In most vehicles resonant frequencies have little effect. It tends to be the general transfer function which dominates. Your best bet is to try to map this by measuring the response of a box in and out of the car on a meter.
Getting 8-10 dB extra isn't easy, and without a meter it would be very difficult indeed as testing can't be done as effectively.
If you can find the right note to play with the current setup (I'm assuming you're not sure of this at the moment) you can typically gain about 2dB, it can be frightening how fast it drops off from the peak value in some cases.
With your current box, if possible experiment with aiming it in different directions at full power and taking SPL readings from 40 to 65 Hz every 5 Hz. Allow some time between tests for the batteries and subs to recover.
If you have better access the thing to do next is to make a box about the right size, start by tuning to 40 Hz, with as large a port as is practical without having the end of the port within about 1.5* the port diameter from the back of the box. Chart the results (frequency wise every 2 Hz or 1Hz if you have the time) for the tuning, then increase the tuning by cutting 1" off of the port. Repeat the measurements and continue doing this. You'll need to do this about 6dB down from your max if you want to not have your results skewed by voice coil heating and battery drain, but it should put you close to where you want to be.
It can often help to retest the box aiming at this point, as the increase in SPL can bring about different effects in the way that the box is loaded and the way that the cabin responds.
The next step if you have more time is to try different box volumes. Sometimes smaller boxes can work better than huge ones due to the way that they interact with the car. Usually a bigger one will give a higher peak, but it's a lot easier to figure out where to tune and test now that you know what your car's like.
If you do this it is almost essential to re-experiment with box aiming. Smaller boxes often work better firing backwards, while larger ones prefer to open up and play with less loading by being aimed forwards.
Once you've got the best tuning for your car, and the best box volume hopefully, the next step is often to increase the port area so that it approaches the size of the radiating area (cones). It should still be kept to a point where the end of the port is more than 1.5* the port diameter from the back wall though, else you can increase port area and decrease SPL.
The next step is to do some testing on this box, cutting off in 1/2" increments and measuring the frequency response. You should try to do this at full power so as to get more accurate results.
You will then need a digital voltage meter and a clamp on ammeter. Set both to peak as it's the same. At 1 Hz intervals measure the in box impedance by playing that note and measuring the voltage and current. Impedance = voltage/amperage. Once you have a chart of the in box impedance it is possible to experiment with the wiring. So if at 60 Hz your in box impedance is making each amp 'see' 8 ohms, and by adjusting the wiring you can make the amps 'see' 2 ohms bridged (if that's what they're designed for) you suddenly have 4* the power, or to put it another way a 6dB gain before power compression.
The final step is to do a little more testing of the box, the more you can do on this the better. If you want to keep your amps safe it's best to chart the in box impedance before playing notes. You can do this well in advance of going on the meter by testing how the impedance varies for different port lengths at home if your using a shop meter and save yourself a lot of time at the shop. If you can do this at full power, and the dB testing at full power you'll be doing yourself a favour as even the impedance can change at higher power levels due to heating.
The total time on the meter is often around 10 hours+ The more complex the install the more there is to test. Due to the amount this can cost (inc. fuel), most people choose to buy their own meters to test with. The Kriss meter www.kriss.com is a great meter for the money and as long as it's kept well charged will not deviate much at reasonable SPL levels. A good battery charger is also important. Leave the battery on trickle charge overnight to allow it to regain its full potential.
Once you've done this you should be about where you want to be. You can now experiment with the little things like upgrading power wires, learning which windows/doors to lean on to gain SPL. Then there's the more useful things like upgrading the alternator and batteries so that more power is available to your amps so that the maximum power can be delivered.
Then, assuming your bank manager hasn't placed a court order on you to keep you away, start looking to buy more powerful amps so that you can drive the subs harder. You will already know the impedance you want to drive, so it's usually the best time to buy amps.
Once you have all the information (usually in excel) you can try to sell it on Ebay to recoup some of the money spent on meter expenses and child support payments to your now ex-wife . "
this was written by the man who owns the opel wagon that was posted a while back he is running 6 dl ur15's with 6 audison amps he does 168.8 not too shabby
enjoy
jon
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hu: kenwood 9018
Front stage: Dls Magnesiums 6.5" with the addition of 10's down the track
Rear: DLS 8" coaxial
subs: dls sp12 (the only 2 in aus)
Amps f/s: Valve Audio class A 4 x 100
Rear amp: Valve Audio Class A 2 x 50
Sub stage Amp: Valve Audio quad monoblock hand made in belgium
Power: yellow top, 2/0awg
Team Gates