by pushing the rubber section out, tearing it from the steel bushing which is left undisturbed. Maximum notes they sell the rear bushing alone, or all three bushings as a kit, but not just the two front bushings. That's because it's best to change all three bushings at once, but some customers already have the two front bushings sourced from another shop.

If hogging out the differential-to-subframe bushings is a chore without a special tool, the IRS subframe-to-unibody bushings are even worse. There are four of these, and they too are best replaced with urethane bushings. The procedure is the same-push out the rubber center with the MMT-6 while leaving the steel outer shell intact-and is made more important because Ford welds these outer steel shells in place, leaving them slightly out of round. This simple MMT-6 press tool works like a charm, far better than trying to drill out the rubber. Interestingly, Ford has used two slightly different sized bushings here. Previously this caused some problems, but now it's carefully redesigned to fit in either Ford bushing shell Maximum's current urethane replacement mean you don't have to worry about it.

Photo Gallery

View Photo Gallery