It may very well have improved, or at least not affected, his emissions, but the EO number is basically the "proof", it's the number assigned to a part after it has undergone the necessary testing that demonstrates emissions are not negatively impacted. In short, in California, you can only (legally) fit aftermarket emissions-related parts that have actually been tested by a state-approved lab. Once it passes testing, the California Air Resources Board assigns an Executive Order number which validates the part for sale and installation for on-highway use. (Off-highway, etc. is typically unrestricted)
The reason for this is that for a long time aftermarket suppliers were making parts that were substantially WORSE than stock, they basically peed the pool for everybody. The EO system was a way to close the loophole created when California mandated stricter emissions standards for cars. Too many people were buying new, clean, cars, then changing out any number of components in the name of performance and rendering the cars VERY unclean. In some cases, the combination of parts made the newer cars with aftermarket parts literally a thousand times more dirty for certain metrics than the older "pre-emissions" cars. Stuff likes oxides of nitrogen, CO, CO2, etc.
The range of tests varies with the parts. If you're doing something like a full turbo kit, it's a fairly involved test that lasts several days (because there are days of evaporative testing inside a sealed tent to make sure you aren't forcing fuel vapors out of the pressurized system, etc.). I believe the testing for a simple intake (or other "air plumbing only" type mods is simpler and only involves measured post-burn (tailpipe) testing.
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As a lifelong Californian who is also a near-lifelong gearhead, let me tell you that overall, it's "worth it". We never had "snow days" when I was in school, but back in the late '70s and early '80s we had "Smog Days" - days where the kids had to stay indoors all day (no recess, lunch at our desks), because the local air quality was dangerously bad. Back in the '60s, my Mom attended USC in LA for 4 months before the first Santa Ana blew the smog out and she saw the mountains for the first time. She was so surprised she crashed her bike. The air is MUCH cleaner now, and the cars are faster than ever, despite complaints from auto makers and aftermarket manufacturers that new regulations would render the automobile untenable in California.
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