i have a stock 74 390 in a f 350. what kind of hp gain would i get if i put headers and duel pipes on it? how big of pipe to use and what make of muffler?
my 79 390 has duel 2.5, stock logs with glass packs coming out behind the cab.it has a nice sound but the 74 will be used for long hauling a 5th wheel r.v.
thanks guys

 

I don't know how much you'll gain but it will be noticable. Putting headers and duals on my 360 F250 was the best thing I ever did. 2 1/4 to 2 1/2 is a good size. If you have a cam and 4 bbl then I'd lean towards the 2 1/2". To keep things quiet I'd use Dynomax Super Turbos. 17734 has a 14" case or 17749 is 20" case. It all depends on how loud you want. You should be able to buy all the parts, headers, 2 mufflers, mandrel bent pieces, etc for an out the side, front of rear tire for around $400.