What will headers give me?

I have a set of tti's ,have yet to put them in. In terms of horse power,torque,what will I gain? 340,air gap,650 carb,355s in the rear, bit of a cam,not sure what it is.Automatic car.
thanks.

Probably about 20.

After you do headers you need to adjust the carb to maximize a/f ratio to get the most out of it.15-20 is a good bet however you will also gain torque everywhere(below peak,peak,and after peak will not drop as quickly).Make sure you have adequate exhaust pipe size and good flowing mufflers as well so that part of the system does not become the restriction.