The biggest mistake people make is buying more expensive fuel. People often have the misconception that how to pass a smog test is to use premium 91 octane fuel. It seems to make sense, right? More expensive fuel is probably purer, which means it probably burns cleaner and gives off fewer emissions right? Wrong. 91 Octane fuel contains higher amounts of combustible chemicals, which means it’s formulated to burn longer, in higher performance engines. If you put it in an engine that’s not meant for high performance, the fuel won’t completely burn up inside the chamber, meaning that every time a piston fires, a little bit of unburnt fuel will be pushed out of the exhaust, causing much MORE emissions to come out of the engine.

Classic cars do qualify for a smog exemption in California. By definition, a classic or antique car is any vehicle that is 25 years or more old. Since cars built before 1975 are exempt from the California smog certification program, owners are not required to have their vehicles checked for smog-producing emissions. This include oxides of nitrogen, hydrocarbon emissions and carbon monoxide emissions. Pre-1975 cars have rudimentary emission control systems that usually consist of exhaust gas recirculation valves, as well as early manifold air pressure sensors. However, this is about the limit of any emissions add-on hardware. The other cure used was leaning out the fuel-air mixture as much as possible.