Saturday, January 20, 2018
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IRS Facts, Fiction, and Useless (and Useful) Trivia

Q. - So what's the difference in the 99, 2000 R, 2001-2 and 2003-2004 Units?

A. - Here's what we know so far:

  • 1999 - small 28 spline axles, 3.27 gear, posi center, soft rubber in subframe mounts.
  • 2000 Cobra R - a true beast. 3.55 gerodisc hydro-mechanical posi center, massive 31 spline axles, improved CV joints, upper camber slot moved in 5mm to improve camber adjustment, rubber bushings are the hardest installed, reduces body hop.
  • 2001-02 - mild improvement in axles - 31 spline on the inside while still 28 on the ends. Still has 3.27 gears.
  • 2003-2004 - a lot of parts from the 2000 R unit make it to the 2003-2004 units. 31 spline axles (slightly shorter) 3.55 gears (standard posi and not the expensive Gerodisc), stiffer but not as stiff as the R units rubber subframe bushings, improve camber and new for 2003-2004 - the inner tie rod mounts are lowered 6mm to improve bump steer.

Q. - Will standard GT brakes fit on the IRS unit?

A. - Yes. GT and Cobra brakes will interchange. The Cobra is a bigger brake unit so it has a larger throat on the calipers. The rotors are E4ZZ parts - they haven't changed since 94. The late model Bullit cars had Cobra size calipers and rotors. So, if you find an IRS unit without brakes, GT brakes will work in a pinch!

Q. - I've heard the IRS units are weak. Is this true?

A. - Yes and no. They are weak if you take them to the drag strip, install a set of slicks on them and pull 5000 rpm hole shots through them. There are five transfer joints in one of these things - you can hurt them if you abuse them. These things aren't the best solution for drag racing anyway. If you are road racing the car you shouldn't have a problem. Also note that in the Ford Motorsport Catalog they state that gearing higher than 3.27 can reduce the lifespan of some of the parts in these things - higher gears mean more torque transfer.

Q. - What about the tie rod connecting rods bending?

A. - There have been a couple of guys who have bent them, but it doesn't seem to be a major issue. If you are worried about them, Kenny Brown sells a set of hardened connectors and that should more than take care of the problem.

Q. - Will a Super Coupe IRS unit work with your kit?

A. - Nope. Although the SC and Lincoln pieces are more plentiful, they are bigger, heavier, weaker in some places and the mounts are completely different. Here's a shot of each for comparison:

Cobra IRS on the left
T-bird IRS on the right