1973 NASCAR International Stock Car Racing Rule Book NASCAR Grand National ® Championship Division Section 20-8 -- Exhaust

a. Muffler must be removed, exhaust manifold optional. Pipes from muffler take-off must be no larger than four inches inside diameter when round exhaust pipe is used. Maximum one 4" pipe per side. Any other system must be approved by NASCAR. Exhaust pipes must extend past driver and to the outer edge of the car, beneath frame, with a minimum ground clearance of four inches. Exhaust pipes must come out on side of car in front of rear wheels only.

b. Extra brackets are mandatory.

c. The chief inspector reserves right to reject improperly mounted or unsafe exhaust pipes.