Firewall design: min thickness vs. sound reduction

I'm in the process of installing a floor to ceiling firewall in my scirocco, the bulkhead is going to be constructed of three aluminum sections bolted to an angle iron framework. The largest sections of aluminum will be approximately 24" x 44"

I will probably be adding a layer of thin carpet or other sound damping material to the people side of the firewall. I'm worried that such a large expanse of flat aluminum may act as a sound diaphragm or vibration amplifier.

The most stringent NHRA rule on firewall I could find requires .040-inch for aluminum.

I'm willing to pay a slight weight penalty for thicker aluminum if it means the car will be more livable, anyone have direct experience with this sort of thing? Wild speculation and crackpot theory is completely welcome as well.

Pictures to come.

Put a stiffening rib on the aluminum.

yeah roll some beads into it.

yeah bead it for structural integrity.

As for livability ive used DYNAMAT before i dont think its that much of a weight penalty. But it sure is ugly if you dont cover it with a dash or something.

Good call, I'll look around for somewhere local with the ability to manipulate aluminum.

Should I have the ribs run longitudinal or diagonal across the long bits?

so I am confused.. why floor to ceiling and not just bend it 90 so it sort of follows the same path the original hatch cover did?

Just get a Cisco Pix firewall.....

Oh wait, wrong type....

Simplicity's sake, with the roll hoop installed there would be a lot of fiddly sheet metal work required to make the thing fit. I would also have to make a removable panel for service which means fasteners and precise sealing surfaces. Way more time than I want to put into the project, I'm ready to finish the car and move on to something else.

I don't have a picture online which shows the roll hoop supports more clearly:

I would likely roll X shaped beads in the panels. Another option is to put a little shape into it with an english wheel. They can still be relatively flat, but the little bit of dish will keep them from flapping back and forth. That is what I did with the aluminum panels on my miata.

I would likely roll X shaped beads in the panels.

That's what came to mind for me as well. Also, you might want to shoot a PM to kcbhiw. I know he used something to block off the trunk of the tracer, because of the use of a fuel cell.

I like the X idea a lot.

Thanks for the input, I should have something in place by next weekend.

I just want to congratulate you for the proper use of the word "damping."

Really, that's rare.

Anyway, I can't see your photos (netnanny stuff) but you'll get a lot of noise if you have large enough unsupported area in the panel. If you rivet it everywhere it contacts the main hoop, it might be enough (remember, I haven't seen your photos yet). The bead rolling, or even just a little bit of a break (the x) will help if needed. Ultimately, you might have to run an aluminum angle somewhere to rivet it to and give it more rigidity.

Clem

And yes, I think for a hatchback car this is a good way to excecute a firewall (just make sure you seal it up so it really is a firewall) if you've got a fuel cell in the hatch area.

The TransAm cars (GT1 style stuff) had a nice lexan window in the bulkhead so you could see behind you with the mirror.

Clem

[threadjack] thats a fancy place for a motor... do you have a build thread or anything where I can see more of it? [/threadjack]

http://grassrootsmotorsports.com/reader-rides/828/

Don't overly pop rivet it to the frame work seal it with fire swelling caulk. Yup it goes down like normal silicone sealer but swells under extreme temps and swells to seal out the flames. Found next to the wood stove chimney pipes in most hardware stores.

A "X" would be my 1st try as well.

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Ansonivan, you really need to put your site in your sig, or at least in your readers rides writeup.

http://www.durocco.com/

been a fan of yours for a few years, BTW, Olentangy HS class of '04 (i know you used to bomb around Delaware co.)

Too bad you couldn't use some aluminum honeycomb. They makes some pretty thin. My dad works with the stuff all day. Another thing you might consider doing is put cork over the back of the Al sheet facing the engine. Cork will probably deaden sound and if the stuff hits the fan it is a char surface. They use cork on satellites and space craft because it provides a char surface and insulates the craft.

Good luck!

http://www.mcmaster.com/#8458k26/=shliq

Looks expensive, but it is cheaper than aluminum sheet from these guys. I know you can find honeycomb cheaper than that. Try your local airport - ask about a scrap or surplus store.

If "Miata" is the answer to all car questions, "aluminum honeycomb" isn't to far off of that for "what should I make this panel out of".

The honeycomb idea makes me extremely happy, I work a stone's throw from the Columbus airport and there are several aircraft repair places in the area.

Begging/authorized dumpster diving in aircraft scrap sounds like a E36 M3 ton of fun.

I took proper measurements tonight, the panels measure 51x15 48x8 53x16

Too large to be considered scrap, I'll have to take the car along to get the point across, ie; really mister, your contribution of honeycomb goodness will be saving me from a horrible death by fire.

I have trouble with coma's and semi colons.

Most sign shops have sheet alum and honeycomb too.

After a fruitless search for cheap honeycomb sandwich aluminum I went with .040 sheeting.

I bought the material fromResearch Alloys, that place is stuffed to the gills with weird metal in all shapes and sizes, everything available for scrap prices. $20 got me what I needed and they sheared it to rough sizes so that I could haul it home in my e30.

After shaping the three sections using cardboard templates I took the sheets to a friend's aviation shop and had a go with his bead roller. The machine was pretty basic and opperated with a hand crank... some of my beads ended up a little wiggly.

My day job is working on bmw's in an independant shop, some of the mid 90's automatic cars use 6mm bolts to fasten the transmission pans, trans filters for these cars come with new bolts. I saved a bucket full of the used bolts with the intention of using them for something nifty... firewalls are nifty. By careful use of the WAG method I determined that the firewall sheeting needed a bolt every 4 inches, this meant I needed to drill and tap about 120 holes. I welded a 1/4" drive socket to a cheap tap handle, tapping was still a pain.

I still need to seal the edges of the sheet, I may paint them and add some sound deadening material as well since the panels are quite boomy despite the beads.

Nice work. I like the bead rolling in those panels. I made some door panels from aluminum and then put an engine turned finish on them, I wonder if the finish would surveve bead rolling? I will have to try it and find out.

How hot do those panels get? If they stay less than 120F or so you could try some of the home depot roll roofing to quiet the booming. Won't be nearly as pretty though.

Oh my you hand taped the holes! a variable speed drill is your friend use a taper tap not a plug tap and you could have power taped them easily.

Nice work.

Are you are going to line the engine compartment with the foil and mat stuff to keep heat transfer down?

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