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Johns180

Exhaust restrictor plate safe diameter, noise reduction

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I'm wanting to reduce the noise from my exhaust in a cost effective manner.

At the moment my car exhaust makes 84dB at idle and 104dB at 4500RPM and with 90dB the legal limit at what I think should be 4000RPM so from what ive read a restrictor plate could be a solution, just replacing one of the exhaust gaskets with sheet metal.

I have a 3" turboback xforce exhaust with a 4" muffler tip, has cat and resonator, since the stock exhaust is 2" would I be fine with a 2" hole restricting air flow? And should that most likely get me under 90dB or will a smaller size be required?

Car has a T28 on an SR20 with stock cams at 9psi but if I do this I'll most likely remove the boost controller and leave it at the stock 6psi.

This is my daily so im trying to avoid defects where possible.

 

John

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I don't think putting a "restricter plate" in sounds like a good idea at at.... You shouldn't really try and restrict the exhaust flow. Look around for a silencer to go into the exhaust tip, they bring down the noise quite a bit but don't reduce flow too badly.

 

- Lance.

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I don't think putting a "restricter plate" in sounds like a good idea at at.... You shouldn't really try and restrict the exhaust flow. Look around for a silencer to go into the exhaust tip, they bring down the noise quite a bit but don't reduce flow too badly.

 

- Lance.

Silencer or restriction plate, same damn thing...

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stuff your muffler with chicken wire, or go get a good exhaust

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Ideally you'd get a quieter exhaust rather than sticking some horrible restrictor in it

 

actually... you could swap your catback section for mine, since I'm after a louder exhaust myself. Heh. I've got a 3" system but just too quiet for me. Haven't seen brand markings or JASMA compliance so it's possibly custom but who knows. It's gotten boring for me, but on the upside, I've never been hassled by cops in nearly 3 years of owning the car.

Edited by Skepticism

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If the cops want to defect you for your exhaust they will, it's the fact that it doesn't look stock not how loud it is.

A restrictor plate is a terrible idea, just replace the cannon with a nice chambered muffler, cannons are what will get the police mans attention.

I went for a subtle twin tipped muffler, relatively stock looking sounds good and doesn't restrict flow, the police have never given it a second look

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An oem exhaust read like 86db at test rpm when i went epa... so pretty much no hope of sticking something in the end to make it legal and not be a massive bottleneck

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A restrictor plate is not a solution, it's a temporary workaround, the same as a cable-operated butterfly valve is a workaround for temporary noise reduction. The stock exhaust has horrible flow, so the fact that the car runs doesn't mean it should be used as a benchmark for suitability, and it's largely pointless to introduce a 2" restriction in a 3" exhaust system... might as well install a stock exhaust and make it entirely defect-free.

 

If you want a cost-effective approach, look at getting some second-hand mufflers in good condition. Irrespective, the only functional solution to your problem is to replace the hardware with more suitable parts.

 

1. Buy a second-hand 3" in/out muffler that fits the mid-muffler space, remove the resonator if it's in the way (otherwise leave it), then get it installed. A resonator is a great tool to help attenuate certain frequencies, but it isn't designed to reduce the overall decibel level. A mid muffler does, and it works.

2. If the level is still too high, buy the biggest 3" straight-through in/out muffler you can possibly find, install a 3" tip, install the muffler, then test. Installing a large tip on a muffler will change the sound slightly, such as the old Milo-tin trick kids used to do to their cars for a laugh.

3. If the level is still too high, either buy an even bigger muffler (if you measured incorrectly and went too small) or buy a tri-flow muffler, which will hurt performance but heavily reduce the decibel level.

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Get blast pipes lol

That's your solution to everything haha

 

Solves all problems haha

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Get blast pipes lol

 

Might as well get a screamer while your at it

 

I've thought about it, but don't wanna get defected lol

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A restrictor plate is not a solution, it's a temporary workaround, the same as a cable-operated butterfly valve is a workaround for temporary noise reduction. The stock exhaust has horrible flow, so the fact that the car runs doesn't mean it should be used as a benchmark for suitability, and it's largely pointless to introduce a 2" restriction in a 3" exhaust system... might as well install a stock exhaust and make it entirely defect-free.

 

If you want a cost-effective approach, look at getting some second-hand mufflers in good condition. Irrespective, the only functional solution to your problem is to replace the hardware with more suitable parts.

 

1. Buy a second-hand 3" in/out muffler that fits the mid-muffler space, remove the resonator if it's in the way (otherwise leave it), then get it installed. A resonator is a great tool to help attenuate certain frequencies, but it isn't designed to reduce the overall decibel level. A mid muffler does, and it works.

2. If the level is still too high, buy the biggest 3" straight-through in/out muffler you can possibly find, install a 3" tip, install the muffler, then test. Installing a large tip on a muffler will change the sound slightly, such as the old Milo-tin trick kids used to do to their cars for a laugh.

3. If the level is still too high, either buy an even bigger muffler (if you measured incorrectly and went too small) or buy a tri-flow muffler, which will hurt performance but heavily reduce the decibel level.

 

This seems like the best option, since I'm not keen on changing the way it looks from the rear.

I'm thinking of adding the second muffler between the resonator and "cannon", how do I work out what size I need for a 14 dB reduction?

 

Cheers.

 

edit:

there doesn't seem to be much room after the resonator, so I'd have to look at one between the cat and res, what are your thoughts on replacing the resonator with a muffler? I know you said not to, but there doesn't seem to be much room to play with. Does the replacement muffler have to be significantly longer than the resonator currently in place?

Edited by Johns180

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Yeah dude there's not much room at all, I've just got the biggest straight through muffler I can fit there, like 18 inch I think is the biggest they come.

If you want to quieten it down significantly the only thing you can do is change the rear muffler

Edited by Coastie

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Yeah dude there's not much room at all, I've just got the biggest straight through muffler I can fit there, like 18 inch I think is the biggest they come.

If you want to quieten it down significantly the only thing you can do is change the rear muffler

 

I got quoted $160 from a local exhaust shop to weld on a quiet one if I reuse the the pipe coming from the cannon to the flange, $240 if I want to be able to bolt the muffler I have on it now back on which I do like the sound of.

I asked for the cheapest exhaust they had, mild steel etc, however I don't know what it looks like.

 

However he did say even though it would reduce the noise a fair amount it most likely wouldn't be under 90dB though which put me off...

Edited by Johns180

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This seems like the best option, since I'm not keen on changing the way it looks from the rear.

I'm thinking of adding the second muffler between the resonator and "cannon", how do I work out what size I need for a 14 dB reduction?

 

Cheers.

 

edit:

there doesn't seem to be much room after the resonator, so I'd have to look at one between the cat and res, what are your thoughts on replacing the resonator with a muffler? I know you said not to, but there doesn't seem to be much room to play with. Does the replacement muffler have to be significantly longer than the resonator currently in place?

 

If there's no space, it's fine to remove the resonator.

 

As for sizing, well you're over-thinking it. The goal is to literally fit the biggest muffler that you can... it's that simple. A resonator is not the same device as a muffler, so don't worry about a length comparison to the muffler. Just remove it if it's in the way, install the biggest [small] muffler that will fit and weld it in place. Do the same thing with your main muffler at the rear; get the biggest one that can possibly fit the cavity space, then weld it in.

 

The only way to really tell the decibel reduction is to actually fit the mufflers and assess it, as the design and packing of the mufflers will change the noise suppression. Ultimately, whatever you install will likely be a big improvement over what you have, as 10dB might not sound like much, but it's a very significant factor of reduction, given the logarithmic nature of the measurement unit.

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This seems like the best option, since I'm not keen on changing the way it looks from the rear.

I'm thinking of adding the second muffler between the resonator and "cannon", how do I work out what size I need for a 14 dB reduction?

 

Cheers.

 

edit:

there doesn't seem to be much room after the resonator, so I'd have to look at one between the cat and res, what are your thoughts on replacing the resonator with a muffler? I know you said not to, but there doesn't seem to be much room to play with. Does the replacement muffler have to be significantly longer than the resonator currently in place?

 

If there's no space, it's fine to remove the resonator.

 

As for sizing, well you're over-thinking it. The goal is to literally fit the biggest muffler that you can... it's that simple. A resonator is not the same device as a muffler, so don't worry about a length comparison to the muffler. Just remove it if it's in the way, install the biggest [small] muffler that will fit and weld it in place. Do the same thing with your main muffler at the rear; get the biggest one that can possibly fit the cavity space, then weld it in.

 

The only way to really tell the decibel reduction is to actually fit the mufflers and assess it, as the design and packing of the mufflers will change the noise suppression. Ultimately, whatever you install will likely be a big improvement over what you have, as 10dB might not sound like much, but it's a very significant factor of reduction, given the logarithmic nature of the measurement unit.

 

Alright so overall do you think introducing a muffler replacing the resonator or the cannon should reduce the noise the most? If I had to choose one to start with and see if it's quiet enough. You seem to advocate the mid muffler a lot more but I'm unsure if you're saying this because it will be more effective at attenuating the sound or because it won't change the overall appearance of the vehicle. There is an appeal to having a more standard looking twin tip exhaust so if I have to change it or not doesn't bother me... and as long as the outlet is 3" or twin 2.25" there shouldn't be a loss in performance afaik.

 

As far as I know cannons are preferred by some as they are louder so that may be amplifying the noise more than necessary and replacing that may be more effective at a guess.

I am familiar with the decibel scale as I've done work with audio before, just never on exhaust systems so I'm unsure what effect the changes will have.

 

 

If the cops want to defect you for your exhaust they will, it's the fact that it doesn't look stock not how loud it is.

A restrictor plate is a terrible idea, just replace the cannon with a nice chambered muffler, cannons are what will get the police mans attention.

I went for a subtle twin tipped muffler, relatively stock looking sounds good and doesn't restrict flow, the police have never given it a second look

 

What size are your exhaust tips? I ask because although my car doesn't have many mods now I would like to have an exhaust free flowing enough to let me reach 250kw with headroom to 300kw... I seriously doubt I'll ever go over that.

Edited by Johns180

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Alright so overall do you think introducing a muffler replacing the resonator or the cannon should reduce the noise the most? If I had to choose one to start with and see if it's quiet enough. You seem to advocate the mid muffler a lot more but I'm unsure if you're saying this because it will be more effective at attenuating the sound or because it won't change the overall appearance of the vehicle. There is an appeal to having a more standard looking twin tip exhaust so if I have to change it or not doesn't bother me... and as long as the outlet is 3" or twin 2.25" there shouldn't be a loss in performance afaik.

 

As far as I know cannons are preferred by some as they are louder so that may be amplifying the noise more than necessary and replacing that may be more effective at a guess.

I am familiar with the decibel scale as I've done work with audio before, just never on exhaust systems so I'm unsure what effect the changes will have.

 

No, I advocate fitting a mid muffler because you don't have one already. Clearly a 20" muffler won't suppress noise to the same extent as a 44" muffler, just based on obvious physics. However if you don't even have a mid muffler, then you're missing out on 20 inches of extra expansion chamber room, whether you change the rear muffler or not.

 

It's also relatively cheap to do because the mid muffler is small, and a somewhat foolproof item to install, because the space limits your options so much that whatever you get can't really be improved upon further; the absolute best mods are ones that have a guaranteed positive effect, and never need to be changed. In contrast, if you change the rear muffler, you have to decide on the size, type, flow design, packing material, inlet/outlet layout, etc. Simply changing your current Cannon for a giant oval muffler will net you the biggest improvement, however you should still be adding a mid muffler for maximum noise suppression anyway, so why not do that first and assess the results?

 

To confirm, yes, Cannon mufflers tend to suck pretty hard at reducing sound. Better than straight pipes, but sometimes not that much better.

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My exhaust was an xforce 3.5 inch turbo back with a cannon I could fit my head in, getting rid of the cannon was a huge improvement mainly at idle and lower revs, the muffler I used has a 3 inch inlet and 2x 2.5 inch outlets with some nice 3 inch tips.

If you want your exhaust to support 300kw it's definitly not going to be quiet haha, it has to be minimum 3 inch.

If your main goal is to not draw attention from the police you're better off focusing on looks instead of sound, you just keep the revs low when you're around any it's not hard haha

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What pmod is trying to say:

 

- more mufflers is better

- cheap china off the shelf exhausts usually have 1 cheap resonator and 1 cheap muffler, and are loud

- stock exhaust is shit

- theres no law that says smaller exhaust is quieter, or bigger exhaust flows better. a better exhaust is better

 

good jap exhausts have two mufflers. I have an ancient fujitsubo exhaust, has two mufflers. was legal noise on a friends car, at 290kw.

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^ Translation complete.

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Thanks a lot for the help guys.

Ended up taking it to another shop (this guy seems to know what he's doing a lot more), had it up on the hoist and was told what I thought was a resonator is actually a muffler, the oval shape just threw me off. In his opinion there wasn't really much room to add in a secondary muffler so I was better off using what was already there.

Anyway, I'm getting an oval type muffler to replace the cannon tomorrow, 3" inlet to dual 2.25" outlet. I doubt it'll get me legal but hopefully somewhere near 95dB at 4k I'm hoping.

Either way it should reduce police attention, which is the ultimate goal.

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What is the guy saying? there isn't room to adda third muffler? or the "resonatorish muffler" can't be replaced with a muffler?

 

x1.jpg

 

x2.jpg

 

these are two xforce exhausts for an s13. One has a mid muffler, the just jap one has a resonator.

 

if you have the just jap style one, I'm telling you, you can fit a second muffler. not maybe, or probably, there is room for two mufflers under the car.

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