Basically Ted is correct in all facets. When my TII was had stock exhaust, it pulled like a raped ape up to about 5800rpm, then fell on its face, lost approx 2-3psi (from 10) of boost and chugged to redline. I upgraded to an 80mm Corksport Turboback, ported the wastegate huge (almost 40mm) and tried it again with a 3" intake also. I couldn't believe my eyes, it pulled so much smoother up to 5800rpm then actually picked up steam all the way to 7100rpm. Boost stayed almost rock on 10psi and never budged. A free flowing exhaust system has got to be one of the better mods you can do to a T2 RX-7, if not the best. IMHO.

 

so on a turbo the exhaust causes loss in hp around 6k rpms will it do the same to a NA?

 

No.
Exhaust tuning on a turbo is simple - run the biggest thing you can afford and fit underneath.

Exhaust tuning on a NA is complex.
You need to balance low end toque with top end power.
If you run the exhaust too big, you lose low end torque, but you gain top end power.
If you run the exhaust too small, you gain low end torque, but you choke the engine on the top end.
This effect is magnefied on the NA, cause there is no turbo to help air / exhaust flow.

As a rule, NA's run very well on a 2.5" exhaust pipe for exhaust.


-Ted

 

I used a magnaflow muffler. Big can and 3" outlet. But its straight through. Doesn't look to shabby.