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By 55blacktie - 7 Months Ago
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Just about all of the aftermarket Y-block cams I've seen are single-pattern. However, I know Ted has used dual-pattern cams on some of his builds. If the exhaust flows less than 80% of the intake, should a dual-pattern Cam be used. Just looking at some of John Mummert's numbers, it appears that the exhaust flow of G heads is about 70% of the intake. I have Sanderson headers/w 1.5-inch primaries and 2 1/4-inch dual pipes/w flow-through mufflers.
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By Joe-JDC - 7 Months Ago
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Even at 70% bias, most modern camshafts are designed with at least 4* more exhaust overlap than the intake. Whenever I port a set of iron heads, I try to get a minimum of 70% exhaust flow to the intake flow. However, if that number goes up significantly, then you run into the problem of the exhaust pulling fuel through the chamber into the exhaust pipe, and burning fuel in the exhaust. That also causes poor performance, and poor fuel mileage. On most BBC heads, it is not uncommon to see 62% exhaust percentage, and terrible rod ratio, but they still make big power. It all comes down to the camshaft design for the for the particular engine. Joe-JDC
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By Ted - 7 Months Ago
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As a generalized rule, I will use single pattern camshaft on a racing application where the headers are being run ‘open’. I do prefer dual pattern camshafts on street driven vehicles where a full exhaust system is being used. There’s enough back pressure being generated by the exhaust system to require a longer exhaust duration (more degrees) than what you see on the intake side in which to compensate for.
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