headers

how much time could be shaved of by replacing the headers

Shaved off of what?

the 1/4 mile time

No one can really tell you how much it'll shave off your 1/4 mile in accuracy, it'll all depend on your engine and how efficient and well its running or maintained. Of course we can give you a ball park figure of 10-25hp which can be expected with a good set of headers, but its hard to estimate how fast you'll be running the quarter just on this info. Also, what other mods do you have, obviously this will affect the 1/4mile time. need more info bro.