Devil restoration

[ejk]

Hi,

I have started to dissect the old devil exhaust I have. The inner perforated tube was broken and most of the steel wool filling had burnt out.

Last year, I had already made a jig to ensure that I would be able to reassemble it. Just last week I received two pieces of perforated tube and a box of steel wool (both stainless steel).



Yesterday I have cut the exhaust apart and discarded the old perforated tube and the leftovers of the wool. The front cap is not in very good shape, so I think I shall replace it with a flat plate. This means that I shall also need a slightly longer outer pipe. I hope to get those after Easter. In the meantime I also want to have the end re-chromed.



To be continued.

Cheers,
Erik

 

[GeoffBob]

Hi Erik

Not sure how true this is but I was told that if you interweave (or layer) your stainless steel wool with high temperature glass fibre your muffler/silencer will last longer. The small amounts of nitric acid that form in your exhaust (particularly if you run lean) will eventually eat away even stainless wool and interspersing with glass wool apparently helps. Just don't use the really dense glass wool that is used in high temperature furnaces as that eventually soaks in moisture, goes hard, and makes your exhaust sound really loud. The high temperature version of the roofing insulation apparently works well.

Good luck with the rebuild. It looks like a really mean exhaust!

Geoff

 

[ejk]

Well, it's ready. I must say it all went extremely well together. I friend of mine did the welding and we assembled the exhaust on the car this afternoon. I hope to take some pictures tomorrow.

Unfortunately the noise is still quite extreme so I may well decide to go back to the stock exhaust soon. The engine seems to pick up slightly better but it is hard to concentrate on that with all the noise

Even if I am not going to keep on using the exhaust, at least it was a lot of fun restoring it.

Cheers,
Erik

 

[ejk]

Some pictures:

 

[GeoffBob]

Nice one Erik.

 

[ejk]

I have replaced the restored devil with the stock exhaust again. The restored devil was really much too loud to be comfortable. I took the car to my work in Amsterdam, which is a 125km drive. When I arrived, my ears were exhausted .

Does anybody know the science behind (absorption) mufflers? I wonder if there is any way of designing the size of the muffler in advance. Or is it really a case of experience? What surprises me the most is that the sound seems to be louder now than it used to be before the restoration. Although I have only made minor changes to the dimension of the muffler.

Any thoughts?

Erik

 

[Nate]

Bible on zorsts:
http://www.amazon.com/Scientific-Exhaus ... 0837603099

Length & size of holes is probably going to be a big thing, also maybe put in chambers of air - i.e. spiral wind the steel wool/packing around the inner, dont completely pack it, kind of like a silencer on a rifle

EDIT:
Silencer design - p122
- hole size as small as possible to maximise friction
- too small & they'll get clogged, min practical is 0.11in at a pitch of 0.25in
- if the holes are stabbed towards the packing material (from in to out, looking inside the silencer) its better
- min thickness of packing 1in, preferably over 1.5in
- consistent shape preferred - cyclinder, not a cone or oval
- no flat surfaces - they drum
- solid outer casing (thick outer metal) will minimise drumming

although absorbing silencers are bad for motorbikes - oily discharge, high velocity - check your filler compatibility with oils/fuels

Back pressure silencers
- cut slots & not holes
- reduce exit area in comparison to entry
- chambers in series
- tapering passages
- flow reversal

FISHTAIL - most effective adjunct to silencing on both car & motorcycle engines

Possibly have the zorst feeding into an "innter tube" which is partially or blocked at the end, then the gas goes out holes in the side of this into the main part & exits.
This stops the primary waves from hitting the outer casing directly.

I can scan pics etc if you need...

 

[ejk]

Bible on zorsts:
http://www.amazon.com/Scientific-Exhaus ... 0837603099

Length & size of holes is probably going to be a big thing, also maybe put in chambers of air - i.e. spiral wind the steel wool/packing around the inner, dont completely pack it, kind of like a silencer on a rifle

EDIT:
Silencer design - p122
- hole size as small as possible to maximise friction
- too small & they'll get clogged, min practical is 0.11in at a pitch of 0.25in
- if the holes are stabbed towards the packing material (from in to out, looking inside the silencer) its better
- min thickness of packing 1in, preferably over 1.5in
- consistent shape preferred - cyclinder, not a cone or oval
- no flat surfaces - they drum
- solid outer casing (thick outer metal) will minimise drumming

although absorbing silencers are bad for motorbikes - oily discharge, high velocity - check your filler compatibility with oils/fuels

Back pressure silencers
- cut slots & not holes
- reduce exit area in comparison to entry
- chambers in series
- tapering passages
- flow reversal

FISHTAIL - most effective adjunct to silencing on both car & motorcycle engines

Possibly have the zorst feeding into an "innter tube" which is partially or blocked at the end, then the gas goes out holes in the side of this into the main part & exits.
This stops the primary waves from hitting the outer casing directly.

I can scan pics etc if you need...

Thanks for the response Nate. As far as I can most things were done according to the requirements you state. The only thing I can possibly imagine is that the wool is packed too tightly inside the muffler. I coiled it around the inner tube until it fully filled the muffler. Does it state anywhere how much wool should be inside the muffler?

Some pictures would be nice as a guideline.

Here is a cross section of the muffler with some dimensions.



Regards,
Erik

 

[Nate]

maybe try spiral wrapping it?
silencers in rifles have washer - space - washer - space

So maybe try to start wrapping it with little space & then get more & more space towards the end?

How about wrapping the steel wool with wire, pack it really tightly together & wrap it.... sorta like a sausage, then wrap the sausage round the tube?

 

[ejk]

Well, your guess is as good as mine. I think it depends on the actual function of the wool. I think it's function is to reduce the energy (pressure pulses) of the gasses by creating many reflective surfaces in a space that would otherwise be completely empty.

I also thought there would be sufficient air inside the wool to allow the gasses to go through it. Perhaps that assumption is wrong and the muffler is simply too full and the wool creates a barrier that prevents the gasses from flowing out of the perforated inner tube. In that case it would be sufficient to remove a lot of wool to allow air to flow through it.

Still, all just guesswork. Any other input is welcome.

Cheers,
Erik

 

[Nate]

Its all about absorbing energy, mostly through friction.

I'm not 100% sure about turbo's, but the book went to great lengths to explain its not much to do with the "flow" of gases as such, but its more tuning in respect to a muscial instruments etc. & creating the neg pressure at the exhaust stoke opening.
having the turbo there is the main source of back pressure though & Corky Bell's book said, after a turbo - as straight & short as possible!

let me know how you get on, as i'll be doing something similar in the future!

 

[r5gordini]

Erik - I think I said this before, but I had a Devil copy. It was just too loud! So I sold it and moved on.

I've no idea how it was constructed - I purchased it as a complete unit.

Andrew

 

[Nate]

nah its a friction thing...

A few other "applicable points"


"sound waves passing down the main tube penetrate into the close-packed absorbent material via the performations & are there frictionally damped"
..."usually very effective on the high frequency waves"
"apparent noise reduction was much better, because of the elimination of the high-pitched waves"
"smaller holes greater frictional dampening"

maybe try a different material? some fibre glass battens & steel wool?

 

[GeoffBob]

Erik

Take a look at http://auto.howstuffworks.com/muffler.htm . The section with regard to more conventional mufflers that employ a resonant cavity (these mufflers produce greater back pressure and thus tend to reduce power output) will not apply to you, so ignore.

The type of muffler that you are using is termed the "glass pack" or "cherry bomb", and is well known to be noisier than a muffler that has a resonant cavity for noise cancellation, but produces less back pressure. It works by a process of allowing the sound passing down the pipe (which of course travels faster than the actual exhaust gasses, and faster than the speed of sound at standard temperature and pressure, since the gasses are hot) to diffract into a cavity around the pipe lined with an absorbative material via holes drilled into the wall of the pipe. The degree of sound absoption follows (without going into too much detail here) an equation called "Beers law" (no, nothing to do with the beverage) which basically says that the sound pressure drops off exponentially and in relation to the absorption cross-section of the sound deadening material. In other words, you must use a lining that absorbs sound (not one that reflects or transmits it) and the more sound it absorbs per unit distance travelled through the lining the quieter the exhaust will be!

The most likely reason your current Devil muffler is too loud is because you may have packed it too tight. By doing this you have created a sound-board for the sound pressure to reflect back off into the the pipe. One way to really get a feel for how the filler should be packed is to cut open an exisiting absorption type muffler and see how it's done, or you could just experiment until you get it right (could take a long time). Alternativley, visit a muffler shop where they manufacture absorption mufflers, that's what I did.

If you want a quiter exhaust - make your muffler longer and fatter and fill it with glass fibre. If you can't make it fatter and longer, then just make it longer, or at the very least - change your filler. Note that steel wool is not highly regarded for its sound absorbing properties (since steel is highly elastic and displays little damping ability). I am using absorption type mufflers on my project filled with high temperature glass fibre.

Also, the outer wall of your muffler needs to be at least 1.6 to 2 mm thick to stop it from resonating like the skin stretched over a drum (thicker is better, but 3 mm thick is just plain heavy). If not, the outside of your muffler can become an effective source of sound. Remember - the sound doesn't necessarily have to follow the same path as the exhasut gas flowing through the muffler. In fact, if your muffler is attached to the underside of your car via a rigid mounting point (instead of a flexible mount) all the panels at the back of your car will start to radiate sound, most likely at certain frequencies at which the system goes into resonance.

 

[Nate]

I dunno if this has been done or not...
but on the outside of the muffler/silencer, how about weld some thin bar along it to stop resonance?

It would be like having a beam of support along it.
I dunno why it wouldnt work...

Maybe give it a go & see if that makes it quieter, if it does, then its probably the sound being transmitted by the outer casing of the muffler

 

[GeoffBob]

Nate, adding pieces to stiffen the outside would definitely help to prevent the transmission of sound from the interior to the exterior. However, I think the problem here is the filler material, and probably not the thickness of the muffler can.

Erik, the attached pics may be of some help. Many moons ago I made up a cherry-bomb for one of my R5's. I doubt I'll ever use it again so I've cut it open so you can see what's on the inside. This was for a 45mm exhaust system (1 & 3/4"), so not very big. The outside diameter of the can is 120mm and the length is approximately 270mm. The interior volume of the cavity (the space between the outside of the perforated pipe and the inside of the can) is approximately 2430 cubic cm (2.43 litres if you prefer). The cavity was filled with what can best be described as a loosely woven glass-fibre rope that I wound around and around the perforated tube until the can was full. For the attached pics I pulled out the filler (by now very sooty after years of use) and weighed it - 895g. This means that you need to fill your cherry-bomb muffler cavity with glass-fibre to a density of around 0.37 grams per cubic cm of volume (370 grams per litre). This muffler worked quite well so I would recommend that you follow this recipe. However, I would recommend using high temperature fibre since your muffler is mounted quite close to an engine with a turbo. Also, for reasonable results I would recommend making the diameter of your can around 2.75 times greater than the diameter of your pipe. It looks to me that you are using about a 2" inch pipe - if this is the case then your can should have a diameter of about 140mm. Also, the longer you make it the better, but looking at your pics you may have a space restriction and I doubt you want the muffler sticking out further than your bumper. I would suggest around 300mm if you can fit it in.

 

[Nate]

FYI - according to the old diagrams from France "Formation Technique - Renault 5 Turbo.pdf" p15, has the temps just after the turbo of 800dec C

So check your materials are OK at this temp.

 

[maxi.man]

geoffbob are you related to westfield :lol:

 

[GeoffBob]

Nope, but I take your point :mrgreen:

 

[ejk]

I had a response from the exhaust specialist from whom I ordered the parts. He asked whether it would be possible to add another muffler. Also he suspected that the trumpet-shaped end piece probably amplifies the sound. Well, the space is very tight so another muffler would not fit before the current one and I would hate to loose the end piece. However, it did get me thinking and it turns out that I actually can add a muffler...... behind the current one! It might also solve the amplification issue of the end piece.

I had a piece of perforated tube left so I managed to make it today:





I have no idea what the effect will be but I am going to try it anyway. As soon as the steel wool is in I am going to try it.

Erik