Needs more DB...
 

To start off, I compete in a Mazda MX3 hatch. For right now I only have a Sony CDX-GT200 head with a 2v out, but I have a Phoenix Gold Line Driver to help boost the signal. Planning to upgrade the head soon.
I have a DDZ1 amplifier pushing a DD3512 4ohm dvc. My problem is that I cannot beat the 142 mark. I've built about 5 different boxes and so far the loudest has been a box 3 cubic feet tuned to 55hz.
I decided to shorten the box by 1 cube and change the tuning frequency to 58hz. It dropped me down by 2db, yet i had heard a smaller box might be better. Both boxes used round ports, which upon building another box I used a slot prot. Big mistake. I did a 136 on the mic wit it, and that box was quickly set on fire after in a poor attempt to make me feel better.
I know my problem isn't much in the equipment because i can vary up to 8 db just by changing the box alone!
Does anyone have any ideas as to which box i should use for this sub and amp to better my score? Big/small? Round port/slot? Tuning frequency? Thanks!

Does this amp not produce full power at 1ohm? If you are only running one dual 4 ohm sub you may only be getting half the power this beast can produce.

Yes this is true. Yet, I've had the same setup in a friends Grand AM and we reached a 144. I guess my main question is why can I not acheive the same dB in my car? What style of box would be best used for the 3512 in my MX3?

All cars are different man. Just because you hit 144 in your buddys does not mean you can hit it in yours. Different cars react different to SPL.

what all sub/port set ups have you tried?

subs up port up should be plenty loud.

Here's a couple things to try.

1. Subs up, ports back.
2. Subs forward , ports back.

Use external ports for both boxes. Aeroports would be the best, but if you can't get any just use 4" PVC pipe. The box should be small and play around with the number of ports and port length for tuning.

Hope this will help a bit.

I've already tried close to everything listed so far. External porting, sub up ports back ports up sub up. I have not tried sub forward port back though. With a slot port I've done sub up, port fired back, (CRX style box) as well as both sub and port fired back.

first off, lose the line driver

now what is your electrical system like... what kind of batt, how many runs of power wire, etc.

are you bulding a box and then burping it a few times and thats it??

i think your problem is that you are not testing white makin a box. normally u dont build a box tuned to a note and then just burp and see how it does. box tuning is a process. u need to get on a meter and play with the port, making it longer or shorter (if u are using aero ports).

2 runs or 0awg from an Optima red top to the Z1. The line driver is in place only because the deck's 2v preout sucks. It does seem to make a difference, but i could be wrong.

I have a term lab mic. That's how I've been getting my score so far. So if the sugestion is to use one box and then just work on the ports, either making them shorter or longer, is there a box size that would be best suited for the DD? I don't think it likes a smaller box, such as a 1.5-2 cuber. I've tried that and it dropped me quite a bit. But once again, I could be wrong.