Originally Posted by falkore24

I can guarantee 100% that below 4000 rpms the 4 cylinder accord is slower with a header. The reason is that the systems are sized around 6500 rpm. At low engine speed the airflow isn't uniform and will not move through the exhaust piping as efficiently. The header will give you less of a restriction at the upper range and therefore will dyno a gain at peak HP, but this is simply because the curves have been shifted up in rpms.

HP = torque * rpm / 5252