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How do spark plugs affect catalytic converters

Hey, i have a 2004 Subaru Impreza and had the catalytic converters removed. i was after new spark plugs and in the owners manual, it has an option of different spark plugs to use if you have no catalytic converters. I was wondering what is the difference it has to the engine and would it cause it to run differently.

The only options I see are for Turbo vs non turbo

Cats provide a small amount of back pressure and hold heat in the exhaust between the spark plug and the cat so it works more effectively.

I’d guess the plug might be a heat range step hotter without cats. The hotter range burns off contaminents a bit more. A cooler plug is normally used on a performance engine because the combustion process builds more heat.

NGKs work the best but I do question why you removed the converters and how you figure the owners manual specifies no converters.

You say you “had” the converters removed. Who removed them?

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I was wondering the same thing, as I was under the impression removing the catalytic converters on pretty much any vehicle made in the past 45-odd years is illegal.


Federal laws make it illegal to remove a catalytic converter if the vehicle was manufactured with one. Fines are as high as $10K for the individual who removed it. Interesting enough, it is NOT illegal to be caught without a catalytic converted, but likely will fail emission testing.

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I think the enforcement would be difficult if you are not subject to emission tests. Not saying it is right.