Originally Posted by r3dn3ck

the independant rear is a wheelhop nightmare almost no matter what. You have to replace all the bushings with non-deflecting types, go to a pretty high rear spring rate (650 for standard type, 550+ for coil over), and generally make your car less streetable.

If you want good drag times and low maintenance then go find someone with a 99-04 GT or better yet a Mach1 and swap with them. The "live" axle is a solid axle where you have tubes extending from the differential to just before the wheels that contain the axles and allow the differential to move with the suspension. Independant suspensions remove that tube and fix the differential to the chassis which reduces unsprung weight and can improve handling. Once you have a new solid axle, then get some weight jacker arms and drag springs, or pop for some coil overs, and a maximum motorsports PHB & TA setup. You'll be moving down the track nicely with wild grip and solid handling.

Level 5 halfshafts won't help attenuate the wheel hop, they'll keep it from fragging your halfshafts so easily and are a requirement for guys that drive their car to the track instead of trailering it if they'd like to drive home. You also stand a strong chance of damaging your diff housing which is a 700 dollar aluminum part if you let the wheelhop get too bad. If it break it replace it with a 92-94 iron case from a T-Bird supercoupe. They're LOTS stronger.

So, from the Maximum Motorsports parts bin to get your IRS useable: Aluminum diff bushings front and back, delrin control arm bushings top and bottom, poly cradle bushings, and a rear diff support (I think billetflow makes one, if not check MT or Steeda (ick.. steeda's yucky)). Also get the stage 5 halfshafts... don't cheap out. Use as stiff a spring as you can stand and it'll help to a point. Stiffer rear springs without stiffer fronts can unbalance the handling so understand there are side effects. Also, pull the front sway bar so the front end can lift and that'll help plant the back end.

In order to launch a IRS car (I've had 2 gt's that I converted to IRS) you can try my method. No guarantees but it worked well for me on the street. (start by getting a really nice warm up on the tires is you're using DR's or slicks) hum it up to about 1800-2200rpm at the green, haze the clutch out as aggressively as you can without letting it spin the tires real hard. As soon as the car starts picking up speed you've got all the traction you're going to have for a few meters so dump the clutch slowly and at the same time start matting the go-pedal very smoothly. If you do it right the launch won't break any 60' time records but it will have you off to a smoother run than you'd otherwise have which helps times.

A live axle is easily 80lbs lighter no matter what than a IRS and that's going to equal almost a tenth in the 1/4 on weight alone. Add that you can build the 8.8 live axle nicely and really beat the hell out of it without too many worries of getting home. The IRS... one problem and you're stuck.