i just purchased 03 cobra  .i 'm told that yhe weak link is the rear . what is my limitation as far as horsepower . how much will this rear end support .  

The limitation is more in how you drive the car than it is a result of horsepower. There are plenty of cars running nine-second elapsed times in the quarter-mile with the IRS, it just needs some beefed up parts, which is really no different than a standard axle.

Aggressive launches and the use of sticky drag radial or slick-type of tires at the strip can result in axle destruction, be it solid or IRS. A fortified solid axle is marginally cheaper to build than an IRS, but if you are planning to strictly drag race your car, chassis setup is a little easier. If you're going to continue to drive you snake on the street, keep the IRS and pick up an IRS bushing kit from someone like Maximum Motorsports, along with a differential brace and some high-performance half-shafts.

Depending on how you drive the car and how much horsepower you are making, the stock IRS will take you quite far, though the bushing kit is highly recommended at any power level.

Steve  

Also, don't 5,000 rpm clutch dump on your IRS all day long and let the wheels hop.  I am sick of guys who drive their IRS cars like idiots, destroy the rear end and then say it was too weak.  How about you listen to the car, when doing a burn out and the control arms are getting bound up and causing wheel hop back out of it!