It is hard to put a fixed number on a single mod. The reason this is difficult is because each mod effects other mods and the whole system needs to breathe. If you have a car with 100% stock exhaust it will not show a crazy gain putting on a dual pop. Same thing goes the other way, If you have a stock air box the exhaust mods are not going to shine. Each mod can only work as good as the rest of the system allows. Anywhere you leave a restriction is going to show a lessor result on the other parts used in the system. If you dyno test a car that has all the best stuff on it to breathe well and leave one last part, that last part before and after will look like the hero and the gain will be huge! It is because that last piece of the puzzle is finally letting the other parts work to their full potential. If you had full 3 inch exhaust and had 2.5" intercoolers with 2.5" intercooler pipes and went to a dual pop from a stock air box, it is reasonable to think the gain could be 60RWHP. The dual pop could not do that on it's own, but because it is letting the other parts work, that is the difference in the restriction it causes at that level of breathing.
