Cat Back Setup

i need some more grunt . so here it is... my first performance mods...

de-restricted stock Intake (thx to visage):
- take off the resonator chambers on the intake... and leaving the stock airbox with stock filter on.
- put a flex pipe from the airbox to the front bumper grille to feed fresh air

benefit: higher air flow, more throttle response, nicer intake sound (esp on higher rev)


Exhaust:

this is stock mazda3 system (2inch):
- 4-1 header coupled with 1st cat
- then a second cat
- a pre-muffler
- a muffler


i use 2.5inch inlet and outlet magnaflow hi-flow SS muffler

then for the piping, i tried few different setups:
1. 2.25inch system with no second cat, with 2.25 resonator, and just 2.25 press bent pipe. still very restrictive.

2. no second cat and no resonator. replaced with 2.25inch mandrel bent pipe. higher flow but still very restrictive.

3. this is the last and the best setup so far. it is 2.5inch mandrel bent one piece straight pipe after the header and the first cat. all the way to the muffler, then press bent 2.25inch tail pipe after the muffler outlet for a bit of back pressure that was lost by opting 2.5inch pipe....