It's been more than 40 years since the federal Clean Air Act went into effect, which among other things means there are quite a few "antique" or "classic" vehicles (usually defined as a car or truck 25 years old or older in most areas) subject to emissions control laws and - sometimes - tailpipe emissions testing, too.

Even after 40-plus years. California, for example, requires that all motor vehicles built since the 1968 model year (the first year pollution controls were installed on new cars) pass an emissions test as a condition of being able to get plates/register the thing - just like any late-model car.

Other states have a rolling retirement system under which cars are subject to emissions testing until they hit 21 or 25 - after which they become exempt from the actual sniffer tests.

But - and this is important to know - even the more lenient rolling system doesn't exempt a pollution controlled motor vehicle (remember: That means pretty much anything built from 1968 onward) from having its original, factory-installed emissions control intact and at least seemingly operational. It may not have to pass a tailpipe sniffer test, but it will have to pass a visual once-over by the inspector. This is part of the state motor vehicle safety inspection that most states have in place, which usually includes checking off that the factory emissions stuff at least appears to be there - and working.

For example: If the inspector notices that a car originally equipped with a catalytic converter doesn't have one, the car will fail state inspection.

There's one possible escape hatch. Most states offer "antique vehicle" plates for cars and trucks more than 25 years old. Usually, these confer a permanent registration as well exempt them from both state motor vehicle emissions and safety tests. But the catch is these plates are usually limited use only and issued with the proviso that the car be driven less than 2,000 miles per year (if generous) or operated on public roads only for "testing" purposes, or to go to and from antique/old car shows - if not so generous.

If a cop notices you driving to work every day in an old car with antique tags, there's a good chance he's going to hassle you.

So it's important to think - and plan - before you make any modifications to an antique or classic car built after '68 - and especially those built after 1974, when catastrophic converters came online and emissions equipment started to get more elaborate with each new model year. Even though the total number of 25-year-old or older cars being used as daily drivers is small and their impact on air quality negligible, "old cars" are an easy - and visible - target for do-gooder political frauds braying about "the environment" and "clean air."

Here are some tips:

Don't be obvious -

Things such as aftermarket chromed valve covers, an open element air cleaner and other such clearly non-stock parts on the engine will make it more likely that the inspector's gonna look more closely at a car he might otherwise have given a casual once-over.
A side benefit is that factory air cleaners are usually big things that cover up most of the top part of the engine - neatly obscuring any mods. you may have made, or the absence of such parts as the EGR valve (which many non-factory, high-performance manifolds lack provision for). Save your factory valve covers, air cleaner and other such parts. These can be easily re-installed for inspection day - then swapped out for the good stuff after you get your sticker.

Camouflage your mods -

* Non-stock aftermarket aluminum intake manifolds can be easily disguised by painting (or powder-coating) them the same color as the engine. I used this trick on my '76 Pontiac to hide the presence of a medium-riser high-performance intake. The aftermarket manifold looks factory at a glance and only close inspection reveals the fact that the original part is no longer on the car. Avoid obviously not-factory carburetors, too. It's worth a little work tuning a stock Quadrajet or Carter AFB vs. pulling it out and replacing it with a double-pumper Holley.

* Reinstall all the factory fittings and hoses - even if they are just dummied up to look like they work and don't actually connect to anything. Many older cars built from 1968 to the 1980s have milesof vacuum lines and several fittings that screw into the intake manifold and the water neck/thermostat housing. Always try to retain at least the factory look of these parts were possible.

* Avoid headers - or use legal ones. They're e a great performance enhancer - but they're also obvious, visually and otherwise. The presence of headers on any pollution-controlled vehicle (especially '70s and '80s-era stuff) is going to cause problems for you. Use headers that are legal (they'll have a California Air Resources Board/CARB or EPA exemption) and that work with any existing pollution control equipment, such as Air Injection Reaction (AIR) tubes. They're more expensive than the generic, one-size-fits-all kind - but they're also not illegal and won't cause you to fail inspection. Another very good option with certain makes and models is to use factory-type cast-iron headers. My old Trans-Am has a set of factory Ram Air manifolds. Well, they were factory-available equipment on some '60s-era and early '70s GTOs and Firebirds. But they fit my car like they were factory-installed - and even better, they look factory installed.

* A reasonably quiet exhaust will help your case just like wearing a nice suit to court instead of cut-off jeans and a wife-beater T-shirt.
Flowmasters (and similar) aftermarket mufflers flow great and are also not-obnoxiously loud.

* Dummy cats. Early catalytic converters - especially the GM "pellet" models used on all GM cars from 1975 through the early '80s - sucked. They crippled performance. Just cutting them out and replacing the thing with a section of straight pipe could free up 15-20 hp on some cars. But now you've run afoul of The Law. A simple trick is to drain the thing.

Those old pellet style converters usually have a plug on the bottom that you can pry out. Then you drain the pellets. Looks right, performs better. Or, if you have a more modern "honeycomb" converter, you can cut the thing open, pull out the ceramic honeycomb inside and then bolt the two shell halves over a section of straight pipe to create your very own fake catalytic converter. It looks perfect - trust me.

The only caveat here is you can't do the above skullduggery on cars equipped with OnBoard Diagnostics, which means most cars built from the early-mid '90s forward. The "check engine" light will come on - and if the inspector sees that, you'll probably fail the inspection.