| Yes that sounds fair, but the header has a taper and this gets thrown into it aswell. As well as the port size. The port size comes into it if you have just thrown in a set of bigger valves and not increased the port size. All that exaust gast escapes into the port at x pressure and velocity. Then it enters the header. If the diameter of the header is too large, the gas then expands into the header and drops velocity and increases pressure, and throws the whole thing out of whack.
The taper, from memory, should be finished 13 inches from the valve. this enables the correct forming of the shock wave. This shock wave is responsable for pushing the the remainder of the exhaust charge back down the port and partially into the chamber, thereby pre pressurising, getting ready for the incoming fuel/air charge. In regards to the std ST4 headers, are they the same ratios as the 888 and SP2/3 headers? My guess is that they are different. Hence the 45mm system works well. But As I don't have any to measure I am only guessing.  |