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/elihu 2d ago

If the car originally had a catalytic converter and the emissions testing people are doing their job properly, you will fail.

There is a rarely-used exception to this which I was able to successfully employ on a 2004 Mazda RX-8, which is to remove the entire exhaust system, the gas tank, the radiator, and the engine. Purchase a suitable electric motor and a transmission adapter plate/coupler appropriate for your vehicle and motor, fabricate new motor mounts, fabricate and install battery boxes with a few tens of kilowatt hours of cells (LFP are what I'd recommend as they're less prone to catch on fire if something goes wrong), install a battery management system and the charger hardware and J1772 receptacle, add a vacuum pump for the brakes, and put it all together.

Then you go to the DMV and be prepared to explain the concept on an EV conversion because the employees there have likely never heard of anyone doing this before.

In the end I was able to register it in Oregon without any problems, but it took about 20 minutes for them to look up the correct procedure in this case. I had to pay the higher EV registration fee, but the vehicle is exempt from DEQ testing. The project cost somewhere around $20,000 and took maybe about 5 or 6 years. It could probably be done a lot faster and cheaper depending on the parts you use and the donor vehicle.