I've a jdm 1997 subaru forester. What type of gains am i able to be expecting if I place an aftermarket downpipe? Mike – Generally, higher move catalytic converters are only vital whenever you add a supercharger or turbocharger to an engine which was In a natural way aspirated. Also, https://cesaraintx.bimmwiki.com/10076955/the_definitive_guide_to_bmw