port displacment calculator?

[ehall8702]

so I need to find a calculator ( or formula for dumb ass to understand) that will give me the displacement of a few different ports. im building 2 separate boxes for under my back seat of my 98 sierra. I need .75 ft3 tuned at about 34hz. as of right now I have 10" ( front height), 6.25" (rear height), 13.25 (deep, front to back), and 13.5" ( wide, side to side). that's .85ft3 using .75" mdf. I cant find sub displacement ( its an AB es1244, its 4.3" deep so it can be very much) and I cant find a calculator that will let me figure out port diaplacment either, round 3" port need to be over 17" long so that would need an elbow and im sure it will take up a lot of room in the box. Slot port at 1.5" wide by 4.25"( I used 1.5" wide slot for my xfl and it was adequate, cant see a shallow moving more air than the xfl did) tall needs to be just over 17.25" long.  so two questions, which has less displacement( and how much does each have)? and can I use 1/2" mdf for the slot port to help lessen displacement?  round port would be 3" pvc cause I have quite a bit sitting around. 

my other option Is to kerf a bottom for the box to stretch the whole underneath from side to side, and then build the box piece by piece to make it fit as perfectly as I can to the space available. my only problem with that is the fact that I would have absolutely no idea how to calculate the airspace and then figure out port length to get desired tuning.  most generic boxes for this truck say there is .95 airspace each side, but that down firing and not a very good fit to the available space. I will be mounting my subs face up ( about 1.5-2" under the seat) just because I could leave my seat flipped up until I needed it down for a passenger. 

so basically I need one of the guys who are smarter than me and model boxes a lot more than I do. also, I will be hiting these subs with almost double their rated power each, I did the same ( 2k rms) with my xfl and it actually liked a bigger than spec box ( called for 2.25 and it worked best in 2.5). any help would be awesome, cause I really don't want to buy a cheapie prefab and port it just to have cheap box come apart when I start pounding it.

[Jessica]

torres. alhough i dont know where to get it anymore. here is a thread on here somewhere. i'll go look for it

[CstrokerV]

For the round just look up volume of a cylinder and you can do it that way. The slot is just how h*w*D / 1728