[cleatus12r]

Jack's on the right track.

With an internal combustion engine, the only way to get more power is to increase airflow and the amount of fuel going into the engine. The downside to having a gasoline-powered vehicle is that a strict air/fuel ratio must be maintained at all times to make the engine run.

The trick comes when trying to increase the volumetric efficiency of your gasoline engine. Without major engine modifications, the VE doesn't change. Volumetric efficiency is expressed as a percentage because it's a ratio of how much air the engine can physically "hold" versus how much air is actually able to get in under wide-open throttle.

If a person is to artificially cause a slight increase in VE, then it is certainly worth it. However, keep in mind that when you increase airflow, you MUST increase fuel delivery.

Long-tube headers can have an effect on VE at certain RPM because the flow from each cylinder's exhaust port isn't interrupted by the flow from another port on that bank of cylinders as is the case with standard "log" style exhaust manifolds. In addition, the laws of physics and fluid dynamics state that there are negative pressure waves that "follow" the high pressure pulses that are sent down each tube when the exhaust valve opens and the spent gasses are expelled. At certain engine RPM, these exhaust pulses from each individual pipe can actually cause a "scavenging" effect on adjacent cylinder header tubes and help "draw out" the exhaust from other cylinders. This helps reduce what are called "pumping losses" and help the engine work less to make the same amount of power.

Now for the reality of it.....
When aftermarket companies deal with an existing platform like your F-150, they have a TON of space restraints and a very few places to properly route the header tubes. Properly built headers are "tuned" to a particular engine for a specific RPM range. When aftermarket companies build these headers, they are built to fit within the confines of the truck so you may end up with a set that operates efficiently on one bank in a certain RPM range and the other bank will operate efficiently at a completely different range.

Truth be told, the sound of the exhaust is probably the biggest change you'll notice by going to headers unless you want to spend the money to have a custom set built JUST for the way you drive. The stock exhaust systems on these trucks are very well designed (believe it or not). It's quiet, flows very well, has minimal backpressure (which is something you DO NOT want contrary to popular belief), and it fits well without vibration or drone.

I can't speak for Ford, but GM's specification for the backpressure on their newer LS engine trucks (99-present) is .5 PSI at full load/max RPM. I can't imagine Ford's being much different.