A long time Vizard disciple, I've found that a moderately worked over head valve engine needs an internal exhast area equal to the cubic inches of the engine, divided by 60. This is based on thousands of emperical dyno runs on vehicles ranging from 50 cube Minis to 460 cube Lincolns. This guy maintained that zero backpressure is the policy for performance, but that header (extractor) selection is best done on a dyno.
Example exhast outlet internal diameters. (This assumes all bends over the differential or about the gas tank and tranny are either mild, or are of a diameter 0.25 inches greater than the size listed below):
144 cid = 2.400 sq inches ( 1.75 inch id)
170 cid = 2.833 sq inches ( 1.90 inch id)
200 cid = 3.333 sq inches ( 2.0625 inch id)
240 cid = 4.000 sq inches ( 2.25 inch id )
250 cid = 4.167 sq inches ( 2.3 inch id single, or two bigger than 1.625 id pipes)
300 cid = 5.000 sq inches ( 2.5 inch id single, or two bigger than 1.78 id pipes)
For full house engines,where low speed torque isn't an issue, you can divide the cubes by up to 35. An old Falcon campaigner, Aussie Dick Johnson, is on record for saying that the pipe diameter in a street engine must be big enough to drop backpressure, and small enough to keep the exhast drawing the spent gasses out of the exhast at the optimum speed. Lots of guys with 351 V8's found that low speed torque plumetts when a big bore extractor system goes on. Don't streach the friendship!
