Dear Ray;
On the later 'smog' engines any modification can be used to fail the test in California.
In the USA (in general) I guess It's up to whoever is testing, what state (is testing) and their regulations at the time.
A few years back a large Co. that did Propane conversions was suied out of business by the State, for just that. It didn't matter that the engines producd less emmissions after the conversion as; they used the (modification) law to "flex their muscles".
It's just "nuts"!!
Most states don't get that 'crazy' about it though.
Vehicles from model years 1974 and before are exempt from testing. So; Our (Hot Rods) are mostly left alone (finally) these days.
