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13 On the left is the 1999-up Tumble Port with a larger 54cc combustion chamber. On the right is the original Twin Port with a 52cc heart-shaped chamber. The Twin Port offers a combined flow of 231 cfm from both intake ports at 0.500-inch valve lift. The Tumble Port yields 225 cfm intake flow at 0.500-inch valve lift. Exhaust flow is the same at 0.500-inch. The later ’03-’04 Cobra/Mach 1 head delivers the same intake port volume at 177cc, but greater compression thanks to a smaller 52cc chamber. Intake flow is 233 cfm. Exhaust flow is greater than the Tumble Port at 169 cfm.