Originally Posted by Lenny

there are two cats; one per header

in most exhaust layouts, the catalic converters are where a lot of back-pressure builds up due to the setup, hfcs free up that pressure and allow for a briskier throttle response, though often at the expense of some high-end power (loss of back-pressure in high rpms tends to lose hp, tho i'm not sure if thats the case with the Z)