How much difference in power and gas mileage would headers make on a 4.7 L ?

 

I would say not enough for the price of them.

 

not sure, tho I am looking into getting them. Don't expect anything higher than 20hp, and that's with the full exhaust. LOL my 14 year old cousin has a 15 year old friend who told him his dad was getting headers for his montego, and that he was gonna bore em out and stuff, so apparently the friend told my cousin that the hp would increase from 400hp to 700hp. Yea...I got a good laugh out of that one. Then when my cousin asked me about that I was like yea..no freakin way. If I could get a 300hp+ increase with just headers I woulda gotten em a long time ago, heh.

 

4.7 stock exhaust manifolds were built to flow within 2% of headers, which means they wont do anything. if you want to do exhaust work, get rid of the Y pipe, and you'll probably gain about 10 horses. other then that, leave it alone.