Anyone here mess have any experience and tips on sound proofing cars? I think for me the engine makes the most amount of noise in my car... next would probably be road noise through the floor and doors I think.

http://www.rodandcustommagazine.com/tec ... to_25.html

The firewall looks like a pain in the butt to do.

Old tricks from stereo sound offs-

Inside the car:
One layer of adhesive back sound deadening from the base of the dash to the trunk lid. Foil backed over the catalytic converter.
One layer of 3/8 inch insulation (foam on one side and fiber on the opposite), between the carpet and the floor and under the trunk carpet too.
Rubberized undercoat from the base of the firewall to the rear bumper.

In the side panels:
Glue 1/8 inch sponge and/or more of the 3/8 inch insulation to the backs of the interior panels.
Adhesive backed sound deadening on the back sides of the door skins, quarters, and tailgate if necessary.

Engine noise can come through the floor, reflected off the ground beneath the car. It can also be transmitted by vibration through the frame and body panels. Road, tire, and wind noise are much more than you think, too.

Wouldn't you be adding something in the neighborhood or 100 pounds for sound deadening per that example?

Not unless you tracked in 80 pounds of gravel and dirt while putting the insulation under the carpet. Two rolls of sticky back audio mat would be the heaviest part. The sponge back fiber insulation weighs next to nothing. A couple paint brush servings of contact cement to glue sponge to the backs of the doors isn;t much weight either.

Ten or twenty pounds of sound deadening probably weighs less than the stack of big gulp cups and burger wrappers in most people's trunks. But if that small amount of weight is a problem, buy ear plugs, they weigh a couple ounces.