Back pressure is the pressure faced by the fuel air mixture as it enters the engine cylinder where it is going to be ignited to generate the power. Now you may ask why should the mixture face any pressure at all. Well it faces pressure because the engine generates power in a cyclical manner. When one lot of fuel air mixture is ignited exhaust gases are created in the engine cylinder. And for the new air fuel mixture to enter the cylinder these exhaust gases must leave. In reality the exit of the exhaust gases requires an extra stroke of the piston that clears the exhaust gases. The easier it is for the gases to leave the less is the power required in this stroke. An exhaust header makes it easier for the gases to leave the cylinder. This reduces the effort required for the stroke of the piston that pushes the exhaust gases out. Since the engine is the only component of the car generating power any saving in the power required to run the engine means there is more power available to run the car. Exhaust headers are relatively easy to install though the installation should be done by a professional. handiweb.