also what about this:http://cgi.ebay.com/ebaymotors/Focus-Zetec-00-01-02-03-04-05-2-0L-DOHC-Exhaust-Header_W0QQitemZ350123315855QQcmdZViewItemQQptZMotors_Car_Truck_Parts_Accessories?hash=item350123315855&_trksid=p4506.c0.m245&_trkparms=72%3A727|65%3A12|39%3A1|240%3A1318
can that be legit, as far as performance goes??
When someone asks a question like you did, I respond with another one: When was the last time you saw an ebay decal on an SCCA, NASA, drag, or rally car? Search the forums for this topic there is plenty of info, but in short, the best header is the SVT long tube header designed by Cosworth.
"full legth headers" or "long tube header produces more power like oandawow said. The exhaust flows better b/c the primarys are all the same lenght. In other words the exhaust from two different cylinders dont meet in the collector at the same time and cause turbulence. In a sence, when that happens twice as much exhaust is going through the pipe than if it were just the exhaust from one cylinder. So it flows a lot better and thats were headers really make their power.
Bolt-ons don't add power, they just free up more power that the motor didn't have. When coupled with a complete exhaust and intake, it should add a little get-up. Numbers are so trivial because they vary from car to car, and the difference between 120 and 127 at the wheels is nothing really. The car will be pretty slow either way. The real limiting factor on the Zetec non-SVT motor is the head. If the head flowed like a Duratec or SVT head stock, then the Zetecs would be putting down 140 or so at the wheels with just the standard I/H/E and a mail order tune. Remember, SVT engineers punched out 170bhp from a 121 cubic inch motor, which when it came out in 2002, was pretty badass.