Comment by wcunning

I work for a competing car company and I once saw a presentation showing that turbos significantly increased CO2 emissions because the cooling of the exhaust air causes the catalytic converter to operate a few hundred degrees Fahrenheit cooler, lowering reaction efficiency. This can be mitigated by hybrid setups, since you can avoid pumping cold air down the exhaust path on decelerations, but without that, it’s actually not clear that the emissions efficiency of smaller, turbo motors is necessarily better at similar power outputs to larger naturally aspirated engines.