what size exaust??????
 

i bought a 87 rx7 witha new engine. my question is what would be the best size exaust for power. i will be installing a header too.. the reason i ask is because it feels like im losing low end torque because of the exaust thats on there now which is 3 inch i think. with cat replaced with a resonater. i know it should be smaller but can i just run a straight pipe where the converter is???

I would say 2.25 or 2.5 for an NA. But you will need find a way to open the aux. ports with your S4.