many questions

[slowfkncar]

If you are trying to run 5k and maximize spl then your gonna want a 4th order blowthrough,otherwise just do a standard ported and cut open a big hole in the rear deck. Bmw's especially the newer ones with fiber deadener and a closed trunk firewall is tricky to get output,unless that is you want your 5krms system only doing high 130's at the windshield.

yeah I'm definitely worried about the ouput with the firewall and all. But cutting into the backseats for a blow though is out of the question.

there is a stock spot on the rear deck that's already cut, its about 4" x 12". I was planning on extending this to around 12"x24". Think that hole will be big enough?

Idk if it will be big enough for two 15's on 5k but it will definetly help your output and sq,just make sure its a "hole" and not a tube or anything port like. Alot of trunk deadener will also help force the sound to the cabin. When i said cut the rear deck i didnt mean cut the back seats,i ment literally cut the rear deck just as your doing.

And just an fyi,my buddies 335i with a bdcp2500 gained almost 4db at the winshield! Which is a crazy gain in my world by just cutting a big ass hole in the rear deck,and this was with 2 12" dc lv4's