I'm interested in a couple of things for the insulating foam:
1. What is the fire rating and temperature range?
2. What is the NRC (Noise Reduction Coefficient)
3. What is the thickness
4. Does it contain Barium Sulphate as a sound deadener to increase the mass?
5. What is the weight per square meter?

I spoke to Clarke rubber, and the specific part they sell is called FormShield (http://www.formshield.com.au/)
Basically this is only a thermal insulation product. So possibly great for underneath the bonnet for example, but it has a NRC of around 0.15. I think also the temperature rating is only up to 80degc....

Acoustop (a specific multi layer composite of foam and barium impregnated plastic) has a NRC of 0.5 to 0.6 which is excellent, but it's BLOODY expensive ($120 for a sheet of 1.3 x 1m)

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1972 Datsun 1600, S14 SR20DET Engineered (204rwkW @ 17psi.)
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you guys are getting too technical.
Go to your nearest carpet place and raid theyre big green bin for used carpet underlay.
Works great for FREE!

I agree with nick

Old or new carpet insulation.

I have used this on several cars and it works well and is free.

If its a race car who cares about the noise if its a car just for driving who cares about the few extra kg. If you are that worried about weight see Jenny Craig or stop drinking Farmers Union Iced coffee.

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The Fleet
71 Datsun 1600 G-Spec
68 Datsun 1600 sr20de
Fiat 124 cc
Property Fleet
200b orange g-spec
71 Datsun 1600 paddock basher
69 Datsun 1600 sitting idle blown engine

Let me answer the weight question for you:
Basically the heavier it is the better it absorbs sound. So the weight per square meter is a rough gauge on how well the material absorbs sound.
If you use a composite, then you can drop the weight considerably, as it is kind of like using an air gap and another layer to absorb the sound.
A few extra Kilo's i'm not concerned about.

Jenny Craig? Farmers Union? Meh. If both of those products could make a good insulation i'd buy it.

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1972 Datsun 1600, S14 SR20DET Engineered (204rwkW @ 17psi.)
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#SR20Datsun @SR20Datsun