My understanding is extractor modification changes the torque curve, eg. 4-2-1 promotes low rev torque at the expense of high rev torque. The total torque and hp of the car doesn't change.
For NA engines, the "improved" flow from exhaust upgrade have little/insignificant boost to performance. Its more to getting the nice sporty bass noise only. At most, the modification probably shave small amount of weight off the stock exhaust depending on the material used.
This post has been edited by shirley_andy: May 16 2019, 04:57 PM
Exhaust questions, make my engine breathe