These just look so cool and are suppose to increase hp, torque and gas milage. I'm not a mechanic (like computers, cars work by magic as far as I know), so how do headers do all that? And what this about "cut your car rear Valance with provided templete".
This isn't a header, it's just a replacement exhaust. I'm guessing that it will give you an increased exhaust note (make it sound "throatier") and possibly reduce back pressure a bit. It depends on how restrictive the stock exhaust system is. You will need to cut your bodywork to make it fit - that's why the template.
I'd be surprised if you see any significant gains in performance or fuel economy. Often it's necessary to remap your fuel injection system to adjust for any changes in back pressure, though the computer can usually compensate for small changes. Mostly this is a cosmetic item.
Real headers come off the cylinder head (that's why they call them 'headers'). An open header (one with no exhaust), like a racing header, helps reflected sound waves from the exhaust pulse to increase cylinder scavenging (of the exhaust discharge) by pulling more of the old exhaust out and also pulling more fuel into the cylinder. The end result is increased power (and noise, since there's no exhaust). If the header is installed before an exhaust (i.e., it's not an "open" header), then you still get the benefit of reduced back pressure, but not as much scavenging or horsepower increase.
If you really want to know more, go to
Header Design
Maybe someone will design a real header for the Smart!
Hope this helps.
