Manifolds vs. Headers

7dswinger said

How big of a gain would it be for me to run headers vs. using the stock iron manifolds. I know that headers are better but with a pretty much stock 318 how much of a gain am I looking at. I'm going to have an exhaust shop install dual exhaust with bigger pipes so I'd have them install the headers as well.

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Not that much 10-15 hp ?? If you don't change the carburetor intake and cam it will not make a big difference , will sound better though . If you plan on changing things down the road it will be worth it . And if cheap headers say goodbye to ground clearance , you'll be hitting every speed bump in town .
here is a chart , not the word of god , but a good guide to what to expect , a bit less on yours because stock intake carb

http://www.moparmusclemagazine.com/...ers_into_300hp_crate_engine/dyno_results.html