Dynamat / Similar - 914 firewall

23e Heure · Post by **23e Heure** » Wed Apr 05, 2017 12:17 pm

There have been a few sound deadening threads recently, so apologies for going over any old ground.

I am removing the back pad from my 914, and carpeting the cabin side firewall instead.
I'm retaining the sound deadening pad on the engine side, it's in really good nick and has been kept dry.
However I still want to provide some sound/vibration deadening on the cabin side too, and am looking at dynamat...

- How much do I need for a 914 firewall? (I don't have my car with me to measure up, so hopefully someone else has done this already on their own 914)

- Can you carpet direct onto dynamat, or do you need some kind of intermediate layer? (I have a grey perlon firewall carpet that I bought from the US)

- Anything better than dynamat for this application?? I saw Dodo Pro somewhere...

Darren C · Post by **Darren C** » Wed Apr 05, 2017 4:06 pm

"Anything better than dynamat for this application?? "

Yep...

Ear defenders

To be fair with the engine side sound deadening and carpeting the bulkhead in the cabin, £ for £ further sound deadening is not going to make much improvement in a 914 other than add weight

LMR · Post by **LMR** » Wed Apr 05, 2017 4:49 pm

James

I am picking up some dynamat this week. I will look to see how many sheets needed for the firewall

23e Heure · Post by **23e Heure** » Wed Apr 05, 2017 5:45 pm

Darren C wrote:To be fair with the engine side sound deadening and carpeting the bulkhead in the cabin, £ for £ further sound deadening is not going to make much improvement in a 914 other than add weight

You are not the first to say that! Might it help with heat transfer, or would that be negligible too?

LMR wrote:James. I am picking up some dynamat this week. I will look to see how many sheets needed for the firewall

Thanks!

Mr Pharmacist · Post by **Mr Pharmacist** » Wed Apr 05, 2017 7:07 pm

With the fan directly behind the firewall suckling air in all the time I wouldn't have thought there was much chance of heat building up and getting through? Mine has no engine bay sound proofing and I only recently carpeted the firewall and I've never noticed any heat (from a 4 cylinder engine). If you do want to buy some dynamat you'll need about 1.5m x0.8m to cost the inner fire wall iirc

Darren C · Post by **Darren C** » Wed Apr 05, 2017 8:55 pm

Stuart is right, heat shouldn't be an issue on the bulkhead above the tinware. Below it the bulkhead is double skinned anyways so excess heat from the exchangers/manifolds is separated from the cabin by clever design.

The sound levels and frequency range of an air cooled engine is your "perceived" issue. With a removable Targa roof and questionable window seal gaps, there is always going to be some noise with roof on. Besides, we all mostly want to drive with it off which defeats the point of trying to sound deaden a bulkhead when you have a mesh engine lid.

Just rejoice in the sound James

Think of it like James Bond in Thunderball when he dodges a bullet while being chased in his Bentley. Fleming wrote that it was the "Cheeky iron rattle" of a Volkswagen that caused Bond to turn, saving his jaw from the bullet at the time it was fired (or something like that).
Makes me smile everytime.

Who'd not want to hear that

23e Heure · Post by **23e Heure** » Wed Apr 05, 2017 9:00 pm

Yes all very valid points.

But I tend to drive long distances. And have done for many years.
Add in a misspent 20s DJing in illegal venues with loud but poor sound systems, and my hearing is now in trouble!

Darren C · Post by **Darren C** » Wed Apr 05, 2017 9:03 pm

No problem James. At the end of the day it's your car. Do what pleases you, but don't play the radio too loud then

Bruce M · Post by **Bruce M** » Wed Apr 05, 2017 9:47 pm

You might consider "silent coat". Similar to dynamat, a sticky rubber butyl sheet with top layer of thin aluminium.

As I understand it, you have three problems:

Bathroom effect (reflected sound) - if you have bare hard surfaces, the sound bounces around the cabin like in a tiled bathroom. You need some form of carpet to deaden the sound & stop it reflecting around inside. A basic RS lightweight carpet will do this job but not much more.

Drum effect (resonating metal panels) - if you tap a bare metal panel it will sound like a steel drum. The metal resonates. To stop this you can use the thin butyl rubber sheets like "silent coat". However you don't need to cover the whole panel. You only need to damp each separate drum-like surface. I fitted a steel bath in my house last year and it rang like a bell. Three or four small sheets of Silent Coat, in different areas, stopped it resonating.

Sound transmission (like single glazed windows) - a metal sheet is rubbish at stopping external sound (like low frequency engine or exhaust noise), passing straight through. To stop that you need sound insulation, a thick dense foam sheet (or similar) that will muffle the sound and stop it transferring from outside to inside.

Mr Pharmacist · Post by **Mr Pharmacist** » Wed Apr 05, 2017 9:49 pm

23e Heure wrote:Yes all very valid points.

But I tend to drive long distances. And have done for many years.
Add in a misspent 20s DJing in illegal venues with loud but poor sound systems, and my hearing is now in trouble!

My hearing is suffering too - I tried driving with ear plugs in but just don't like the loss of one of my senses (even if it's a dodgy one!) whilst at the wheel

jackstowers · Post by **jackstowers** » Wed Apr 05, 2017 10:12 pm

Double glaze the rear window! All the roar from my carbs comes through the glass I reckon. Ear defenders (when I remember) certainly help when driving full throttle for extended periods.

23e Heure · Post by **23e Heure** » Wed Apr 05, 2017 10:25 pm

Thanks everyone.

I already wear specialist earplugs for long journeys in the 911. They are designed for motorcyclists, and do this neat trick of cutting out only certain frequencies, and leaving others (which enables you to talk to passengers without having to shout).

The 914 cabin will be noisier, and I'll just stick with the earplugs, as it sounds like the dynamat won't really cut out much of the extra dB.

LMR · Post by **LMR** » Thu Apr 06, 2017 8:40 pm

Just in case you decide to go for it, this is one sheet of dynamat

23e Heure · Post by **23e Heure** » Thu Apr 06, 2017 8:58 pm

Nice one! Looks like 2 sheets would cover it, with spare left over?

LMR · Post by **LMR** » Thu Apr 06, 2017 9:29 pm

Yes, two sheets would be plenty

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