Pilo- when you changed just the exhaust at first, the ECU had the ability to compensate, and add a little fuel. With a new air intake, the restrictive inlet horns are gone, and the engine has the ability to flow more air than the ECU can add fuel for. The mixture becomes too lean. A lean mixture burns hotter than a richer mixture, and can create 'hot spots', on carbon deposits or valve edges. When a fresh mixture hits the hot spots on the intake stroke, it can detonate, causing the backfiring through the throttle bodies. Adding more fuel with an aftermarket device will richen the fuel mixture, and cool the cylinder temps down, eliminating the intake backfiring.....