They are still eligible to compete, however they haven't fared very well against the newer cars from Corvette, Aston, and Ferrari in recent years. Much has to do with development, its all but ceased on the Viper while the heavy hitters (P&M Corvette, Aston Martin Racing) are constantly updating their kit. The Vipers are all in privateer hands these days, with no new bits coming from Chrysler. With each passing year its going to get harder & harder for teams using Vipers to get a nomination (unless they're French of course).
Fax