Some really good data on the difference an aftermarket header makes on an EJ25. All testing done on the same 2016 STi, fuel, turboback, EWG intake etc, only difference is stock header, Agency Power UEL and HKS EL. Same dyno and same tuner. You can see that the most beneficial header was HKS EL like we always preach, surprisingly the AP UEL worked very well! Remember however that this is at “lower” power levels, as HP rises the benefit of EL becomes more apparent. Always have headers in stock, if you’re looking for the next mod for your EJ25, look no further! @basicsti