Both of the people who responded to my comment seem to have missed my point about ethanol being part of this. High-ethanol (E70 or higher) fuels have roughly h

Both of the people who responded to my comment seem to have missed my point about ethanol being part of this. High-ethanol (E70 or higher) fuels have roughly half the emissions when burned that gasoline does, while having a higher effective octane rating. That said, from an energy density perspective their energy density is lower, so it requires tuning so the fuel injectors on your car inject more fuel volume at a given speed density (or other tuning method). There's been multiple studies on this. From an environmental impact perspective, biofuels are generally considered worse than gasoline due to land-use, however at the point of use, when burned, they are massively cleaner than gasoline and don't produce the same quantity and types of byproducts. In point of fact, burning high ethanol biofuels without catalytic converters produces cleaner emissions than burning gasoline with catalytic converters. Also, my parent comment was not an argument against catalytic converters, it was an argument against the encryption of ECUs, banning of tuning components, etc. by the EPA in the name of emissions, when these things are necessary to convert a car that runs on gasoline to run on high-ethanol biofuels, which are better from an emissions perspective at point of use. You can use downstream high-flow catalytic converters with ethanol fuels for final cleaning, such as in the mid-pipe of the exhaust, but you cannot use them in the exhaust header itself as the temperature of the exhaust gas is significantly higher with high-ethanol fuels, which is part of why their byproducts are less emitting.