The GS Class (as well as the ST class) is a production car-based racing series. According to the GrandAm literature, this class was created to show the reliability and durability of production cars in a racing environment. The cars competing in this class are more or less stock. While major modifications are allowed when it comes to safety, the engine and the rest of the car remains as it did when it was still on the showroom floor. In other words, this is real stock car racing.

So all these body panels you see are factory sheetmetal. No carbon fiber or non-OEM fiberglass or aero pieces are used here. While safety rules dictate that the driver and passenger door glass must be removed, all other greenhouse pieces are OEM glass. No Lexan allowed!

Modifications can be done to the suspension, provided the parts bolt up to the standard mounting points. Brakes can be replaced, but the discs can't be more than 5% bigger than the stock units, and no special ducting is allowed. The engines can be blueprinted and balanced, and some stand-alone engine management systems can be used, but crazy overbores and headwork isn't permitted. Still, this "minor" tuning can still get 350-400 reliable horsepower out of these engines.

Even the gas tanks must be stock, unless safety rules dictate that another unit must be used. All the fuel lines can be upgraded, of course, but this means that the cars have wildly different fuel capacities. The Porsche 966s used have 16.5 gallon tanks while cars such as the Ford Mustang are permitted to have 20 gallon tanks. Teams must really plan ahead when it comes to fuel consumption here!

Due to sponsorship reasons or something, all GS Cup cars must run approved Hoosier tires. However, as Hoosiers are known for their stickiness (and grip), we're sure that this is one mod that everyone is happy with.

For the GS Cup, all cars are required to have at least two drivers. Of course, this means that at least one driver change has to occur during the race.

Of course, the cars used in the ST class have smaller-displacement 4 and 6-cylinder engines. Forced induction cars are allowed to keep their blowers, but they cannot be modified to make more power (no cranking up the boost...). Cars in this class make between 170 and 240 horsepower and have a top speed of about 135 miles per hour.