My 71 Olds 442 came from the factory with 2 1/4 headpipes and 2 inch tailpipes.I had changed the cam in the motor with a Kenne-Bell Mark 2. The best I could get out of the car with stock exhaust was 13.9 at 103 mph. I took a long extension with a 9/16 socket and loosened the headpipes off the exhaust manifolds about 1/4 inch to relieve some backpressure and ran it down the 1/4 mile. The car went 13.4 at 107 mph, backfiring out of the carb at higher rpm from being too lean. If I would have had richer secondary rods for the Q-Jet to put in, I'm sure it would have improved even more.
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