I have used sound deadening products in over 10 projects, and have changed my approach radically.
One big reason I sold my last FD chassis was because I layered the whole thing in dynamat (actually brownbread the better stuff)
and was disappointed in the result. It is virtually impossible to remove and weighs a ton for limited effect.
My recc:
read this:
http://www.sounddeadenershowdown.com/and use stuff from here:
http://store.secondskinaudio.com/firewall-liquid-thermal-barrier/For an FD:
1 - remove factory crap (wish i would have done this- sounds like you are way ahead already)
2- for mass loading use a liquid; many like lizardskin. On my FD I sprayed Spectrum Firewall on the external firewall surface, but ROLLED/TROWELED it in the wheelwells/inside/trans tunnel. Use their thickening agent and you can make it like putty and layer it on thicker. Once it dries it is quite light, easy to work, water clean-up with and very impressive in it's sound deadening ability.
3. Use a closed cell foam of whatever thickness you want (I went with camping mats on my FD, and stuff from a local foam wholesaler on my Caddy) and on top of that throw a layer of tin foil with 3m adhesive- try to minimize the holes.
3+:If you are really worried about heat substitute the closed cell foam/foil for one of the spectrum thermal mats on the inside central trans tunnel (I did).
4. Then throw on your carpet. make sure to seal areas like the tunnel with a trans tunnel plate so you don't have a volcano of hot air.
Or make the whole inside of your car look like a space shuttle and wonder why it is still loud/hot in a couple months.
