After 46 years in the automotive repair business, I can tell you a couple of things about aftermarket catalytic. First the sound level will increase with a less restrictive and cheaply made part. Secondly you will more than likely experience check engine light coming on with various oxygen sensor codes. This is because the converter will not run as efficiently as a factory part. I understand the predicament you are in, however get yourself a factory part ordered as soon as possible to eliminate the problems you will begin to experience.
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