I have a '75 Dart that I never had time to work on, and when I went to change the muffler I realized it has a catalytic converter! It must be trashed, as I have been running the car with an old Holley 1920. I had no idea a '75 would be equipped with one. I'll check it out this weekend and see if it has been hollowed out.

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Never seen a Valiant I didn't like!

yup 75 was the 1st year for them.

Yup, '75 was the first year some A-bodies had cats (meow?). Cats and carburetors (any carburetors) don't play well together. If you have to have one because of laws, which I doubt because you are already running non-stock carburetor, post that situation on here for recommendations.

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

Dan,

What recommendations are you offering up for converters that play well will carb'd inductions? Or is specifically the lack of gizmos like air pumps (if lacking in a 1975 rig), etc that you are referring to when it comes to catalytic converters and carburetors. I realize that the lack of very precise fuel metering and control in a carburetor can "load up" a converter to the point where it no longer functions as design. Fuel injected rigs perform better as far as getting along with the catalyst, but regardless catalytic converters DO require replacement eventually. Do you have good experience with a specific brand?

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Slanted D150

You need one with heavy catalyst loading (to clean up the extra-dirty, poorly-controlled exhaust) and robust substrate (to handle the extra-high heat that comes from operating with poorly-controlled exhaust).
It's difficult to find a 2-way (HC/CO) converter any more; you tend to find 3-way items (HC/CO/NOx). The NOx reduction doesn't work very efficiently at all with non-stoichiometric exhaust, but it can work somewhat to add extra oxygen (one of the products of the reduction reaction) which then helps a little with the HC/CO oxidation reaction.

So: a converter meant for a heavy-duty truck with a large engine in California. A Catco #712005 (2¼" inlet and outlet) for example; sourced at a good price here, for example. (Why California-specific converter which is more costly than 49-state? Because the CA converter has much higher catalyst loading which will do a more effective job over a longer period of time with dirtier exhaust.)

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