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intake:
if you add a cold air intake it will draw cooler outside air from a different placement than the stock location (most of the time from an area near the front of the car away from the warmer air in the engine bay) through a less restrictive filter media and less turbulent intake tube (the stock tube and stock style paper filter can be restrictive), in which the cold air is denser and less turbulent to some degree. this will add a marginal performance increase in power (horsepower and torque) and this increase may add to better fuel economy (if you can take your foot off the gas). the sound is not stifled like in the stock setup which is meant to reduce cabin resonance, so inside you will hear a throatier engine note as you run up the rpms with a load on the engine. the same can be said for a short ram intake. the idea is the same but the tube is short and may breathe warmer air from the engine bay but still offers a marginal performance gain in the less restrictive filter and tube. the short ram is ideal if you frequent bad weather and don’t want to suck up water from large areas of standing water which can severely damage your engine

header:
the stock headers are made to reduce cabin resonance and exterior exhaust note while being easy to manufacture yet dependable for the life of the vehicle. typically the manufacturer uses a log type setup where the exhaust ports share one pipe flowing to the mating area. aftermarket headers are made to not only sound better, but marginally increase power by reducing friction of the exiting exhaust gasses and backpressure. through different designs this is accomplished by individually reworking the paths of the exhaust gasses from each cylinder to the mating area where the rest of the exhaust system is bolted on.

letting the engine breathe more air through these mods may increase power and milage, but I suggest that if you install headers, put on a performance cat back system with a freeer flowing exhaust. if you free up restriction with performance intake and headers you need to continue the restriction reduction by installing a freer flowing exhaust system. straight through mufflers are key to continuing the reduction in exhaust gas restriction and sound great when the engine is at full song, yet similar to stock at idle. s-style or baffled chamber style mufflers may sound louder and throatier, even at idle but may compromise performance even more than the stock setup. performance catalytic converters may also help reduce restriction, but may not meet c.a.r.b. emission requirements. I recommend any mods be carb approved or pending approval, since your car may not pass an emissions test without approved aftermarket parts. Don’t hollow out the cats, since you will mess with your cars emission control system which can compromise performance since the computer uses data from the downstream oxygen sensor (the one after the cat) to adjust engine parameters and that is affected by how the cats work on the exhaust gasses to reduce pollution… it’s also not cool to pollute.
Hope this helps you in your decisions and your report!