I’m aware that EU-spec cars are equipped with gas particulate filters (GPF/OPF) which mute sound compared to cars exported to the States and Canada. Does anyone know if Japan-spec 992s have this OPF?
Secondly, does the filter affect performance/top-end acceleration at all?
Correct, any horsepower gained from removing the non-functioning OPF could be attributed to natural variance, there’s maybe 2-5 hp in it. With that said, replacing the OPF with standard over axle pipes helps the sound tremendously.
Going to an catless or high flow cat header will give moderate gains that are more than just variance.