it sounds like youre trying to do a hairtrick...idk what kind of car you have but i think its gonna be hard to do that with a single 15. either way, a tuning like 28-30Hz will theoretically move the most air
you dont make a port a hertz. you make a port a size which produces a frequency. and 1 15 is not near enough cone area to make a wall and have it be benefecial.. just my .03