Dyno room exhaust setup question 2

[thrashercharged]

I'm not sure if this is the right forum for this, but I'm setting up an exhaust system for an engine dyno room and have questions. Here's what I have:

1. Room purpose - dynoing 4-cycle gas car engines, anything from 4 cyl to V8s, up to 1000HP or so, both blown and NA. No sustained durability tests, just quick ramp to redline pulls and some brief steady state at various rpms/loads for calibrating the engine.
2. We have residential neighbors within oh, 100 yards
3. The dyno room walls are poured 8" concrete
4. Room has conventional 10' drywall ceiling (hanging on resilant channel to avoid transmitting sound directly to trusses)with asphalt shingles outside.
5. We have (2) Nelson 300 dyno silencers (mufflers) 12" opening, side in, end out, about 30" in diameter, 112" tall rated 25-35 dB of silencing depending on frequency.

http://westernfilterco.com/critical_300.html

Why 2 and why these particular mufflers? We found them on eBay and they were a good deal.

Our main goal is to operate the dyno as quietly as possible to keep the neighbors happy. Preferably, we'd rather that they not even know we're running a engine. I'll point the chimneys straight up.

My plan is to keep the mufflers inside to keep even more of the noise inside the concrete walls, stand the muffler on end and run a chimney through the ceiling/roof. The mufflers themselves should get plenty warm, but I think the room fans circulating the air 10x/min should be able to suck the heat out of the room sufficiently. How warm they'll get I don't know - anyone have any input on this?

Questions:

1. Since I have 2 of these mufflers, I might as well use both. Should I use them independently and run 2 chimneys? That is, give each bank of the engine it's own muffler?

What are the pros & cons of this setup? Will the muffler be too big for just a single bank (4 cyl at most) and not provide enough silencing? I ask this because of a statement from another dyno muffler's website:

"If the silencer is much too large, then the exhaust noise simply passes through using only the initial large expansion for attenuation."
- quote from

http://www.eiwilliams.com/steel/index.php?p=EngineSilencers

My understanding is that if the muffler is too big for the volume of exhaust input, the baffling inside isn't very effective and the muffler essentially becomes a large unbaffled chamber - is this correct?

2. Or should I put the 2 mufflers in series (I can easily do this by standing one up and laying one on its side and running its end output into the side input of the other.)

Will this provide more silencing? I realize this may increase the backpressure, but these mufflers are so large already, I'm thinking the increase in backpressure will be pretty small, but I'm no expert on this (which is why I'm asking these questions here!)

3. Question about exhaust temps. At the header, temps will be in the 1400-1600 *F range, but the muffler is a huge heat sink, so what are realistic temps at the chimney? How much lower?

4. Outlet sizing: My understanding is that the outlet should be smaller than the inlet for good sound attenuation. Both openings on my mufflers are 12". This probably already is overkill for the engines I'll be typically running, but just in case I get a blown big block or diesel I guess I'll be ready. I assume with the exhaust gases cooling, they'll have less volume and a smaller outlet pipe/chimney is ok and won't increase backpressure? Ideally, what size should my outlet & chimney be?

5. Another reason for wanting an 8" chimney is cost. A 12" double wall chimney stack is pretty expensive and custom while 8" double wall chimneys made for wood stoves and such are readily available. Would a double wall chimney rated for wood stoves and furnaces be ok to use?

After I get through the roof, the exhaust gases should have cooled even more, could I even adapt down to 6" for more attenuation or will this create too much backpressure? The reason for this is I'll need a raincap, and 6" is about the largest that are readily available, anything larger is custom again.

Thanks for any help. I think I've given all the info I can, but ask if I've left anything out.

 

[turbocohen]

Geez, a few of the dyno facilities I have been to were very quiet. Aim the pipe the exhaust as it will be installed in the car into the inlet of an exhaust evacuation inlet. There are several tubeaxial blowers out there that will suffice, just make sure they flow at least 4x the exh volume to dilute the gas temps. Grainger sells one with an external motor.

http://www.grainger.com/Grainger/wwg/itemDetailsRender.shtml?ItemId=1611631292

 

[franzh]

Dont forget the harmonic noise at low frequency levels. ULF sound travels through almost anything, strangely just where an engine begins to ramp up around 1000 rpm. Any rattle within the cell will be noticeable, even shake some things loose. As the engine ramps up in speed, the harmonics within the piping will develop standing waves accentuating the sound. Call it a trombone effect if you wish.
One test facility I visit frequently has about 50 single cylinder lubrication study engines. Every once in a while they all fall in syncopation and it sounds like a helicopter hovering about 30 feet overhead. Wait about 5 minutes and its eerily quiet!

Franz

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[MWPC]

Thrasher,

I just helped install a dyno for performance automotive V8 testing. I do not have technical data but here is what we did and the results. Sprint car headers into 4" galvanized flex pipe about 4' long. These ran into a heavy (1/4") wall pipe about 8" in diameter. The pipe was about 10' long running horizontally out through the wall. It connected to what looked like a giant glasspack muffler. 8" id, 16" od, 6' long. Inside was expanded metal and some type of packing. The test mule engine was a 400 hp SBC. Standing at the muffler outlet it sounded like a small aircompressor running. Very quiet. The galv. melted for the first couple of feet on the flex pipe but the heavy 8" pipe did not seem to get excessively hot. I can ask them to check the temp after a run if it would be of interest. Ken

 

[fueliefan]

Hate to thread-jack, but...are california building codes posted anywhere that describe what measures have to be taken for noise, exhaust, chemicals, etc? We want to install an engine dyno for r&d purposes and we're wondering what it's going to take. we'd hate to build a "nice" room only to find out our walls aren't thick enough and have to tear it all down.

 

[GregLocock]

MWPC sounds on the money. I don't really agree with that eiwilliams quote about 'too big', we run everything from 60 hp to 700 through our truck mufflers. We do have a butterfly plate in there so we can set the back pressure correctly.

8" pipe sounds ample.


fueliefan, you need to talk a consultant, or at least start your own thread.


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[thrashercharged]

Thanks for the responses & info. GregLocock, I've worked in dyno cells that use a butterfly plate to adjust backpressure. Who sells these? What size are your truck mufflers, and how effective are they - how loud is it outside? Could you describe your dyno exhaust - what size inlet pipes, output chimneys, etc.

MWPC - thanks for giving me some details about your setup. I'd be interested to know how hot the exhaust in the 8" pipe is. How long/hard was the engine ran? is this 8" pipe before the muffler? When you say the galvanized melted, are you saying the flex pipe was still usuable and just the coating melted, or that it ruined the pipe? Where's a source where I can buy this flex pipe?

It sounds like the mufflers I have will be more than enough to attenuate the noise to a very acceptable level, so is it overkill to mount these mufflers inside? What about using both - overkill and not worth the trouble, using them seperately per bank, or tied in series?

Sounds like an 8" chimney is the way to go. I think the eiwilliams quote about mufflers being "too big" is saying that the baffling will be essentially a don't care and the muffler will just look like a large expansion chamber if the muffler is grossly oversized. I can kind of picture this - if for example we ran a 5HP lawnmower engine into one of these baffled mufflers, and another similar engine into an unbaffled expansion chamber the same size as this muffler, the output noise will probably be the same. They're implying that a correctly sized smaller baffled muffler would be more effective in attenuating noise in this example. Or am I not understanding this eiwilliams quote correctly?

thanks for reminding me about ULF noise. What can one do to minimize this? No muffler will minimize ULF will it?

I'm not sure I understand the first response's comment
"Aim the pipe the exhaust as it will be installed in the car into the inlet of an exhaust evacuation inlet. There are several tubeaxial blowers out there that will suffice, just make sure they flow at least 4x the exh volume to dilute the gas temps."

Where's the muffler in this system, and how large must it be if we're mixing in 4x ambient air? A formula I've run across to estimate exhaust volume is 2.4 times the intake volume. Thus if we estimate a max intake volume of 1000 CFM, exhaust volume will be 2400 CFM, and 4x that to dilute with enough ambient air is almost 10,000 CFM. I don't see many axial blowers this large.

 

[Warpspeed]

Thrasher, I have been giving all this some very serious thought myself in recent times. I plan to install a roller dyno in an undeground suburban garage when I rebuild my house, unfortunately very close to fussy neighbours.

Placing mufflers in series will definitely result in cumulative noise attenuation, and also cumulative back pressure. Some back pressure after the muffler is definitely required to attenuate the lower and fundamental frequencies effectively.

What you require is some sort of flap on the system outlet to hold a small amount of fixed back pressure, fairly independant of flow volume. The reduced sound exit area will definitely hold in a lot of the noise. Something along the lines of the standard rain cap fitted to the vertical exhaust stacks of diesel trucks suitably weighted, should do the trick. It absolutely must not flutter, but with a suitably high (but balanced) total mass, flutter would be unlikely.

If the additional imposed back pressure is objectionable, I see no reason why a centrifugal air blower could not be added to make up for this. I once watched a film clip of a Formula One engine on an engine dyno where the exhaust collectors fed straight into the open intake of a massive air blower, so that idea is not original.

I have also been thinking how to build myself a suitable muffler system. This is one area where size IS important! The most practical idea I have been able to come up with so far, is to buy a fairly long length of concrete storm water pipe perhaps two feet in inside diameter. This can be filled with old automobile tyres over it's entire length, maybe twenty to forty feet or more, and buried under the garden.

At suitable odd distances heavy baffle plate flame cut from 1/4 inch steel plate with a central hole and pipe stub, would split the pipe length into multiple chambers. These baffles could be packed between sections of old tyres inserted down the concrete pipe. The individual chambers should all be made very different odd non related volumes to spread any possible system resonances.

For even better high frequency attenuation, these old tyres could each be stuffed with fibreglass insulation.

I have not tried any of this yet myself, but I believe twenty to forty feet of buried concrete pipe with perhaps a hundred old (free) bald tyres, and some heavy baffles should have a pretty high acoustic attenuation over a very wide frequency range. Especially if the outlet is fitted with a variable restriction.

Just like to again stress here guys, these are only ideas, and have not yet been put into practice.

 

[GregLocock]

The mufflers are about the size of a domestic hot water service. We use 6 or 8 inch pipe. You can stand next to the wall with the mufflers and they aren't too loud. The nearest house is about 3 minutes walk away.

The butterfly is about as crude as you can imagine.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Warpspeed]

My neighbours are not minutes walk away, but metres away, hence my paranoia.

 

[patprimmer]

When I did national service in the very early 70s, we set up field generators to run silent.

We dug a hole and kept it all below ground level and dumped exhaust into a long rubber hose which lay in a trench that was covered by heavily leaved tree branches.

I know, it is at a different scale re power output, but we had a very strong motive to not disturb the neighbours.

In my opinion, that experience adds weight to at least some of warpspeed's suggestions re buried concrete pipe idea. I think the baffles are not required, but a blocked end and lots of small holes facing down could work along with a heavy but porous covering. Gravel comes to mind.

Regards

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[Warpspeed]

Yes the militay are very noise aware. I believe the special service guys go to rather a lot of trouble to silence outboard motors. Commando raids are very serious stuff indeed, and the silenced outboard motors are really quite something.

That is the sort of thing you need for your dyno. Someone yards away making real horsepower, and almost silent.

The clue here is sound absorbtion through friction. In mufflers fibreglass or steel wool seems to work well at the higher frequencies. But chambers in series with small apertures between (baffles) would seem to be much more effective at mid and low frequencies.

Rubber has a fair bit of hysteresis, it turns rapid motion into heat fairly effectively and so is quite good for sound absorbtion. Old tyres would make great muffler stuffers, free too and almost indestructable!

Check out the design of the Nelson muffler mentioned in the original post. It not only has chambers in series, but they are coupled with tube stubs, to add pipe friction between chambers. A lot of thought and testing has obviously gone into it.

 

[franzh]

Slightly off scale, but do not overlook inlet noise, it can be horrendous, especially when blowers are added! You cannot have just an open room with exhaust being dampened, you need to look at the total picture. Now, toss in environmental condiderations, structure integrity with harmonics rattling walls and windows, room inlet air filtering and noise control, cooling water absorbtion, and the list goes on. I recommend visting a commercial engine development lab and see what they go through.

Franz

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[thrashercharged]

Thanks for all the responses and discussion guys. Just a quick post before I leave to go camping with the kids for the weekend.

Franzh - thanks again for reminding us about all the other noise considerations besides the exhaust. I've 8" poured concrete walls, concrete floor, no windows, the roof is conventional with shingles, 6" rolled batt fiberglass insulation on the attic floor, and the ceiling is 1/2" drywall mounted on resilent channel to avoid attaching the drywall directly to the trusses so as to not transmit as much noise. For room air circulation, my fan doesn't blow outside, it blows into my shop area. This is done to avoid having a large hole in my ceiling/wall that noise could escape out of. I think I've covered everything - is there anything I can do differently? I haven't put in the door. Obviously, the heavier the better, but I'm trying to do this on a budget - any advice on making a cheap yet heavy door that seals? My other concern is my air inlet to the room - I haven't figured this out yet, I was thinking a large vent above the door, but wouldn't this be just a big gaping hole for noise to escape? What do people do to keep noise from escaping through their air inlet?

Warpspeed - I sympathize with your situation. Besides the 2 dyno exhaust manufacturers I've already mentioned, (eiwilliams & Nelson), Maxim Silencers is a 3rd maker an they have a nifty sizing program you can download from their site. Links are:

http://www.maximsilencers.com/

http://westernfilterco.com/critical_300.html

http://www.eiwilliams.com/steel/index.php?p=EngineSilencers

I've heard from other dyno owners of a guy who used some 55 gal drums, filled them with old car tires, and welded a few together in series. I've been told these worked very well. I don't know how many in series, and how they were attached (i.e. if he used some smaller diameter pipe, effectively creating different size expansion chambers tied together). So your idea of using old car tires has merit. I can try to find more info or get you the guys name if you want.

What will you do about noise escaping out your ventilation system? You'll need large openings for airflow to cool the car. You also say this is a chassis dyno, so there will be tire friction noise with the rollers, which is considerable, but high freq. Also, how will you couple your exhaust system to the vehicle - if you slip on hose over the tailpipe, won't a lot of noise escape from that coupling? I've a bit of experience with this as I have an operating chassis dyno. I picked put a medium sized Maxim Silencers M31 (I believe - it's been awhile) from an industial auction. I modified an old welding cart and attached this muffler on it. I use flexible rubber dyno hose and attach it over the tailpipes of the vehicles (they already have mufflers of some sort on them) I'm dynoing on the chassis dyno to keep it quieter for the neighbors. I've no measurements, but there is a noticeable decrease in exhaust noise, but I believe a lot still escapes at the primitive coupling to the tailpipe. If you have questions about my experience with chassis dyno noise just ask.

http://www.maximsilencers.com/products/specs/m31.html

From your post, it sounds like a reduced sound exit area is very important for noise control. So dropping the 12" outlet to 8" or even 6" is a good idea, right? Why must the raincap not flutter? Obviously, I'm trying to minimize backpressure. I'm assuming with very large muffler volume, choking down the outlet like this doesn't affect backpressure as much?

Hypothetically, if a muffler provides 25dB of noise reduction at a certain freq and 1" H20 of backpressure, if you put 2 in series (at the same operating conditions), this doesn't simply translate to 50dB noise reduction and 2" H20 backpressure does it? It can't be that linear, can anyone give some ballpark estimate of what happens when you put 2 mufflers in series?

Well, gotta go - kids are yelling for me.

 

[Warpspeed]

Technically, fitting two identical mufflers in series should double back pressure and halve the measured sound pressure level, but the practical results are never quite like that.

The problem is that the anoyance level of the noise is not always directly related to the direct measured sound pressure level. It is easy enough to get rid of the harsh bark and crackle of an open exhaust port with some fairly simple acoustic absorbtion. The first muffler always gives the largest subjective improvement because it directly changes the character of the sound. It gets rid of the harshness and knocks the edges off the explosive shock waves. The second muffler will certainly reduce the measured sound pressure level further, but it will not subjectively seem to have as large an improvement as the first muffler.

That is why muffler design is so very important. It is the lower frequencies that will be most difficult to atenuate without introducing some deliberate additional back pressure at the exit end of the system. Some experimentation will be required, but fortunately it will be an easily accessible place to change things.

I have also heard about the success of 55 gallon drums filled with tyres used as short duration dyno mufflers. But suspect the wall thickness of the drum may be insufficient to prevent the drum itself from radiating quite a lot of sound. Buried 55 gallon drums in series would be much better, but they would probably rust away fairly quickly. I then thought of a long brick pit divided off into several sections and filled with tyres, that idea has progressed to using round concrete storm water pipe with suitable baffles placed at intervals.

It may even be possible to divert the silenced exhaust down into a large stormwater drain if you have one available. I now do this with my airflow bench. It now runs about as silently as a window airconditioner once all the exhaust air roar dissapears underground into the suburban stormwater drain system. My neighbours even complained about my airflow bench to begin with, but then, it was rather loud.

As my proposed future chassis dyno will be in a faily spacious open underground workshop/garage area, the fresh air ventilation system is the next part to plan. The first step will be to absorb as much noise inside the immediate dyno space as possible by fixing sound absorbant material to the walls and ceiling. The idea is to completely kill the natural reflective echo. Even heavy lead weighted curtains (don't laugh) or floor to ceiling sound absorber panels on wheels could be placed around the vehicle to deaden the acoustics. Maybe I can buy a pile of old office divider panels cheap, something like that perhaps. Turn the whole underground dyno area itself into a giant absorbtion muffler.

A very large volume of forced external fresh air ventilaton will be required, and provided the entering air and exhaust air travel down reasonably long lengths of duct that has an acoustic lining, not much sound should hopefully escape to the outside of the building.

The main vehicle entry door is going to be my biggest sound leakage path. I still have no real idea how to solve that.

 

[MWPC]

Thrasher

Any truck and trailer type parts shop should have the flex pipe. We used galvanized due to lower cost. It is also available in stainless steel. The 4" flex pipes dumped into the 8" heavy wall pipe. The other end of the heavy pipe was bolted to the one 8" i.d. glasspack. They had another muffler of the same size but it seemed pointless to install it. I have not talked to them on the temperature rise. I will ask next time I am there.

Ken

 

[whitevette]

Hi, Thrashercharged!
Two mufflers? In series, they will work well together. In parallel, you'll have half the volume of gases (at the same noise level) going thru each one...so you'll end up with two noisy mufflers! Nothing much accomplished! A tip: using a length or 14 gauge mild steel the same width as the ID of the muffler inner pipe...and by clamping one end in a vise, pulling on the other end with a come-a-long or similar tension device, twist it 380 degrees (a helix). then slip this in the perforated bore of the muffler...tackwelding on one end only(allowing for expansion). This muffler mod will surprise you! Zero restriction to flow...and very quieting!
Regarding mounting the muffler(s) inside the room...DON'T! They get hot! These "heaters" will drive you out! Plus, when they leak...you will die! Put 'em outside! BT DT! -whitevette

 

[turbocohen]

Run many an engine dyno with exhaust drawn out by axial and centrifigul fans with no noise problem. If there is enough cool air entering at the same area the exhaust expands into a lot of noise is attenuated via dilution and if the fan is a small enough high flow rate type it reduces pressure pulsations, standing weaves etc pretty well.
An oem contact sent me this link.

http://www.cincinnatifan.com/centrifugal-spb.htm

They run GM LS2 and other stuff at full load for extended intervals with one of these fans with a single stage large dia attenuator inline. Standing outside of the boom room it is not objectionably noisy for a lab environment.

 

[thrashercharged]

Thanks again for the additional responses. Since I've response from various people and have so many additional questions, I'll split my posts up for ease.

Warpspeed & MWPC, I understand the diminishing returns in sound attenuation as you add mufflers in series. From MWPC's experience it sounds like it may not be worth the trouble of using both. Muffler design is complex, in my case I've the luxury of using commercially available mufflers that I assume were designed correctly by qualified engineers instead of designing my own!

I am curious about the temperature inside the muffler. Obviously, if people are successful stuffing old tires inside barrels for homemade mufflers, the exhaust temps must cool really quickly. I don't know what the flash point of rubber is, but to even consider using them inside a muffler tells me the temp inside must not be much over 200 or so degrees F? Over 250 degs F and you'll smell baking rubber won't you? I'm assuming the mufflers will probably be within 10-15 feet of the header collector, that would seem fairly typical, can exhaust really cool that quickly from about 1500 deg F at the port?

I agree that putting things underground and turning your room into a large absorbtion chamber will really help. I see your point about long ductwork for inlet and exhaust air minimizing radiated noise. Can you elaborate on what you mean by acoustic lining for the ductwork? How's that lining take the exhaust heat? Again, this brings up my question of how fast the exhaust gases cool.

My experience with my chassis dyno is that superchargers and the roller/tire contact create a lot of high frequency noise, which is louder than the exhaust because our test vehicles all have street legal mufflers and then I attach the Maxim M31 silencer to the tailpipes, which helps a lot.

I've worked in million dollar NVH quiet rooms which had ceilings and walls covered with acoustic material. The garage entry are expensive doors that open in layered panels, each panel is about 2-3" thick and heavy - they didn't open overhead like a conventional residential garage door would, each is on a seperate track that stacks one in front (or behind) the other. They have a rolling panel of acoustic material which was placed in front of the garage entry door, effectively making it as any other wall. The material appeared to be batts of fiberglass insulation folded and stuffed into wire mesh structures (wire fencing with 1/2" square holes) shaped into a 3-dimensional isosceles triangles, the equal sides being about 18" long (i.e. they protrude out of the wall about 18"),the base being about 8", stretched out about 16-18". These were arranged in groups of 4, then these groups of 4 were placed perpendicular to each other on the wall (a square of 4 running left to right, placed adjacent to one running up and down, etc.) I'm sure there are more complete and technical plans available, this is just what I observed. If you've ever worked in one of these rooms, they're totally silent, absolutely no reflected sound, it's hard to hear people talk. Hope this helps with your room.

 

[thrashercharged]

Oh, one additional question Warpspeed (or anyone else), can you answer this:

From your post, it sounds like a reduced sound exit area is very important for noise control. So dropping the 12" outlet to 8" or even 6" is a good idea, right? Why must the raincap not flutter? Obviously, I'm trying to minimize backpressure. I'm assuming with very large muffler volume, choking down the outlet like this doesn't affect backpressure as much?

Again, I assume the answer has to do with how much and how fast the exhaust gas cools?