I put a K&N air filter and had a Flowmaster 50 series exaust (single) on my truck and it made a big difference. My truck is only a 2.3 but you can imagine what it would do to a 4.0. I dont think a 50 series would be good for a 4.0. Probably a 40 or a 44 would sound better.

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I'd rather push a Ford than drive anything else.
1990 Ford Ranger 2.3L