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Well, tomorrow I am going to build a new box for competitions. My two conficts are:
A. Build a box to fit behind the third row of seats in my van (largest box possible + large port)
B. Build a huge box to fill up the back (only two rows of seats in)

This box will have a huge port, but I'm not sure what to tune it to (WinISD shows a good gain at 45 hz). I'm aiming for about 6 cubic feet. For USACi, I could go under stock 0-300 with A (above). With the other, I'd be in modified. I'm not sure which route to go. With my old head unit, I hit 137.4 dB in my current box. My new head unit does make my sub a bit louder, so I'm guessing I've got a little over 140 dB.

Do you guys think I could break 150 dB with the right box? I'm going to build a slot ported large box to try to beat this!

I have a Brahma 15" and a JBL 600.1. My current box is 4.5 cubic foot, slot-ported at 30hz.

It's been raining so I won't be building it today.

damn 150 with one sub, thats a lot, cant help you dont know anything about decibels or compoteting, good luck though.

You won't get anywhere near 150 db's with 600 watts. It takes a lot of engineering to hit 150 db's with a single driver.

If you are hitting a legal 137 db's right now, set a goal for 140 and go from there. Find the Fs of your vehicle, build a box tuned to that frequency or slightly below it, and have lots of port area. Then trial and error :P