I have a 4port 13B with bridgeport and currently running header and 2 1/4 with RB presilencer and blow through muffler and its loud but sound restricting on the engine. I have holley set up and this week I recieve my borla muffler and I wanted to know if I should go dual to match the header with 2inch pipe to the rear with y to the rear 3 inch and to the muffler ? I havent tried different exhaust but one thing for sure borlas sound nice.

 

two seperate 2" pipes running back, with a Y collector after the rear axle, and then a 3" muffler make awesome power for race ported rotaries.

The dimensions of the collector are critical; the throat of the collector needs to be about 2-1/4" diameter, then gradually expand to 3" diameter.