Audio Systems: BIG, midbass drivers, inch woofers

Question
most of my freinds around town carry 2 15's either diamonds, kickers, or rockford. im familiar to what 15's can do. my cousin and i wanna get into competition but want to go very extreme... how do 18 inch woofers perform.. say 18" titaniums from eclipse or the solo x from kicker.. do they pump out good for SPL? appreciate it

Answer
While I am not specifically familiar with those models and their enclosure loading capabilities, I will say "They can".
I used a pair of 18's (along with 15 other speakers) in my competition system from 1987 till '94.
Of course, back then that was a lot for the 250-501 Pro class (and a TON for a GTi), but as to pure SPL, you will get owned by people that have a dozen 15's, 12's or even 10's installed.
As long as the vehicle has the room, the fastest and most efficient way to loud is with more speakers.
Double the power = +3db.. double the amount of speakers = +6db.
With 465 watts RMS and a 100% stealthed, 17 speaker system, I blew countless R/F systems out of the water, even an R/F demo van that had 2 RF1000 amps and four 18's and I have never heard anyone play "The Ghetto" so diligently (and so long) trying to outpump me as that van did. ^^

IMHO, a pair of ANY speakers these days is not very extreme. It is however, a good starting point. Going with 18's or 15's usualy means that you'll need midbass drivers (four 8's or so) and an amp to drive them, if you are after SQ and SPL.

I could go on and on, but without knowing much more specifics about your intentions, I think you get the general  idea by now.

J Mael