aapman

back pressure is checked by the obd2 system fitted to the car. smaller diamiter would have warmer gas and better moving gas. but the heat would not be good for the steering unit. the backbox on this car is a 20kg lump that has plenty of the backpressure in it. or you could refit a multi chamber midbox that would help the pressure. but in my words. its fine

Hi. i decat the system on my is200. i never looked back. did it at a exhaust fitment centre.(yanked the manifold and mid pipe and removed cats) the diffrnce is notable and no change in tone. however i fitted a back box with multiple chambers (sum bs about the back pressure) i`d say a increase of 20 horses just mind the sensors(heated oxygen) fron one is easy by removing the plug and unscrewing it. the manifold (headers) are tricky to get out as the steing unit is a bit in the way. remove 2 12 cm bolts. pull out and refit after wards . but the rear(under) is fixed. just remove the mid pipe. two bolts front and back. these have special heads. so grind and fit new. repack the gaskets after wards 3 hour job but lifetime of joy