Reader Question: Cat Delete?

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Here’s the latest reader question, along with my reply!

Allison asks: I’m a female so this may be an unexpected question but here goes: What do you think about removing a car’s catalytic converters? Will it mean noticeably better gas mileage? I’m thinking it will which is why I am thinking about it.

My reply: I’d think carefully about this!

First, if the car is “modern” – which I’ll define as any car designed with integrated emissions controls (so cars designed since the late 1980s) rather than cars designed before emissions controls but had emissions controls grafted onto them (like my ’76 TA) then removing the cats will probably not result in much difference, either MPG-wise or performance-wise. Chiefly because modern, lattice/honeycomb-style cats are pretty efficient and because the engine was designed to work with cats.

Also, if you “delete” the cats your car will fail emissions (visual and functional). If you live in a state/county that requires passing an emissions test to get/renew registration, you’ll be unable to legally drive the car. There are, of course, ways around this – but you have to ask yourself whether it’s worth the time and trouble. For a modern car, it prolly isn’t.

That said, if you own an older car – one made before the early ’80s – then deleting the cats will give you a significant increase in power/performance as well as MPGs – because the early cats (roughly, 1975 – the first year – through circa 1983 or so) were horribly restrictive and the engines were designed years (decades) before emissions controls came online and so don’t work well with them.

Also – compounding the problem – the rest of the exhaust system in the ’70s and into the ’80s was usually horribly restrictive as well. For example, my Trans-Am’s factory exhaust was a single exhaust and just 2.25 inches in diameter… which worked like a chokehold on the 455 cubic inch (7.5 liter) V8 under the car’s hood!

By deleting the whole exhaust system and replacing it with headers and 2.5 inch dual exhaust without cats, I clawed back 20-30 horsepower without any other mods. Plus the car sounded a lot better!

Got a question about cars, Libertarian politics – or anything else? Click on the “ask Eric” link and send ’em in!

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5 COMMENTS

  1. Anything made in the last 25-30 years will see little to no gain from deleting the converter(s). The old stuff that Eric mentions is a different animal. Converters will screw with the wave tuning effect, but so does running mufflers and full length exhaust. I.E. don’t sweat it unless race car…
    You can get an MPG increase if you tune out the cat overtemp protection routine, it dumps excess fuel to ‘save’ the catalytic converter :facepalm: Anything made after ’96 (with post-cat O2 sensors) will need tuning or an O2 simulator (resistor) to avoid setting the check engine light.

  2. Didn’t entirely read it, but depends on your state

    Here in Dirty Jerzy, delete the secondaries and run a High Flow Cat if you don’t wanna deal with emissions, otherwise keep a test pipe and either your old cat or a hfc handy for inspection’s or find the shady garage that’ll give you a sticker for $$$

    • Now that I read it, will say the Cats Rob power and sound, so hfc if you don’t wanna deal with swapping every few years or again, Uncle Joe’s Garage

      This is especially good for a turbos, biggest gains bolt on wise next to a tune

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