[25stevem]

I don’t know if you folks have ever seen this guide chart for exh header tube size, but I have found it extremely accurate!

The only tempering I add to this is if I need more exh flow then the port is providing in order to get the exh to intake ratio better .

Any exh port I have ever worked with of any make on my flow bench ( and even long ones like Pontiacs ) will pick up 3 to 5 percent more cfm above .350” lift if the header tube that bolts to the exh flange is 1/8” larger in it’s perimeter.

This why especially with a race motor that the exh side is flow tested with at least a 6” long tube in place.