An Improved Production race car is defined as a modified road car intended for race track use, and is made popular by the fact that it is fast, and relatively inexpensive (as race cars go...) Based on a production body shell, and then carrying out limited modifications to engines, brakes and suspension proved to be a popular formula with both beginners and experienced competitors alike.
You are allowed to install a larger or different engine from the same manufacturer as the body shell, with freedoms permitted with head design, internal specification and capacity. The same style of fittment must remain, though. Popular conversions include late model twin cam 4 cylinder engines into early model 4 cylinder cars, and 13B rotaries into early model mazdas. Turbo's are also often fitted to these engines.
The car can be lowered to within 100mm of the ground (excluding the exhaust), and can change dampers, springs, sway bars and fit urethane bushes.
Wheel width can be 9 inches for vehicles over 3 litres of engine capacity if the cars production date is later than 1985.
All Improved Production Cars must run on the same tyre - the Yokohama A048R or A050 Race tyre.
Brakes can be upgraded.
Close ratio gearboxes and locked or limited slip differentials are allowed.
Body add on's such as wheel flares, front spoilers and rear spoilers are allowed, but sizes are limited.