wow! thats above my head :)

Sorry, but really once you understand each of the parameters its not bad. Manufactures always list thier Thiele Small Parameters so you can make a suitable enclosure. What a manufactures suggest for enclosure size is not always optimal but more of a starting point. Its usually better to have too large of an enclosure than too small of one. If you are going sealed its not that big of a deal and you can plus or minus 25% of the recommended enclosure and still be in the park. Vented enclosures need to be much more precise in order to properly tune the enclosure.

down firing subs
 

I am not sure of the acronyms.

Could you please elaborate?

I am contemplating 2-8" down firing subs located behind the metal bar in front of the spare tire well. Not sure whether to do ported or sealed sub box. Any suggestions are welcome.

TS Parameters are simply measurements of a driver that tell us everything we need to know about the specific driver and how it will behave in certain enclosures and it allows us to maximize the drivers performance by designing an enclosure that is optimized for a specific driver. This is why pre-fab boxes are less than optimal for a given driver. You can get away with this for a sealed enclosure but never use a pre-fab ported or bandpass enclosure if you want to get the most out of your subwoofer.

All manufactures of speakers provide all the necessary TS parameters to allow you to custom build your own optimized enclosure. I have attached an example from JL Audio website showing these measurements. In the program I have attached you can just plug in these values and then key in the desired size enclosure and the program will tell you exactly how the subwoofer will perform granted you understand the values. I will be very basic but I will tell you what they mean.

F3 = the lowest frequency the driver can play in the enclosure before it begains to attenuate. (The lower, the better!)

FRMAX = this is the frequency where the woofer plays the highest output in the given enclosure (Again, the lower, the better for a subwoofer)

Q = how tight or accurate the subwoofer will play in a given enclosure. (Anything >1 will be a bit on the boomy - Looser side and anything = or <.6 will be very tight and accurate at the expense of a bit of efficiency.

I tend to prefer a Q of .7 to 1 but anything above 1 is too loose for my taste. Less than .7 is a bit to dry.

BTW I am not sure a pair of 8"s will fit where you want them in a down firing config. I would recommend using (3) 6-1/2 " subs if you are dead set on that location. The JL 6"ers are damn impressive for thier size. Parts Express also has some Tang Band 6" ultra long throw subs that are very good as well and much cheaper than the JLs.

Downfiring boundry loads the drivers and your bass output will be awesome! You will really feel them kick too!

I would port them in order to get lower bass response. These subs would also play some sick midbass duty as well. 200 to 300 watts would the max I would use for them as small sub drivers just wont take a lot of power and they really dont need a lot to do the job.

Don't be intimidated, I have already done all the math equations for you with my software program.

6" JL Sub
 

The car audio store mention 3 - 6" JL subs located in the 370Z hatch area behind the aluminum bar where the foam piece is located. Pricewise 2-8" JL subs appear to be are more cost effective than 3-6" JL subs.

To understand the sub box parameters; would you simply add, either the 3 individual 6" JL sub or 2-8" JL subs specs, in your software?

I am not sure the air space needed in front of the sub in order to downfire either of these subs. The depth is approximately 7" from the bottom to the top of the hatch floor. Any idea of the depth needed in front of the subs to the bottom of the sub box in order to optimize the subs efficiency if the sub box is ported or seal?