NA means Naturally Aspirated, which means increasing HP output by improving on an engines inherent HP creating abilities. Flowing a larger amount of air through the engine via larger intake, throttle body and IM diameter, larger cams and a larger diameter header and exhaust system. Just to scratch the surface...
FI means Forced Induction, which means either a Supercharger or a Turbo is force feeding air through the engine.
And once again, brand isn't as important as the legnth and diameter of the secondaries (secondaries are the 4 pipes coming out of the flange that bolts to the head), and the diameter of the collector (collector is the hole at the end of a header). All of these factors will determine where the header's maximum volumetric efficiency is along the RPM range, which will determine where its powerband is located. Different header companies all make different headers, no two are exactly alike. Now, legnth and diameter of the secondaries is hard information to find, namely because the companies know that 99% of the people on the market for a header don't care about those numbers because they are uneducated as to why it should matter. The companies think we're dumb! and unfortunatley, most of us are when it comes to this stuff. But one dimension of headers that is fairly easy to find out is collector size, which is very important.
If your going NA and doing bolt-ons only, a 2-2.25 inch collector is ideal. You'll maintain good low end and get good gains in the upper rpm's. If you plan on building up the internals of your engine, you will want to plan ahead and get a header with a 2.5 inch collector. However if you do, you will eventually want to have an exhaust system that is also 2.5 inches in diameter. Stepping down in diameter size as you move back in the exhaust system is bad, maintaining the same diameter throughout or stepping up is what you want to do. Just one example of why you need to have a plan for your car...
Don't get me wrong, brand has its importance, because the saying "you get what you pay for" rings true here as it does for all things in life. I just want you to understand how headers work and to make an educated decision based on what you need and not on advertising hype or what someone "says" or "heard" is good just because they say it is. Maybe whats good for them and their engine package won't be good for you and your planned package.
If you have more questions please ask...
