Is my box too small?

I have  2 Audiobahn 1051Q subs in a L-shaped bandpass box that measures 28 x 13 x 13.  The subs are mounted facing eachother w/ approx .75 cuft behind them.  It has a 3" port for each chamber that is 5" long.  The manual says zip about this typeof box.  The manual reccomends .88 cuft. for ported boxes and 0.53-1.06 cuft. for sealed.  It also says length using a 3" port should be 15".  Does that mean my port should be 15" long?  The subs really bang but don't seem to move that much, is this typical of Audiobahn-that there's not much excursion???  thanks in advance!

3" is width, 15" is length... It sounds like your box is a tad too small and might be affecting performance but i also dont know alot about audiobahn---- 1996 Chevy S10 ----

1 Lightning Audio S2.600.2 Amp

2 Lightning Audio 12" Subs

1 Lightning Audio 1 Farad Cap

Too small is safer than too big.  If the sound is acceptable to you, it shouldn't be a problem.

I agree with you on that DYohn but cant you blow your speakers by not being within factory recommended volume?  I was told to always keep it within factory specs---- 1996 Chevy S10 ----

1 Lightning Audio S2.600.2 Amp

2 Lightning Audio 12" Subs

1 Lightning Audio 1 Farad Cap

in GENERAL, going smaller will limit the low frequency response but enable the woofer to handle higher power levels.  So it tends to be safer.  I always shoot for the small side rather than the large side of my calculations.  If the bass response is not deep enough, add some acoustic filler and the enclosure can act as much as 25% larger than it really is.  That's a  good way to compensate, by the way...

I allready bought the MDF to build a sealed box.  I'm giving each sub 1.07 cuft., which is just .01 cuft. larger than the manual reccomends.  It's been a while since I built a box, what kind of spray adhesive is the best for carpeting?  Also, what type of sealant (silicon etc.) do I use?

if u havent built your box yet go a little smaller on size... the best adhesive for carpet is spray glue... sealant use "Beats the Nail" its just like liquid nails---- 1996 Chevy S10 ----

1 Lightning Audio S2.600.2 Amp

2 Lightning Audio 12" Subs

1 Lightning Audio 1 Farad Cap

Unfortunately I allready have the wood cut.  Can't I decrease the volume of the box by adding some braces to the inside walls and subtract the volume of the braces from the total cuft.?  Also should I use polyfill in the box, the manual says fill 50%.

Yes, you can remove space inside the enclosure by adding more braces, or adding a piece of MDF screwed to the inside of a wall (which effectively moves the wall "in" by 3/4".)  Also, you can remove space by simply adding solid filler to the inside of the enclosure.  I've even used bricks inside enclosures that came out too large (just glue 'em down to the bottom of the enclosure with RTV.)  A good rule of thumb for acoustic poly filler is one pound of filler per one CUFT of enclosure volume.  This will increase the effective enclosure volume and enable the woofer to have lower frequency excursions... but if your enclosure is already too large, you may not need it.