I know I know the six sounds great in a 914 but I would like to calm the noise a little inside the car.
Is it possible to fit 5mm thick sound deadening under the backpad? I already have the factory tarboard ? in place so not sure if theres room.

Yes, you should be able to depending on how you attach it to the firewall. It will probably not quiet it much, as much of the sound comes through the window.

Do you have a firewall pad on the engine side?

Put dynamat (and/or a similar product) on the back of mine, and the stock backpad fits fine. I did it a little for the sound, and maybe a little more to block the heat. Sweatyback is bad enough in the summer without adding engine heat to the equation. My car is loud AF anyway.

Yes, there is at least 5 MM between the back pad and the sound deadening pad. You can however find better sound deadening materials that spray on and are 1 to 2 MM thick. On my 74, I had the body shop who painted the car spray the cabin side and the engine side of the firewall with Wurth undercoating when they undercoated the car. The car was then painted the OE color inside and out. The OE tar soundeading mat installs over the undercoated firewall in the passinger compartment and the heat pad installs over the undercoated firewall in the engine compartment.

I did a Google search for "spray on sound deadening for cars" that resulted in products from every major vendor.

Greg

Also make sure the lower butyl tape seal on the rear window is good. I would think that vibration there would make it louder as well.

I found under the seats help. Evidently sound bounces off the road and thru the floor too.

Ok,thanks for the replies, lots of info,I recently moved my engine forward in the car because PO had copied the engine carrier mount wrong and it was to long,now it is a lot better as there was noise transfer from the motor/gearbox because of body contact in 3 area's.
Will try to answer all replys in no particular order;
No soundpad anymore more don't really want one back.
The reply about the floor I think is a good one and makes sense ,food for thought.
Back window has been refitted with correct sealing medium.
I have peeled a lot of weight off this car because I'am a power to weight freak,I will however look at some improvement in noise levels as I'am not the only one in the car,cheers all for replies

Firewall Heat-Sound Reduction Mat

I knew a guy with a 2.7 and webers maybe PMO's doubled up the glass and it made a difference . But as Nathan said make sure there is no vibration in the glass itself