I came up with the idea with a friend of mine and pretty sure it will work great...
we want to make a custom sub box for my 2 12" jdl audios and want them to hit hella hard but not rattle the car for that nasty half *** sound
any idea if for the port hole in the box if i connected a tube that went straight up and into that little cover where the middle light in the cab is (also the 2 rear speakers ((this is for a 88 civic sedan)) turning the cab into the port for the box? as much as i understand the airflow of a sub doing this will make it hit harder in the car and the box will be made and fit tucked against the back seats so its not taking up my whole trunk

 

Short of removing all panels/seats/carpet in the vehicle & installing a bunch of rolls of Dynamat, nothing is going to stop your '88 Civic from rattling.

And no, it's not a good idea to port a box like that.

 

reasons? and im just saying im trying for more internal thump not external rattle

 

More internal thump = more external rattle.
The port should be tuned for the sub, having the whole car act as a "port" is going to sound shitty and screw up all the dynamics of the system.
However, if you don't give a rats ***, generally, a bigger box = bigger boom. Bandpass boxes also have a higher output than regular sealed or ported boxes.
You could also set up a box into the "push-pull" configuation since you have two subs.

 

X100000000 The more sound pressure you "create" inside the vehicle the harder it will try to escape...through every little crack, crevice, and dry-rotted gasket.

It also depends on what you are going for soundwise. Usually, A bigger box equals bigger boom in theory. But if the box is built way too big for a specific sub...it'll sound like poop. There are reasons why manufacturers give you spec sheets with all the info for the specific subs...so you have all the info to build the correct size box for it.

Sealed boxes are generally smaller and have better/smoother transient response because the sub can "fight" against itself and the enclosures internal volume. Generally they are relatively inefficient and usually need serious power...but have "tighter" punchier sound. Usually the sealed boxes are the smallest and simplest to build with the largest frequecy response.

Ported boxes have more output than sealed at the same power levels and are more boomy than sealed boxes...making them more efficient. But building this box is more involved as you have to tune the ports to the specific sub you're using. They are usually larger than the sealed style and have a narrower frequency response.

Bandpass boxes DO pound hard...but within a narrower bandwith than the other 2 major styles of boxes...and if you overdrive the sub the box design filters out the distorted sound so you can't hear it so you usually end up blowing subs and/or amps. These are usually the most involved to construct as you essentially now have 2 enclosures to tune to 1 sub. These style boxes also take up the most amount of space for the relatively small frequency response you get in return.

Another major thing to definitely take into account is a basic question that I have not seen asked...nor answered in this thread.
What kind of music are you listening to??? Mostly...like 95% of the time what are you listening to?

If you're listening to rock you could do with 10's or 12's in a sealed or ported box for tight/punchy/semi-boomy bass.

If you are listening to CRap or Hip Hop you'd want to go with the 12's you already have or super-sloppy 15's for that boomy/sloppy "I don't care as long as my brain rattles" bass probably in a ported or bandpass enclosure.

Basically if you care more for quantity of bass...over quality...and are simply trying to be the loudest on the block just go with a ported or bandpass with a decent amount of juice. Those little sedans pound with a pair of 8's in it! 12's will rattle like crazy

 

well thank you for all the info my specs are a 420watt amp and 2 12" jbl subs (i know the amp is wimpy for these subs but it will have to work) i listen to hip hop/rap and im going to make a custom box that will go with wat you guys suggested and not port the cab anymore i said that for more info on it if it would work but the box will be formed against the back seat and as far to the front of the trunk as possible

 

I don't think I fully understand what it is you are going for here...
Are you thinking of building the biggest damn box that can fit in your trunk?
And you need a new amp if the subs are rated at more than 210rms each, or if you cannot wire them into a stable load. Merely knowing the type of music you listen to as well as your posts tell me that you are going to turn this s.o.b. up real loud. And given the fact that your amp is probably grossly underpowered, you will ruin all your equipment and come back in here "Recommend me a good sub/amp combo."

 

i know what they can handle and my limits and the box is no where near "make it as big as possible" im trying to make my box so its tucked against the back seats and out of the truck space as much as possible so i still have a "trunk" yes i listen to rap but im not going to blow the windows off my neighbors houses thanks for ur concern but my only quesiton about the box is if porting the cab instead of the airspace around the box make it hit harder inside i'll post pics of the box when its bein made

 

You do realize that you need to build the box to a specific internal air volume, correct?

You can't just throw a box together that's aesthetically pleasing and gives you adequate trunk space.

Airspace requirements will be in your speaker's instruction/install manual.