Tony, you have to understand the mentality of these people. They are in no way shape or form audiophiles. They are strictly competitors, and the only grounds for competition is "Mine is bigger than yours."
In some of these vehicles you cannot even identify the song that is being played, regardless of volume level. One that comes to mind is a big Chevrolet Dualie truck owned by the operator of a local stereo shop. The only speakers in the truck are twelve 15" subwoofers. The speakers are all wired parallel mono (or something like that, I don't really get into the technical aspects that much) and the amplifiers are so over-driven (through six one-farad capacitors) that he is momentarily outputting 18,000 watts using six large truck batteries for a power source. He uses a 100 watt amplifier as a pre-amp to feed the inputs to his main amplifiers. He has generated in sanctioned competition 164 decibels, and unofficially over 169 decibels. (The world record is, or was the last time I checked, 172 decibels). These volume levels are measured near the base of the windshield, not with the microphone stuck into a speaker port or something. All the glass has been replaced with 1/2" Lexan.
I have been told that if you were to actually be inside the vehicle when it was cranked up to something like full volume, it would kill you -- something about cavitation in the chest cavity or something. This is probably just one of those urban legends, but I, for one, am not interested in finding out for sure. I have seen people don ear protection and sit in the truck at considerably less than maximum volume, and come out of the truck disoriented and exhibiting difficulty in walking. At the last competition where I saw this truck, stores in the adjacent shopping mall, over 100 yards away, had problems with merchandise vibrating off the shelves.
SPL competitors are a different species from you and me, I think. I guess there is some status in having the loudest car around, but it's not a distinction that I covet. By way of comparison, in my power class most cars are generating about 130 decibels. My car maxes out at a mighty 112 dB. But very few cars, regardless of power class, outscore me in sound quality. [pats himself vigorously on the back]
tanstaafl.
"There Ain't No Such Thing As A Free Lunch"