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Thermal Air Cleaners became available in 70 for CA bound vehicles and were included among GM emission control systems in 71 to meet standards. Yet, weren't not used on Corvette motors until 1973...anyone know why?
The obvious answer is GM met HC and CO emission standards but did not have to revert to the thermal air cleaners until 73 when new standards were set.
Thermal air cleaners were not used on Corvettes until the 1976 model year, although they were used on other Chevrolet cars as early as 1968. I don't know why they were delayed so long appearing on Corvettes. But, that's the way it was.
Starting with the '73 model year, emission certification standards were established in grams per mile, an absolute standard, rather than the previous proportional standard of HC in PPM and CO in percent. (federal NOx standards were established for '72 models, CA in '71)
A light car takes less power to run on the dynamomomter certification cycle, which means less total air consumed and less total exhaust gas. Thus, a lighter car can have higher proportional emissions than a heavy car, both of which have to make the same absolute bogeys.
This may explain why a Corvette V-8 of a given year may have less emission control oriented equipment than the same basic V-8 in a heavier car such as the full size Chevrolet.
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