I’ll be doing the manual 4wd twin stick conversion so my exhaust setup should be able to shoot straight down and back since it won’t have to go around the frame anymore. I just don’t want to regret not doing a 3 if I ever swap the engine to something a bit more powerful.I'm pretty sure stock is 2" and the general consensus is that 3" is complete overkill for a 13BT and harder to run.
Generalizations on straight pipe size, obviously for muffler and bends/etc this number will be lower.
A 2.5" exhaust would be good for over 230hp.
A 3" exhaust would be good for over 330HP.
I also see that stock 2" pipe is good for 140HP.
So I could see how turning up the boost and more fuel and a better intake, you would start to see some restrictions/EGT/heat issues on a 13BT that's "rated" 120HP factory.