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This text came from the http://www.professormotor.com/ website, I hope they don't mind if I re-post it here.
Following is from the John Ford Slot Car Dictionary (SARN Magazine) :
Gp12 - Originally named for its cost. A Gp12 had to cost under $12.00 ready to run out of the box. Today it is a stamped mass production chassis with Group 12 motors. This class has been around since the early 70's.
Gp15 - Originally named for its cost. A Gp15 had to cost under $15.00 ready to run out of the box. The original concept of this box stock class is no longer around in USRA racing. It has melded into a new class of car that remotely resembles its predecessor. The only thing that remains the same is the armature which still retains 29 wraps of 29 gauge wire. More or less. Remember boys and girls, every car is legal until it is proven illegal.
Gp20 - Originally name for its cost. A Gp20 had to cost under $20.00 ready to run out of the box. An almost non-existent class today. It never was very popular. In its beginning stages, the lack of popularity was because of the rigid rules placed on the chassis. A car with the speed of a group 20 was held back in its progress by a chassis barely suitable for group 12 racing. In Texas, Jim Honeycutt is credited with solving the problem by forming a local class using group 20 motors and group 7 chassis. See Gp27.
Gp22 - In the early 70's the Group 20 motor was popular due to its high speed and low cost. There was quite a movement to take this motor and make it more efficient. Hand wound versions of the Gp20 armature were made by Thorp and Steube from California. To tell them apart, they had tags marked 22, merely a number to show it was a hand wound 20. If you come across any of these 22 tagged arms today, they are quite rare. Hang on to it.
G27 - The hand wound Group 22 arms were becoming quite popular, and in Texas especially in the local Group 20/7 hybrid class originated by Jim Honeycutt of San Antonio. The popularity of this hybrid class grew to the point that it actually became a class, and the hand wound group 20 arms began to carry the tag 27. The rest, as they say is history.
Following is from the John Ford Slot Car Dictionary (SARN Magazine) :
Gp12 - Originally named for its cost. A Gp12 had to cost under $12.00 ready to run out of the box. Today it is a stamped mass production chassis with Group 12 motors. This class has been around since the early 70's.
Gp15 - Originally named for its cost. A Gp15 had to cost under $15.00 ready to run out of the box. The original concept of this box stock class is no longer around in USRA racing. It has melded into a new class of car that remotely resembles its predecessor. The only thing that remains the same is the armature which still retains 29 wraps of 29 gauge wire. More or less. Remember boys and girls, every car is legal until it is proven illegal.
Gp20 - Originally name for its cost. A Gp20 had to cost under $20.00 ready to run out of the box. An almost non-existent class today. It never was very popular. In its beginning stages, the lack of popularity was because of the rigid rules placed on the chassis. A car with the speed of a group 20 was held back in its progress by a chassis barely suitable for group 12 racing. In Texas, Jim Honeycutt is credited with solving the problem by forming a local class using group 20 motors and group 7 chassis. See Gp27.
Gp22 - In the early 70's the Group 20 motor was popular due to its high speed and low cost. There was quite a movement to take this motor and make it more efficient. Hand wound versions of the Gp20 armature were made by Thorp and Steube from California. To tell them apart, they had tags marked 22, merely a number to show it was a hand wound 20. If you come across any of these 22 tagged arms today, they are quite rare. Hang on to it.
G27 - The hand wound Group 22 arms were becoming quite popular, and in Texas especially in the local Group 20/7 hybrid class originated by Jim Honeycutt of San Antonio. The popularity of this hybrid class grew to the point that it actually became a class, and the hand wound group 20 arms began to carry the tag 27. The rest, as they say is history.