WinISD or Mfg suggested enclosure?

Hey all.

Thinking that I was doing a friend a favor by designing him a box with WinISD, I've come across more conflicting and confusing data. I'm using a Clarion ProAudio PXW1242, (see attached image for spec sheet) and WinISD tells me that a box volume of 3.5 ft^3 with a 4"x16.85" port is needed to smooth out the gain curve allowing this sub to hit 21.5 Hz before running out of 'oomph'. Is this box size too big for a 12" sub? The manufacturer suggests a 1.84 ft^3 with a 4"x16" port! This curve looks horrible when plotted by the program. Space requirement in the vehicle isn't an issue as he's ready to sacrifice the entire trunk if necessary (Mercury Mystique). The plan is to build 2 separate ported enclosures and fire them towards the rear of the car. As always all input is greatly appreciated. 

Speaker: Clarion PWX1242 (large graphic cut by moderator)

And it probably is fairly horrible. Car audio manufacturers recommend small sized boxes because people want them. Further, they jack up the tuning frequency because your average new driver doesn't care about the design, he just wants IT TO HIT! I'd throw what the manufacturer has to say straight out the window in most cases. Tuning freq. of 52hz? Ouch. In this case I'd talk to your customer. He wanted good sound, follow your educated WinISD plot, he wants more trunk space, make something smaller. New Project: 2003 Pathfinder

I suppose what I really want to know is whether a large (3.5 ft^3) box will destroy this woofer prematurely?

as long as the box is built with enough support and wont destruct, and the port isnt a massive opening that will almost make the box free air. then no, I think the demensions you are are just fine.    Might be a little boomy.   You might want to experiment with something around 2.5 cu ft2009 0-1000 Trunk WR 154.0DB 2009 1001+ Trunk WR

2007 USACI World Champion

2007 World Record

2006 USACI Finals 2nd Place

I will always check out a sub with a program before building an enclosure for it....from now on.  So often I notice the recommended enclosure specs have a built-in peak where the average buyer might think it really rocks, but it's just a type of bass boost.  It really kills the sound quality.  Even those who are striving for pure volume should want a reasonable representation of the frequency range.  Besides that, a lot of box specs allow for too small of an enclosure, or a sealed box when it really should be vented.......just to make it a more desirable product for the masses.