0.707 or 0.85 Qtc?

What make and model sub are you referring to?

OK, do you know what you are talkin about?  Qtc is a t/h parameter of speakers, in this case subwoofers.  The qtc of the sub you have will be a determining factor in the type of enclosure you want to build.  enclosures don't have a qtc, as far as i know. 

Dude, sometimes spec's are not the be the all end the all. If you r doin a spl setup, go into a specialty retailer as they are the ones to put you on the right path. You will need to do a sinewave sweep to find the loudest resonant frequency of your vehicle and then build and tune the box for that loudest resonant frequency with the appropriate speaker. Personally, these things absolutely wail, but the trade off is that if this is your daily driver, then you will get mighty bored with this in a hurry. If you want to build a system that plays loud that you can listen to, tell the dealer this. Be aware that volume levels in excess of 110db will cause permanent hearing damage. The box in this case will have the most to do with the output capabilities of the sub. Biuld it too small and you get nice tight bass but no depth, build it to big and the amplifier will lose control of the motion of the driver, and you will have no tightness to the bass, which btw will probably sound like a wet fart. Build that box bang on and hold on, you should get shaken like a bowl of jello in an earthquake.Top Secret, I can tell you but then my wife will kill me.