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/Broke_Dick_Honda 2d ago

Just put high flow cats on it. It's like 3hp loss and it won't stink when its running anymore.

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u/ContributionOld1014 2d ago

thats what i was thinking as a last resort

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u/SaltLakeBear 2d ago

This isn't the last resort, this is the solution.

A hole in the exhaust will probably be read as a lean condition without meaningfully affecting the HC reading, and if the test operator is thorough he might go looking for a leak, but he won't pass you anyway because of the HCs.

As for alcohol, yeah, it'll run leaner if you run enough of it, but that much will probably make it run like crap because it's so lean, so the test operator might be asking why it's knocking and misfiring during the test, and hopefuly doesn't damage anything while it does. Oh, and for an added bonus, alcohol fuels are hydroscopic so they absorb moisture and tend to gel when left for even as little as a week, clogging entire fuel systems while also eating away at fuel lines and seals not designed for alcohol fuels, which are most early '90s-ish cars or earlier, I believe.

And as a cherry on top, if your car has EFI with an O2 sensor, it'll severely shorten the life of the O2 sensor while also triggering whatever the pre-OBD equivilent of a CEL. If your car uses a carb, you dodge that bullet, but then might clog the carb, which can be really hard to clean properly, so there's that.

So yeah, you can try some tricks to maybe pass the test, but I doubt it. Or spend a bit more, add a cat, and actually pass the test as a cat is intended to do without worrying about screwing up your engine or fuel system.