I'm making a 4.5 cubic foot Tempest sub for my car. I have my port sizes and tuning all figured out, I just have one question. My box requires me to use 90 degree elbows in my 4" PVC, and I'm not sure how to incorporate these elbows into my specified port length. My port length is supposed to be 12.5" long, but how does that work with my elbows? Anybody have any formulas or tricks to calcuting the length of 90 degree elbows in PVC? Thanks, Andrew.