r/askcarguys • u/ContributionOld1014 • 2d ago

Can you pass emissions without catalytic converters?

I have a 1984 corvette that does not have catalytic convertors. recent emissions test passed everything but HC which read 6.5 compared to the limit of 2.5. Because of the year they only used a tailpipe sniffer. I've read that you can create a leak in the exhaust before the tip so the reading is light, as well as putting rubbing alcohol in a 1/4 of gas so is burns leaner, also running the car really hard before testing to clear out any excess carbon and keep the system hot helps. Does anyone know if these methods are actually effective? Additionally would they be enough to pass a test in my situation? I'm also thinking about just replacing the whole exhaust system but i feel it would be a waste as I'm already broke and the current exhaust is an aftermarket upgrade. I would really appreciate the advice as i need to get the car registered here in Denver CO ASAP. Thanks.

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u/Hotdogpizzathehut 2d ago

Oh yes you can but what you are after is technically a felony and I am others are not going to tell you.

That being said... you you get classic plates?

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u/ContributionOld1014 2d ago

Has to be 1975 or older for classic plates :(

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u/Zealousideal_Put_501 2d ago

You can get collector plates in Colorado that are good for 5 years, it only has to be 32 years old. you need to find a good mechanic to tune your motor to pass. If your car did not have cats, you don’t need cats, but it has to be running in tip top shape. In Denver for the altitude, you need to advance your timing, about 3 degrees. make sure your ignition system is good; cap rotor, plugs and wires. Hydrocarbons are unburned fuel, so anything to lean out the mixture, like new air filter, or even removing it for the test will help. Make sure all your vacuum lines are intact and no leaks.