Headers are part of the Civic Si's exhaust system. After oxygen and fuel are combusted in the engine, the waste (exhaust) is sent through the headers. From there, the exhaust travels through the catalytic converter, then muffler, then is released into the air. In general, performance headers are good for a horsepower or two by reducing the resistance (or "back pressure") that is inherent in all vehicles built for the road.
For four cylinder engines, there are two basic header flavors: 4-2-1 and 4-1. 4-2-1 headers start with 4 pipes (one for each cylinder), then taper down into two pipes, then finally into one pipe. A 4-1 header has four pipes that taper into one pipe. In general, the 4-2-1 headers are going to give you a little more power in the low end, and the 4-1 headers are going to give you more power in the high end. Some people report having problems with 4-1 headers scraping if their vehicles are too low.
Fortunately, there are a wide variety of headers available for the Civic Si, many of which will add some horsepower, and a nice look to your engine bay. Have a look at these:
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