I have 40 series flows on my stang, they sound good with the stock h pipe, slight drone i just turn the radio up louder. I recently put a bbk off road h pipe on, fit quality was good sound was way way loud with the flows. I ended up putting the stock pipe back on, i was not happy with the performance of the off road pipe, lost low end power and, it was not worth it for the sound, but that is just my two cents take it as you will dan