How much horsepower do headers give the engine?
The amount of horsepower delivered by headers depends on the size of your engine and the amount of restriction presented by your stock exhaust system. For instance, a good set of race headers on an otherwise stock Honda B16A four cylinder (which already has a good exhaust system) is only worth about 7 horsepower at 8,500 RPM. A set of race headers on a stock 5.0L Mustang engine is worth about 24 horsepower.Source:Honda Tuning Magazine: Header Dyno Testing & Comparison, Tri Y vs.
