Removing cats permanently - is a test pipe all I need?

I’ve often wondered what the ACTUAL emissions looked like before and after a CAT napping on my old Honda NOx went up as did CO but it seemed like the overall pollution wasn’t much different before and after

YMMV

 

Here's a graph based on the World Bank data:
How do catalytic converters work? - Explain that Stuff

CO is 1/7 of no cat, HCs are 1/6 of no cat, and NOx is 1/3. That big of differences can have a serious impact on health and without cats would literally have killed thousands and thousands of people since the introduction of cats on cars in the US.

 

on the older lean burn Honda’s that came both with and without cats from the factory the difference was not that dramatic, much of the emissions controls are due to “other” changes with the engine valves and ECM controls working with the cats behavior .

the reductions seen Recently are simply impossible just by strapping a cat on

real data off his car would be ideal, my guess is that specific emissions will go up a lot but others won’t move much

ah well

 

A lean burn engine probably also is running a two-way converter, which is much less efficient and would create a smaller difference. Pretty much all car engines use three-way converters, which have a much larger impact.