i saw a full exhaust on rx7.com that was made by GReddy it is 3.75in. my ? is 3.75 too big? im planning on doing it with a single turbo also. i know on a piston engine that back presure gives you tq but does it on a rotary?

 

I would think 3.7 inches would be ok (you have a dp and cat, right?). Stock is around 3 inches, anyway. Regarding torque, yes, there is an ideal amount of back pressure required for rotary engines to deliver peak torque. Do a search for the exact numbers. On a practical side, I would think your installation of a single turbo is likely to cost you more torque than the 3.75 inch exhaust. Prevailing wisdom on this board seems to suggest stock or M3 sequential twins will give you the most streetable torque. A single large turbo will generate more boost at higher RPMs. There are some smaller singles that reduce some of this negative effect. Anyone else have an opinion?