[Stan Weiss]

Originally Posted by 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.

Haven gotten flow number from people like Darin Morgan I have found that they do not flow the exhaust with a pipe.

While I don't know how well that formula works. He does state there is a range for FD of from 80 to 90. The Graph uses 80 which produces a larger pipe size than 90 will.

We can see at 160 cfm the difference is a little over 0.09" in diameter
► cfm=160
► ((cfm*1.27)/80)^.5
1.59374
► ((cfm*1.27)/90)^.5
1.50259

Stan