That exhaust extension can play havoc in tuning some engines, idle mid, high or any combo. You just have to do a with and without test, re tweak the needle of course. It was just terrible on the 60NX with blue performance muffler, yet a Webra 55, APC 9X10 and red Jett muffler gained about 300 rpm with the extension.

If you get the performance muffler on that thing, it may scare you.