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The 2-inch header primary tube size on this 289 is too large for a stock engine. This engine needs 1 5/8-inch or smaller for improved back-pressure and exhaust scavenging. It's always a delicate balance to get tube size so it's not too small (too much back pressure) or too large (not enough). When there's too much back-pressure, an engine suffers from exhaust restriction and power loss at high rpm along with excessive heat. When header tubes are too large, you lose back pressure and torque.