Neither is perfect but both could be used. Leaning towards only putting insulation on the inside and using a more modern materials but.....
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Mine was in better shape and I made garbage can fill out of it
If it is concourse quality maybe someone would buy it but there are many better modern options out there

If the goal is a highly original car, I'd re-use one.

If not, I'd consider moving to more modern stuff and may do so with my next real work back there—whether it's just to add a door to service the engine under the factory back pad or to switch to a carpeted firewall.

Would love to hear more about modern materials being better. Lighter? More effective? Or?

I would make a paper template from it and then trash it.

Thanks for the responses gentlemen.



Will do exactly this - excellent idea. Thank you @mepstein

Less likely to hold water would be the main thing, but I think things like dynamat are better at NVH control for the same weight.

Or that is what I want to believe anyway...

@horizontally-opposed
@tdskip

"Constrained layer dampers are generally higher performing damping systems than asphalt melt sheets."

http://lightweightingworld.com/noise-vibra...lightweighting/

Dynamat is a constrained layer solution.

Done more NVH work in my career than I'd like to have done.

The glued on rubber/asphalt OEM sheet stuff that is weakly glued to the bulkhead is more of an outright mass damper than anything. Way better than nothing but Dynamat will outperorm it by virture of the tight bond between sheetmetal, the butyl, and the aluminum constraint layer. NVH solutions have evolved substantially since the 70's. Modern materials and tecniques can be better, lighter, and cheaper to boot!

When I reassemble mine it will get Dynamat and I'll also try to get some thin(er) shoddy material in between the Dynamat and the bulkhead trim panels to absorb some of the higher frequency attenuation too. 3M Thinsulate is also turning out to be a pretty decent NVH reduction at less weight than shoddy though I haven't investigated availability or pricing in aftermarket.

And to add to what Phil said, the heavy pad on the inside of the engine bay is NOT designed as a constrained layer damper. It provides some vibration damping but also damps the airborne drone of the engine. Different types of sound deadening material are designed for different types of noise. A lot of people think that ripping out the pad in the engine and replacing it with Dynamat will be an improvement but that is doubtful. 914Rubber sells a lighter weight pad replacement.

My pad was too far gone to reuse so I went a bit overboard by installing a layer of Noico (cheaper version of Dynamat) with a modern foil faced fire resistant foam pad for under hood deadening. I haven't driven the car to know how quiet it is yet so can't comment on how well it works but the total system weighs in at 5.5 lbs. Much lighter than the original pad.

http://www.914world.com/bbs2/index.php?s=&...t&p=2674370

Man, my thanks to Phil and Brent. That's some great information.

Looks like I may be redoing my interior mat sooner than later, as the one thing I've noticed since converting my 914 to six cylinders is it's plain lovely with the top off but just too noisy with the top on. Anything to cut some of the noise and vibration transmitted (there is plenty!) is interesting to me. Been thinking about different sound mat for the interior firewall and possibly something on the bottom of the engine lid (front half, but…).

First time I've heard of using thinsulate as sound insulation. Did a little research, and it sounds (no pun intended) like it's very effective in that application (and very light weight, too).

Just for giggles, I look on Amazon, and found some (https://www.amazon.com/Thinsulate-Acoustic-Thermal-Insulation-SM400L/dp/B07J3Q373K/ref=sr_1_3), but it is surprisingly expensive.

@BillC

Nice find. I'm so lazy I hadn't even looked on Amazon! Pricing isn't too bad actually. Thinsulate is pricy for OEM's right now and is being used strategically where thicker shoddy won't fit and/or more thermal insulation is needed. 60" x 30 feet is probably enough material to do 10 cars. I'd totally be game at $30 retail for an interior overlay to the Dynamat.

Now we have to find a group buy

@horizontally-opposed
Think about what you don't like with top on. Is it low frequency boomy noise or higher frequency engine whine, chain whine, cooling fan whine, or could it even be even higher frequency wind noises from window seal leaks?

Think about your engine mounting scheme and structure for lower frequency boom. High frequency noises come though small holes (of which there are several in the rear bulkhead) and past window and targa top seals.

Target the noise if you can (use iPhone microphone Apps for freqency spectrum ID) and then you can tailor your solution to what bothers you.

That looks pretty ideal but ironically, might be a bit thick. Descriptions says it expands to 1-3/4" thick. Should make things nice and quiet though!

I am very interested in this subject.
@Superhawk996
What does the word "shoddy" mean in this context?

@Gint

It refers to the chopped fiber and/or foam materials that largely are composed of recycled materials held together with a binder. Since the fibers are largely recycled that is probably where the shoddy moniker comes from. Typically found under carpet and/or soft trim.

https://pacificdiecut.com/automotive-nvh

that felt and flocked tape looks like it would be perfect for under the gas tank where the factory put a felt pad, but most cars its gone.

@Superhawk996
Excellent! Thank you very much.

Just make sure it doesn’t absorb/hold water!

On my BFH project I reused the stock pad but I did have to cut it up and modify it.
It's "fitted," in a couple places it has screws and the the frame of the hole rolls over the cut factory pads edge.

Does a real god job, Weber 3.0, 7K red line, headers

About 26 years ago I got some multi layer foam with a mylar foil facing on it from (of all places) JC Whitney, for an engine bay sound pad. It had two layers of closed cell foam with a layer of neoprene in between, and then the mylar. You could also order a roll of high quality mylar tape for the edges. I doubt if it's still available, but it's held up very well. I just used a factory sound pad for a pattern, and then carefully taped the perimeter before I put it on.

Looking at the front of the underside of that engine lid makes me really wonder about how much something to deaden sound instead of smooth sheet steel might be. And, if so, what?

Consider a 914 targa top with and without the headliner, or an old Fiat 500 with a plain steel top inside vs one with some carpeting up there. Thought it can't matter much with the mesh, but the mesh might actually do something...and also let sound out.

But…I am no acoustical engineer!

@horizontally-opposed .

More importantly mesh lets cooling air in!

You could put some sound absorbing material on the front of the lid but per your observation, it won't be hugely effective since you need the mesh for cooling. That open mesh path will completely dominate what you hear with top off vs. what you hear through the bulkhead with the top on.

This is where side window seals and Targa seals come into play. With top on, you're depending on sealing to block the mid to high frequency noise. Minor gaps in window seals create a lot of high frequency wind noise on their own in ADDITION to whatever outside noise get transmitted right though the leak path. Basically, you want air tight seals.

When doing NVH work, cars are measured for airflow leakeage. More leakage = more NVH.

You've got a long uphill battle to get a 914 anywhere near modern NVH standards but I'm sure there is room for improvement. What you want is a Pareto Chart of major noise sources vs. transfer paths. Attack the big ones 1st for easy gains. After that you're up against diminishing value. Spending more time and $$ for less and less improvment.

Someone just mentioned Pareto - it’s going to be a good day!

Hmm. I think insulating that front area might be worth a shot. Seems like it could cut the sound coming out right behind the glass. I would expect only an incremental improvement over the firewall deadener but seems worth a try. Thinsulate seems like a great choice for that area. You could get some cheap rock wool insulation to test if you get an improvement and swap it out for a permanent solution like Thinsulate if it does. If my car were running, I'd play with it.

BTW, I've always been curious what a Dynamat style constrained layer applied to the underside of the top with the headliner glue over it would do for noise. Not practical because it would be too fuching heavy for one-man removal and install but I bet it would be quiet.

Man, this is a great thread.

Agree on seals, and there is work to be done there. Back in 1991, my 914 leaked like a sieve at the forward door/targa seal. After a paint job in 1992~, and a new windshield seal, it was remarkably tight and leak-free. Almost coupe-like. After the last paint job, and all new seals from Porsche Classic, the doors don't seem to fit correctly anymore, and there is a lot of wind noise. The new windshield seal is also failing at the top right, and I've heard others have had trouble with this part de to the wrong durometer not allowing the doors to close all the way. Makes me wonder about my door seals as well.

Yes, there is much work to do…

(And, fwiw, engine noise from 2.2 with factory airbox and sport muffler is perfection with top off. I'd just like to get the car back towards the exceptional long-haul comfort it offered with the Type IV when the top was on. That's gonna be tough with a six, but it sounds like a good part of my issue is related to the paint job/seal replacement. I knew that when it came to wind noise, but didn't think about it re: engine noise.)

This is what I was/am concerned about. It would kind of be a big time bummer to get the 3.2L installed and then not be able to drive it all day.

In my case not much...I have a GT lid now.
But yes, I think insulating the lid panel would help.

The wife's DD has a cover "beauty" plate over the engine, I pop it off at service time because it's easier to get at the oil fill. I forgot to put this cover back on one time and the wife phones me from work to bitch me out, because she thought I had modified her DD. Seems the insulated foam injected cover does more than just look pretty.

To me the pad has to be an air space sandwich or a thick foam, like the factory pad. Some kind of open cell foam or glass mat with a hard rubber like outer. Even the double wall area of the firewall does a better job at being a sound barrier than dynamat alone.
I have a '67 bus that's done in two rolls of brown bread and it's still a tin can, but it does work when used as one part of the sound barrier system.

I've driven the car with the 2.2 down to LA, and it's definitely doable—though I started out with earplugs and moved to full-fit earplugs to my smartphone for some tunes…mainly because the wind and engine noise was just plain unpleasant. A conversation could be had, but I suspect it would have been tiring.

Once my ears were sealed off (which isn't an ideal scenario for a few reasons!), the drive was lovely. My biggest concern there was missing a change in engine or car noises.

I'd love to get to a better situation in this regard, and it seems more than possible though I doubt it will ever be as quiet at 70 mph as it was with the Type IV. That 911 fan alone...and the cam chains...and...

I really-really do not want to have to drive with earplugs in.

Question: does anyone make big enough sound editing sheets where I could simply trace the pads I removed in those pictures they provided onto the new material? Doesn’t even matter if I use multiple sheets versus one continuous/unbroken piece of material?

You know the old saying. If it's too loud . . . you're too old.

I say that tongue in cheek fully admitting I'm now getting too old also.

Back in 93' when Boxter was 1st unveiled at Detroit Auto Show I got into a discussion with a Porsche engineer. I was motrified to hear about how much work was going on with the 911 and the Boxter to improve NVH. Primarily so that owners could better talk on their Cell Phones . . . which at that time were large analog bricks and/or bag phones.

I knew then, the end of Porsche as I once knew it, and loved it was over.

Oh well.

Nah - I can jump in a convertible or on to the CB750F if I want more exposure to everything. Part of what made the various 1500+ tours/events I've done a good experience is not feeling overly beat up in the process. I have the "SF rule" I've mentioned elsewhere here - if I can't look forward to driving the car from here to SF and back it isn't the right car for me. I can't imagine having to wear ear plus the whole time...just my personal preference.

So - anyone make large sheet materials or does that not matter? Ideas/experiences?

Continual coverage is preferred. However, if you had to seam it with an appropriate tape for the material that would be acceptable and better than nothing.

Just go by the rough rule of thumb guidline:

Low frequency boominess (20-100Hz type stuff) is usually a structural path resonating. Floor pan, front dash, rear bulkhead, Road rumble, Tire imblances, etc. Responds to mass damping and constrained layer aproaches.

Mid fequencies (100Hz - 500Hz) and high freqencies (500Hz+) are usually more related to radiated noises and air leak paths. They repond best to sealing solutions (i.e. no pass though holes, tight door & window seals, etc.) and absorbing solutions (shoddy, fiber mats, etc.).

Love some of the comments—and more than that, strategies—discussed here.

I still love loud cars, and it's funny because I think the Porsche engineers did a pretty great job with the 986 and 996. I'll take a stock 986 2.5 over a stock 993 any day of the week for engine noises. The 2.5's howl through the midrange slays any noise a 993 makes (and I like those, too)...and it sounds better than a lot of Porsches since 1999 short of GT3s and CGTs. But sound is personal, and there are lots of good sounds. And lots of bad ones. And lots of people with different tastes. Many car nuts want that bass, and Porsche started giving it back to them with PSE options that (I feel) are too loud—particularly more recently. They're tuned for others to hear rather than the driver...and that can make them obnoxious for both. A U.S.-spec 981 Spyder can be loud enough when you're on it that I wondered how it could possibly be legal as I drove one through Hawaii. Felt seriously anti-social (though it sure was fun).

So there are different kinds of noise, and the way we use a car has a lot to do with how we "view" it. An old hobby was live sound mixing for concerts, and one thing I learned is you can crank a sweet sound to the point you're turning up trim and they can hear it in the next county, but all you can do with a noise that grates is try to downplay it...because you can't enjoy the other sounds. It's why bad speakers often render songs you know so badly you have to turn them down.

Took my 914 out for a two hour run yesterday with the top off and was reminded that 2.2 + factory airbox + Dansk Patrick Motorsports 57mm sport muffler is Porsche audio perfection. With the top off, I'd say a 914-6 ranks with some of the very best-sounding Porsches of all time. The only audio offender was the wind noise around the inside of the targa bar...maybe around the seatbelts? Got me to wondering if there's a tiny bit of beneficial work to be done there (if at all possible), as that's the kind of noise that wears you down...and also distracts from the lovely parts of the cacophony, which are all about that engine.

Top on, I find my 914 just isn't that nice. Love the sound of that high-strung flat six, but it can be overbearing. This thread has given me a number of things to look at/play with/think about, but the fact is 911 engines are loud things (always were, and Porsche fought with that all the way up to 1998...as its customers often complained about how drained they felt when they arrived at their destination after time on the autobahn). The 914 places the 911 flat six and its primary noisemakers (fan, cam chains, etc) so, so close to your ears where they're quite a ways back and facing away from you in a 911. The proximity is indeed a big part of why so many of us love flat-six 914s, but it comes with a downside and I suspect there's only so much that can be done when it comes to making a 914-6 or six conversion nicer for on long trips or when you're not "on it" around town. A screaming 901/911 six is never going to be a pushrod Type IV flat four quietly thrumming away at 70-80 mph in fifth gear. A lot of the old-school 914-6 drivers, like Bruce and Steph Anderson, kept earplugs in the car for their trips to races, Parades ,etc. I keep a box of them in my driver's door—something I never did with the Type IV—but have only reached for them for 4-6+ hour trips.

But the nice thing with NVH, wind noise, and engine noise is that even 5-10% improvements can make all the difference. Getting all the seals right, improving the interior firewall sound deadening, adding the right kind of sound deadening to the forward section of the engine lid, and considering that rear glass might be enough to really make a nice difference. Might not add much in the way of weight, either, given how much the factory interior pad weighs!

Very helpful thoughts and experiences - thank you.

Don't forget this when considering the sound mat.

http://www.914world.com/bbs2/index.php?sho...=346153&hl=

Yikes! Missed that thread.

Here is something that I have never seen. I do know a bit about acoustical engineering and if someone used a round hole screen on top it could significantly reduce the sound depending on the frequency to the size hole. My quess is that a great bulk of the sound is transitted by the unibody itself.

The physics that surround this are dumbfounding.