3 cubes sealed gives you a sealed resonance of 42. You'd tune the ported section to 42 hz. The ported chamber would need to be 3-6 cubes AFTER displacement depending on your goals - closer to 3 cubes for a wider bandwidth, closer to 6 cubes for more SPL. Optimal port area is somewhere between about 120 and 175 square inches. I would stay on the lower side and also recommend using a large aero unless you plan on doing a lot of testing and cutting back on port area. A 12-14 inch flared PVC aero would be the most efficient.. or a pair of 10" aeros.
1:1 to 2:1 (ported:sealed) ratio is generally more oriented to having a better bandwidth or being daily. 2:1 and up is more SPL oriented and 5:1 is about where returns start diminishing from what I understand.
For port area:
It depends on the vehicle and box, optimal port area is almost always between 1/3 and 1/2 of cone area. It's usually better to go with 1/3 if you're doing a "one and done" box. If you're going to adjust, it's easier to start at 1/2 and slowly cut down.
Tune at Fc for daily and "most" SPL purposes. Some people will play with the tuning for a flatter response or for more SPL at a certain frequency although it's again a lot of testing. Also, aeros are always better than slot ports if the design allows for it. 4th orders are tuned high so the port length isn't usually very long so they work great.
Running it through winisd real quickly 3-3.5 cubes for each chamber tuned around 40hz should be fine. You can always built it a bit larger in each chamber and tune a little lower and then chop the port and add something to take up space as needed. So I'd start with 3.5 in each half chamber tuned at 40 and you can always work it up or down once you've heard it. It's MUCH easier to know how to make a design like this after you have it in the car and know what you want to change.
One more question. Im only running 800 rms to them which is way less power than they need. They sound suprizingly loud off the small amount of power im putting to them. would that effect the 4th order. I plan on buying a new amp when tax returns come in but i wanna build the box now.
From what I understand, a 2:1 ratio 4th order is around the same SPL level (generally) as a ported box. More ported volume = more efficient but less bandwidth. Conversely, less ported volume is less efficient but gives you a wider bandwidth.
im going to be posting up my deisned 4th order with an adjustable port on it.
Okay so here is what i got. the port is adjustable up to a 12x12port. I figured I could make it this way, unless im tottaly wrong and adjust it to my liking with tuning the port. sub are firing into the sealed section as ive heard it may be a good idea so you can smell the coils heating up. I would love to get your thoughts and ideas about this design. I took all your suggestions into though in my design.
You may want to shrink the sealed section to about 3 cubes to raise the Fc a little, although it would be fine at 40 for a daily box.