California since the 90s has had very strict emissions standards. Smog is terrible and air quality in southern California is horrible around any densely populated area on a hot day above 80 degrees. The majority of vehicles that meet California emissions standards and differ from the engineered version that is mass produced for the rest of the country are those vehicles using engines with technology developed prior to the late 90s. Engines engineered during the 90s and put to use in vehicles 2000 model year and newer are very clean and efficient low emissions engines compared to those engineered in the late 70s and early 80s that were put to use in vehicles well through the 90s.
Just about every vehicle 2000 model year and newer meets California emissions standards with the factory engineering and nothing else required. Vehicles pre 2000 especially those still using pushrod designs were sometimes equipped with extra catalytic converters or different timing/different ECM program usually resulting in less power because of less fuel injected in cylinders.
A great example right on this website is a California emissions equipped Mark VII. I believe it was 1991 and 1992 years that actually received Mass airflow sensors instead of the Map sensor and speed density program the rest of the states ran. Mark VIIs in the rest of the country are only speed density. The Mass airflow was equipped to the California models only to allow a more advanced degree of fuel management for a more efficient combustion due to the mass airflow sensor able to adjust to atmospheric events as the speed density program while reliable can only work within a confined parameter with little to no adjustment otherwise.
Mass airflow, direct injection, and variable valve timing have done a wealth of good for the internal combustion engine as we see them today utilizing all these features that help manage fuel/air ratios and keep the engines efficient making the most of the fuel ignited.
