MadBill wrote:I'm not advocating that approach, merely providing a numerical answer to your flow-based pipe size question: "So is a flow chasing 53cc 1.6" valved exhaust that flows 170 cfm oversized for a relatively high rpm 289? Flows about 190 cfm with a 1 5/8" pipe. Thanks, Charlie"
As per my previous response, I believe pipe size comes first. Theories on the subject have certainly evolved over the years. The factory race system (developed in the late forties/early fifties) for the Vincent 61 c.i. V-twins I raced in the sixties was a pair of 2" x 54" pipes and a number of 30.5" singles in the pre-war era ran dual pipes of ~ 1.5".
A friend of mine talked with his old, late friend Mark Donohue at length about the AMCs. They tried all sorts of sizes on primaries on the Javelin motors with no difference, so they used what was most easily available. We know WAAAYYY better now and engines of that size with similar power are running primaries two sizes smaller. This totally means the ports they were using back then were way too big.