I built a sealed box for my car already but i need to build a ported box for comps. Ive got 2 15'' d2s with 1200 watts rms My cars resonant frequency is about 35 from what ive read. Should i tune it to 35? or to what the subs are reccommended to? or ive heard to tune about 10-12 hz above my cars resonant freq.?

Unless your car is a LARGE sedan or SUV, I would tune a lot higher. Hollis's Explorer (the red one on our web page) has a resonant frequency of around 47 Hz. Unfortunately the best way to find out what your actual res. freq. is is to test. Make a box with a changeable port and change it constantly if you can borrow a mic and see what posts the best results.

What kind of vehicle do you have? I suppose that would be a good place to start.

Do a search on Termpro for your vehicle make. If it's a small sedan I wouldn't be surprised if it's in the upper 50's. If it's a hatch, expect low 60's...SUV's...right around 50.

Are we looking to compete? If so...see above. If not..I'd tune to mid 30's for daily..and higher if you aren't the SQ type

its an rsx, ive heard that the resonant frequency is anywhere from 45-55. i want spl. i'm looking to compete. should i tune to my cars resonant frequency or should i tune to what SI says to tune to?

Tune just below your car's resonant frequency...I would advise you to pick up a couple of Aeroports from PartsExpress and design an adjustable ported box. Start with a low tune (35-40) and work your way up...Use a Mic (reliable one) to record your peak frequency by playing all sorts of different tones near 35-45. Cut the port down a bit to increase tuning frequency and continue until you find your peak frequency..

You want to tune the box to PEAK at the resonant frequency. Say you car peaks at 30, you want to tune it so the enclosure will peak at 30, not the actual tune itself.

Not to rain on anyone's parade, but the whole "finding your resonant frequency" thing is a bit overrated. Use the guidelines I8aPony19 suggested, and just pick a number. Humidity, altitude, temperature, and about a thousand other variables will make your setup hit different numbers at different frequencies. Contrary to popular belief, there is no golden note: Just well designed boxes

its an rsx, ive heard that the resonant frequency is anywhere from 45-55. i want spl. i'm looking to compete. should i tune to my cars resonant frequency or should i tune to what SI says to tune to?

If it is 55 (which is probably close), I would tune no lower than 45 Hz. SPL is, afterall, nothing more than trying to pressurize the 1" square area in front of the mic. Tune high, shove TONS of power, have massive port area, and set your gains correctly (slightly clipped).