How much horse power does a flowmaster exhaust add on 4cylinder car?

A Flowmaster exhaust system will not add significant horsepower to a 4-cylinder car. While it might give a very slight increase (perhaps 1-5 horsepower at most, and often less), the gains are usually negligible and within the margin of error of testing. The main benefit of a Flowmaster is improved exhaust flow, which can lead to a slightly better throttle response and a more aggressive sound, not necessarily substantial power increases. The engine's limitations, not the exhaust, are the major factor in a 4-cylinder's horsepower output.