OK, I am about to install new carpet and put the seats back, I'm trying to save on weight though. I was told to leave the insulation out because it adds alot of weight. I know it will be noisey but I don't mind that.
When I put new carpet in my car I went to a
home carpet store for the insulation. They had some apx. 1/2-inch foam backing and I used it. It was very light and does a decent job of keeping the noise down. Plus they only charged me $4.50 for the stuff.
What are you building? If it's a 1/4 mile beast I could see it possibly. Too often guys go to a lot of trouble to drop a few lbs and then always have a buddy in the car, a big stereo-12disc changer etc and a suit case full of CD's, you get the idea.
Insulation provides more than just sound deading and you may end up with hot feet as well.
I guess you know that stock you car weighs about 3550-3650 lbs. Before you spend a ton of money to reduce weight you might consider
starting with a lighter car.
I have a '78 Z that I put a butt-load of money in, however I wish I would have started with a lighter car now. Of course if you go with aluminum wheels and heads you could knock off about 80 pounds, you could remove your rear seat(told they are about 50lbs) Another what maybe 10 for the aluminum intake, throw your spare tire out too.(I just carry a can of fix-a-flat and pray)
Early second gen cars are about 300lbs lighter to start with and first gen are I think near 400lbs lighter.
I added 80lbs. of mastic, a tar based acoustical damperner similar to factor stuff, to my 69. Two layers on the doors, floor and firewall. I tell you it is "good" weight because the interior is noticably more quiet. I bought the stuff from McMaster-Carr. They have more expensive plastic based dampener that is more effective and weighs less. I will put that on the roof when I replace the headliner. I do all this because I like my tunes and also relative driving comfort when I don't feel like listening to music.
Dennis, your probably right. its not going to be a 1/4 mile car. I want it to be fast but it doesn't have to be the fastest car around. I want it to be a nice older car that I can take to car shows and enjoy. Don't get me wrong I want it to be fast just not crazy fast.(all though I have yet to build my new engine for it )
I understand what you mean about the stereo systems and speakers and all that. I'm just going with a basic cd player and 2 6x9's in the back deck. I love my music but my music is for me to hear not everyone else.