My ex's dad used to have this little green box with a digital readout that (in theory) could tell you what your mixture ratio was. It operated off the vehicle's battery and sat right at the tailpipe like a butt sniffer. Has anyone heard of these things before, or even know what I'm talking about? I don't even know if the thing worked since I never got the chance to use it, but the idea of being able to get an approximation of your mixture ratio without welding in an O2 bung certainly intrigued me.

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'74 Duster w/ HEI ignition, beat to snot suspension, A904, 8.25" 3.55 SG rear, still being tuned up and gets 17 MPG

Know how they always build a better idiot? That's me

A CO meter. They kinda sorta work on cars without catalytic converters.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Not sure about green box but they do have wideband o2 sensor units that fit right into the exhaust.

For example this clamp puts the sensor at the end of the exhaust pipe. They make handheld boxes for the readout or you can feed it to a computer.