In general catalytic converters can have the exhaust flow through them in either direction. If this weren't the case there would be a visible marking on the converter that would indicate which direction the exhaust should travel through it.
If it is a direct fit converter for your 1991 Nissan 240SX like the # MF22767 which is a straight pipe with the same flanges on either side that is confirmed to fit your vehicle it would be reversible. view full answer...
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