The Racing Turbo chassis was released in late 1985/early 1986.  The chassis material was ABS plastic and included a can motor making it easier to work on than the pancake motors.  There were two versions of the chassis with variations of each.  The short version (wheelbase 1.5"/ 38.1 mm) had tabs molded into the side of the chassis to allow the bodies to clip on.  This chassis was used for all bodies except the formula/Indy bodies.  The long chassis version (1.625"/47.4 mm) had slots molded in the side of the chassis to allow the tabs molded on the formula/Indy bodies to snap in place.  (Jim Russell called this long chassis the "mono-posto" meaning single seater chassis, but this designation did not catch on.)  When the RT chassis were first released, the ceramic traction magnets were square and did not extend beyond the inside edge of the tires.  This is referred to as the "narrow" chassis.  In about 1988 an "L" shaped traction magnet was used in place of the square one.  This is referred to as the "wide" chassis.   In order to fit the formula body around the "L" shaped magnet a small notch had be fabricated in the bodies.  There were also several variations of the guide pin mounting boss (the area where the guide pin snaps in).  In addition, the bulkhead width between the traction magnets also changed once or twice during the life of the chassis. All versions came with a 7 tooth pinion and a 22 tooth crown gear.  My thanks to Brad Bowman for the majority of this historical information.  I have also seen some cars released by Irwin (Canada) that refer to a Turbo Plus chassis.  This appears to me to be a Racing Turbo chassis, but I could be wrong.