Mufflers - Design Input

[R31Nismoid]

Ok.

Been having issues getting my car past the EPA test, needing around 90dB to pass.

So far its pulled 93dB at this point, Benno/Racepace have done a top job, considering the car has a 3.5" exhaust on it… it’s a fairly big gauntlet that’s been thrown down

The car is no louder than a HSV, I cant even hear it @ cruise, but when you have a 5200rpm test point for the noise test, it’s a ripe pain to get passed as some of you will know.

I've been doing lots of reading on muffler design and so on to get my head around it all a bit better as to what could work, and what does not.

Plus having started reading i need to keep on reading as im finding it all quite interesting so ill fit it to my purpose also at the same time

My requirements.

1. Not worried neccesarily about performance at this point.

2. Just need the car to be at a legal level - which does mean dropping 3dB

I know there are a few engineers lurking, does anyone have some diagrams I could look at that?

Or even some links that don’t have too much jargon speak in them?

At the moment i have:

Compliance/stock size cat (for a GTR i think)

Also two mufflers, larger and a smaller

Im not looking for the quick fix like sticking a baffle/restrictor plate

A restrictor was put in right @ the end (jap exhaust style) and it didn’t drop the dB reading at all, so there must be some forces at work I don’t get, as logic would tell me it should be quieter but the meter says it was not the case!

This thread could probably help a number of people, i know there is someone in QLD having issues too

Cheers

Ash

[tridentt150v]

Do you have a resonator and cat? what size, that will affect the final dB.

[R31Nismoid]

Currently

Compliance/stock size cat (for a GTR i think)

Also two mufflers, larger and a smaller

stuck that in the first post now too!

[tridentt150v]

3.5" exhaust and stockish cat, anyway.

Mine has 90mm OD with 4"catco cat, resonator [brand unknown], inline exhaust [brand unknown] and HKS Hyper 6" cannon type and no power choking issues that I'm aware of.

Sounds like you have almost the same. Maybe try a resonator?

[Ronin 09]

do you have space enough in a straight section for a hotdog/resonnator?

[R31Nismoid]

trident - do you know how loud yours actually is?

Because if it isnt under 90dB then its not gonna help me a lot, and with a cannon i'd imagine it wouldnt be close

This is what i have now... so the answer be nope

[Ronin 09]

you're going to have to get the cat moved closer to the dump - ie where the merge collects, then go a longer muffler and get it angled toward cat and repipe it i'd guess...

what a farker to fit the necessary size mullers under the car!

the mufflers look slightly smaller than mine.

[R31Nismoid]

I think there is some ruling about how far the cat has to be from the exit... i'll have to go and re-read my documentation when i setup my PC's this week. I've got it all somewhere im sure.

I dont agree with the moving/extending though either way as i dont believe marginally longer muffler wouldn't fix it at all.

The other mufflers are pretty freaken big, im still surprised they fat under there

When you think about it, another 10-15cm extension at the most, remaking the whole exhaust just for something like that... is not going to work.

See at this point i have to assume that just an extra 10-15cm wont knock off 3dB if a welded restrictor @ the end did not

[Ronin 09]

remember it's all about mass to absorb the sound. an extra 15 cms of muffler may well do it - consider that the hot dog in my factory 32gtr exhaust kept it down at 87db (everyone kept telling me that the factory 32gtr exhaust does 92db).

you may well be able to get a hot dog in there, but where is the question

[R31Nismoid]

It might, might not... see cost vs a potential (zero) gain isnt my idea of play unfortunately.

Im running low on funding as it is, making what would be almost a new exhaust isnt going to happen in the next few months just to 'test'

[NickR33]

geez that exhaust looks mint and very sensible, can't believe its still 93db:(... only option would be to go larger with both mufflers but it looks like there isn't anymore room

if you put a restriction further up the exhaust away from the tip (and the tester) that may help... like changing the straight through muffler after the cat to a baffled type? that would restrict performance tho I would assume

[TiTAN]

Do you have the space to fit in a 16" offset barrel muffler (might have to get a truck one to keep it 3.5") at the rear in place of the cannon? it wont look pretty but should drop the 3db required.

[R31Nismoid]

I've actually printed a design out earlier titan that i think is what you mean

Something to look into that one for sure, being performance is not the issue at present

[Omurru]

On the topic of restrictors, the 'jap style' ones that go in the end of the exhaust in my experience do bugger all, I have seen them even make the exhaust note louder, all they are is a whistle really. Unless your gonna cut the exhaust up and put in a different muffler which to be under 90db is probably gonna be restrictive as hell and permanant, the only option is to use a baffle somewhere further up the exhaust. I've used them in a few cars either before or after the cat and depending on design (single smaller hole, multiple holes etc) they make a big difference, but still it's not a permanant solution. I've found with mufflers the more restrictive they are the more they quieten.

Although at the 93db it's at why don't you want to just use a restrictor to pass the test, as you said the average hsv is louder anyway.....

[GeeTR]

Im not looking for the quick fix like sticking a baffle/restrictor plate

Nice exhaust, shame to have to cut it up to drop just 3dB. Why are you so against a temporary restrictor plate? It would easily knock 3dB off, and if its already as quiet as a stock HSV, i doubt the popo will notice with it taken out. I was pulled over with a PEII system many a time back in the day (breath tests etc) and they never paid attention to 97dB@1234RPM...

EDIT: Damm you Omurro ... and my lack of refreshing the page

Edited June 2, 2008 by GeeTR

[Omurru]

EDIT: Damm you Omurro ... and my lack of refreshing the page

lol

Found this in one of my books, it relates to turbine engines but I don't see why the principle would change with piston engine exhausts.

"The noise suppressors in current use are either of the corrugated perimeter or the multitube type that breaks up the single, main jet exhaust stream into a number of smaller jet streams. This increases the total perimeter of the nozzle area, and reduces the size of the eddies created as the gases are discharged to the open air. Although the total noise energy remains unchanged the frequencey is raised considerably. The size of the eddies scale down linearly with the size of the exhaust steam. The simplest means of reducing noise levels would be to reduce exit velocity for a given engine, but this is impossiable as thrust is directley related to exit velocity."

Maybe why the restrictor in the end doesn't reduce the meters reading? Also an exhaust system is reducing exhaust velocity by means of mufflers etc but also absorbs some of the noise (soundwaves etc.) Hence a restrictor further up the exhaust will slow it there instead of at the tip where the muffler will then be reducing the level of an already reduced note.

P.S. This may all be bullshit, I really have no idea so feel free to correct me if I am wrong.....

[GeeTR]

Those end things are useless. I was talking bout the ole "plate full of holes" - cut to the flange shape, placed in between two sections

I wonder how they muffle the turbine gas discharge then..without hurting velocity hrmm.

Edited June 2, 2008 by GeeTR

[tridentt150v]

trident - do you know how loud yours actually is?

Because if it isnt under 90dB then its not gonna help me a lot, and with a cannon i'd imagine it wouldnt be close

This is what i have now... so the answer be nope

Sorry, no, but I don't call it loud, more of a subdued rumble. But I realise thats no good to you, you need hard evidence.

My second muffler is a wider oval shape than your's, the inlet vs outlet is also more offset.

[Omurru]

I wonder how they muffle the turbine gas discharge then..without hurting velocity hrmm.

There's other factors too, hot and cold air bypass ratios, minimising turbulence, hot section design and acoustic materials in the ducts, but thats all kinda irrelevant to this topic.

[rb26s13]

one thing thats cheap that you could try to see if it knocks a bit of noise out of it.. get an exhaust flange cut the same as wat your currently running.. but with a 1.5inch hole in it pass the test.. and pull it out