What size box do i need 4 a jbl gt4 1000w 12inch sub??

There are lots of suggestions, specs and plans for loudspeaker enclosures available online with accompanying formulas for calculating sizes of boxes, ports, etc., but successful results tend to be achieved by experience as much as following plans and tuned port reflex enclosures are akin to the black arts - hifi purists would change the port size after a "bedding in" time for the enclosure and drive unit.

Your subs are rated at 300 rms so clearly you are overloading them at 350/400 and you should be aiming to max them at around 250 in a reflex cabinet. If you really must drive them so hard you really need to consider an infinite baffle as an alternative enclosure.

For optimal performance there is really only one size for the box .Firstly, you get two different types of subwoofers, an open air, or closed air sub. If it is an open air sub ( hole with mesh in the center of the magnet) no ports are required, and the box should be sealed on the inside with silicone sealant. If it is a closed air sub ( no hole in the magnet) you need a 50mm port about 120mm long. If it is a 12 inch speaker, the box size will be 13 inches x 13 inches x 13 inches.

http://www.soundmancaraudio.com/how-to/category/sub-box-building

if your subs are dvc 1000w max and 500rms wiring up to a 1 ohm load you would need a 1000w max 1 ohm stable amp. to put this in a pickup truck, i would not put them in a box, i would get 2 12inch tubes to put them in. cause a wedge truck box would make your system sound like garbage and will be distorted, your sql will be poor, basically you would need the right amount of air space for the subs.

Really it all depends on how you want them to sound, and to be set up. There really is not specific air space, but there are general air spaces that work, and some times the manufacture will list some specs for different set ups. It also depends on the size of the sub, which you did not specify.

Kicker recommends as follows.

0.4 cubs per sub for a compact box, and 1.8 for an SQ box. SQ stand for sound quality.

So if you want a small box, do .4 per sub. If you want a better sounding box do 1.8.

Rule of thumb and something to think about.

The more air space in the sub, the less power you can run, the less air space, the more power you can run.

For example you can run 200 watts RMS per sub at .4 cubes, but only 140 watts RMS with 1.8.

As well, the smaller box will be louder, But not sound as good.

try this link will give you everything you need to know about your sub

http://www.jbl.com/car/product_support/default.aspx?Language=ENG&Country=US&Region=USA

this is from pioneer  http://www.pioneerelectronics.com/pio/pe/images/portal/cit_11221/126763081TRD1217-A.pdf

Maybe, but my question would be will the truck alternator carry the load without frying. Also you will need one hell-a-chous size of battery and ground leads in order to feed that much power from the front of the truck to the amps in the trailer in back. I would think that some really big boost capacitors placed at the amps would be in order too. Also, you didn't mention how many amps you were using and what amperage they would draw. That is criticial information before you can figure out every thing else. Good luck.

If the subs are dual 4 ohm voice coils and if your amp is good down to 1 ohm, your best bet is:

1. Connect the voice coils in parallel ("+"s together, "-"s together). This will make the impedance of each sub 2 ohms.
   
2. Connect the subs in parallel.
3. Bridge the amp to one channel.

This should max the amp out at 2400W, which means about 1200W per sub.

If your subs are single 4 ohm voice coils, do the same steps, but you'll only be able to load the amp to 2 ohms, which means about 600W per sub.

Also keep in mind that box size, type, and quality make a big difference in how subs sound.

First off I have some questions to ask you? what size are your subs and what is the max power on them and the brand name?