doubt about mufflers

[TiTiKo]

we fabricated our muffler with this dimension:
large: 60cm
outside diameter: 15cm
inside diameter: 5cm

the muffler is type cherry bomb or glass pack, and we had a problem in the noise test we made ourselves, we obtained 115dB at 11krpm,
To solve it, how should we approach this issue? a large muffler with same sound insulation density or or same size muffler varying insulation density?.

[drivetrainUW-Platt]

I would read some books about car exhaust design since a "cherry bomb" muffler is probably the least engineered muffler on the market(aside from a 6" fart can on a Honda Civic) They seem to sell quite well to the redneck crowd, usually meaning not good for a formula race car.
A muffler of that style is pretty simple to fab up, so make one end removable and adjust your density, inner flow diameter, inner pipe to glass packing hole size, outer diameter ect and test away

[Big Bird]

Also try to keep your exhaust outlet as far away as you can from the engine itself. At the event your noise is measured at a prescribed distance and angle from the exhaust outlet - so if your outlet is right next to the engine then you'll get engine noise adding to the reading. I've seen many cars who failed the noise test end up passing later on after simply adding an extension tailpipe to their muffler. (I remember Wollongong won FSAE-Oz in 2002 using this simple add-on)

Cheers,

[threehondas]

Has anybody seen a correlation between spark advance and dB? If you do your test again try adding some advance in those cell(s) on your map and see what happens.

[Rezwolf88]

When you go to look for books try using the word silencer. There are many theory books that are great but use silencer instead of muffler. Also books that have intake manifold theory often have theory on exhaust too.

[Marc Jaxa-Rozen]

Originally posted by threehondas:
Has anybody seen a correlation between spark advance and dB? If you do your test again try adding some advance in those cell(s) on your map and see what happens.

We tried that after some trouble at the FStudent noise lottery last year, didn't seem to register a change though (same for mixture, FWIW).

Marc Jaxa-Rozen
École Polytechnique de Montréal

[threehondas]

I thought I made some small gains... (noise reductions). Maybe my tests were not adequately controlled...

Considering noise test is run at low MAP pressure and high rpm, you need more advance than most people think.

[VFR750R]

Do you think the judges would notice if you cut two cylinders (4 cylinder engines only)at the point of MAP and RPM you see during sound test? It is pretty far out on the map, you would almost never see that on the track...and if you did, you obviously had your foot at something less then WOT.

[raska]

Originally posted by Marc Jaxa-Rozen:
didn't seem to register a change though (same for mixture, FWIW).


Granted it may have been a bit rich in the first place, but sounding the same to the human ear we have noticed signifigant chances on the sound meter by leaning it up.

[Cement Legs]

Originally posted by VFR750R:
Do you think the judges would notice if you cut two cylinders (4 cylinder engines only)at the point of MAP and RPM you see during sound test? It is pretty far out on the map, you would almost never see that on the track...and if you did, you obviously had your foot at something less then WOT.

They might if the sound level suddenly drops as you are reving up...

[absolutepressure]

Originally posted by VFR750R:
Do you think the judges would notice if you cut two cylinders (4 cylinder engines only)at the point of MAP and RPM you see during sound test? It is pretty far out on the map, you would almost never see that on the track...and if you did, you obviously had your foot at something less then WOT.

Can you hear the difference between a V8 and a V6? Between a V6 and a four banger? Now the difference between a four banger and a 2cyl? I'd hope so, especially if you're the "engine and turbo guy" at cornell. You know what? Go out to your car, unplug 2 spark plug wires, and tell me you can't hear a difference.

Everything sounds like a V8, the only difference is the number of beats it's either missing or adding.

[absolutepressure]

Better yet, you might as well put injectors in your exhaust manifold and run your engine backwards. That would quiet down your muffler.

[Bryan Hagenauer]

Of course the judges would notice- but so what? If the rules only require the noise test at that rpm with that throttle position, and every single time they retest you, you pass with the same thing, whats the problem? It is no different than tuning the exhaust for the test rpm, but more extreme.

Do you meet the rule? Yes. Do you meet the intent of the rule? No. Don't be too suprised when you don't get to run.

[drivetrainUW-Platt]

why does Cornel care, they have the turbo muffler going on, plus the smallest muffler in fsae....lets not discuss cheating, children....

[Bryan Hagenauer]

Who's discussing cheating? I'm talking about meeting the rules perfectly. They could just as easily do the noise test from a certain distance away at a certain point on track, like other forms of racing.

[Bill Kunst]

OKay, if you have a big bang 4-cylinder firing only two cylinders, would you notice that? If it didn't significantly change the tone, you could easily get away with it. Or you could make it like DOD, shut off the injector and ignition on two cylinders at low load (idle) for "better gas mileage". Just some thoughts-
Bill

[BeaverGuy]

Its not cheating in any way, and if you look he is a Cornell alum. And there was an explicit change to the sound rules this year, if you are above the sound limit at any RPM below the check RPM you will fail. As far as circumventing the rules, disabling the two cylinders isn't any different than adjusting the fuel and spark in those regions to meet sound only a much more extreme version of it.

[VFR750R]

Settle down guys, I'm not advocating cheating. As far as I'm concerned, it's something everyone can do, and it innovative thinking. I would count it as cheating if you were going to change it back to run on the track. But in this case, it's a design intent that you could run successfully all weekend.

Absolute Pressure...I didn't mean that they couldn't tell the difference, just-would they care. Would they look at it as design intent to work the rules, or as 'cheating'. Either way, they have obviously changed the rules to reflect they don't want to see things that are quiet only at the test rpm.

[mario_rbr]

How exactly do they measure the engine speed at competition?, just by looking at the tach? or do they attach something to the coils or something, to measure real speed, beacuse if it's just by looking at the tach, you can set it up to lower than real values, son when it shows 11000 rpm, the engine it's only at 9000 rpm

[B Lewis @ PE Engine Management]

In the past, they have allowed you to show them your ECU tuning software to get RPM. We usually have one or two teams asking to borrow a laptop for just this reason during the noise tests.

[kwancho]

Originally posted by mario_rbr:
How exactly do they measure the engine speed at competition?, just by looking at the tach? or do they attach something to the coils or something, to measure real speed, beacuse if it's just by looking at the tach, you can set it up to lower than real values, son when it shows 11000 rpm, the engine it's only at 9000 rpm
That would be closer to stepping over the rules line than most of us would like.

[BeaverGuy]

Originally posted by Alex Kwan:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by mario_rbr:
How exactly do they measure the engine speed at competition?, just by looking at the tach? or do they attach something to the coils or something, to measure real speed, beacuse if it's just by looking at the tach, you can set it up to lower than real values, son when it shows 11000 rpm, the engine it's only at 9000 rpm
That would be closer to stepping over the rules line than most of us would like. </div></BLOCKQUOTE>

However, if you search this forum you will find people admitting to doing this.

[Malkovich]

Instead of trying to cheat (or call it as you want), did someone ever try to make is own muffler and succeed in it?

As I see, many of you are like me. A little bit late on the countdown and you're trying to make a muffler on the side, as fast as you can. Do not really have the time to test a lot of designs.

Wich kind of wool is the best and can resist to that heat? I don't want to blow it off at the face of the competitor during the race...

[Matt Gignac]

We use chopped fibreglass packing, as many teams also do. In a pinch, we've tore up some fibreglass mat to make muffler packing.

We've made our own muffler, because the turbo has a certain silencing effect, allowing use to run a very short muffler.

Matt Gignac
McGill Racing Team

[Pete M]

Yeah, i'll second the "Who needs a muffler? We've got a turbo" argument. We were recorded at 105 at West last year and 106 at Aus. And that last one was with a straight through perforated core muffler, a bit over 200 mm long, 2 inch core ID. Turbos are amazingly effective silencers.

[ad]

Originally posted by Pete M:
Yeah, i'll second the "Who needs a muffler? We've got a turbo" argument. We were recorded at 105 at West last year and 106 at Aus. And that last one was with a straight through perforated core muffler, a bit over 200 mm long, 2 inch core ID. Turbos are amazingly effective silencers.

I'll second that Pete. However i liked the sound without the muffler, chirp chirp...

[Chris Allbee]

Malkovich,

I have seen several teams design and succeed with their own mufflers. It is not easy, and it is very difficult to do properly without a lot of "guess and check". However, as has been mentioned, adjusting your AFR and ignition can have just as much an effect. We recently ran a test and found that adjusting the AFR could drop sound as much as 3-4dB and we stopped as soon as it read 109dB. Probably could get it quieter...

[jakez]

We're running a turbo and without any muffler, we were at 120dB. I made a muffler, 2.5in ID, 4in OD, 12in long, and run of the mill pink panther fiberglass insulation. It's my first muffler, but we're at 105dB. I'll probably make a smaller one before competition.

[VFR750R]

There is muffler packing for dirtbikes that is tailor made for a custom muffler. Yes, it's more expensive then pink panther fiberglass, but is denser and has the right design intent.
Off hand Moose racing makes some and they sell it in Dennis Kirk catalogs but I'm sure it can be found at the local motorcycle shop.

The trick I know from repacking dirtbike mufflers is take your core out from the can. Wrap the core tight enough that the wrapping is all touching but don't crush it much. Wrap enough so it is a snug fit into the can, but don't over do it. Saftey wire the packing 3 or 4 times down the length of the muffler and slide the core back into the can. Loose packing can hurt both noise suppression and hp as the exhaust gases see the extra area in the muffler.