I wonder if anyone has compared HF cat real efficiency, as to what actual resulting emissions content is after all these aftermarket cats, wherever in exhaust path they actually might be placed. Is one HF cat enough? If not - are two enough? (for real work cleaning up / burning excesses, not just to be there for visual checks). Are HF cats of specific CPI enough to match stock? Do they still are less resistant for airflow even if of maching efficiency?
Also if stock tune adresses heating up cats for better emissions during cold starts .. maybe there could be made some tune targeting emissions for use during checkups with even underperforming cat configuration? No clue on what principles it should be tuned by. By limiting rpms even at WOT? At some specific richness/leanness? Such 'legal' tune could also limit max noise at that
