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The story: I have a turbocharged 2l engine in my car, and a 3" straight through exhaust, that is very very loud and drones.

So, I bought a cable operated inline butterfly valve, that can be opened and closed and install between the test pipe and mid-pipe.

Currently valve looks like this. enter image description here

Obviously, for me to be able to drive around, I need to drill holes so that when it's fully closed, I could still drive around.

Question is: Should I drill a bunch of little holes, or one larger one. How large should it be for optimal sound?

I am looking to remove the drone(deeper tones) and eliminate sound as much as possible, while still keeping the car at least 50% efficient when closed.

Stock exhaust is 2.5" in diameter and does a fantastic job quieting the exhaust, but it also has a resonator and pretty restrictive muffler.

migrated from engineering.stackexchange.com Aug 23 '16 at 11:29

This question came from our site for professionals and students of engineering.

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    Recommend closing and/or migrating to either music.se or physics.se, as those sites will provide better info on tuning pipe lengths to match sound frequencies, flaring pipes to improve impedance matching at the exit, and so on. – Carl Witthoft Aug 23 '16 at 12:03
  • I've looked into doing the same thing, and it gets complicated. I would try searching car specific forums. I think for my 3.5 NA v6 w/ 2.5" exhaust I needed something like 42 inches of resonator tubing? Which is why I stuck with upgraded OEM hardware. – MooseLucifer Aug 23 '16 at 12:58
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    Have you thought about adding a Sports Cat - this will cut a lot of noise, have only a small effect on power, and really help your emissions! – Rory Alsop Aug 23 '16 at 17:04
  • What's a test pipe? Why don't you create a Y intersection and dump out before the muffler with the other section going through a muffler. So when you want to open it up, you can, but otherwise, it's nice and quiet? You can use your valve to control this. – cory Aug 23 '16 at 17:40
  • @cory Dumping is an option, but stock exhaust can't be heard much. Aftermarket exhaust is too loud. So the sweet spot is having the aftermarket exhaust with a valve where it's either quiet or loud, but not obnoxious like dumping. – Alexus Aug 23 '16 at 18:15
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Generally, cars that have a valve in the exhaust have 2 exhaust paths. One path goes through the silencer, the other path bypasses it. The valve opens/ closes the bypass. In both valve positions the exhaust gases can flow freely.

Just sticking a valve in your existing exhaust system will not reduce the noise: the gasses still flow through the same undamped set of pipes. With the valve closed, the engine will experience more backpressure and run less efficiently. And you will get more noise because the airflow around the half-open valve will be turbulent.

So you'd have to create a second exhaust path, maybe using your stock silencer. Place the valve in your straight-through pipe just downstream of the point where the stock silencer branches off. Valve closed: exhaust flows through the silencer. Valve open: exhaust flows through both, with the majority going through your straight-through system because of its lower resistance.

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I don't see the need to drill holes here, as you can get an equivalent effect by keeping the butterfly valve partially open at idle. That's how it's usually done on cars like the C5 Corvette, Gallardo and F10 M5.

The added benefit of this is you can adjust the amount of opening as much as you need during the course of testing. Once you're happy with the minimum angle, locking it in with a physical stop on the cable should be quite easy.

Why measure twice, cut once when there is no need to cut at all? :)

  • I like this idea, but wouldn't a round hole in the butterfly valve flow better than an angled butterfly itself? – Alexus Aug 23 '16 at 16:36
  • @Alexus depends on the geometry of the hole vs throttle plate. Both approaches will result in exhaust restriction, but it doesn't really matter at part throttle since your air flow is minimal. Where it matters more is when the plates are fully open, in which case there isn't any difference between the two approaches. – Zaid Aug 23 '16 at 18:58
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The car is forced induction, specifically turbocharged. Placing an obstruction in the exhaust will prevent the turbo from spinning up quickly because it relies on the flow an gas through the exhaust to spin up the veins of the unit. This butterfly valve will impede the flow of gas through the exhaust system and thus quite likely add turbo lag or potentially even stall the turbo.

A better option would be to opt for a "resonated" performance exhaust system is the objective is to make the car quiet. That said, the most civilized exhaust system available is almost certainly the OEM system.

  • Agree with the recommendation here, but not the reasoning. Manufacturers of turbo vehicles do employ butterfly valves in the exhaust, the F10 M5 is no slow coach :) – Zaid Aug 25 '16 at 10:28
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    The vehicle "map" will be tuned to operate the butterfly valve and the boost levels will make use of this. I don't know much about the F10 but I know for certain that E36 had a butterfly but it only covered the second exhaust pipe of the twin pipe exhaust at low RPM, not blocked the entire exhaust. – Steve Matthews Aug 25 '16 at 10:38
  • That's the point. If the valve is drilled to say 2.5" diameter, that would be a hole in diameter equivalent to stock exhaust system size. When fully opened, it's 3", but when closed, it's 2.5. Aftermarket exhaust has a resonator and a muffler, but problem both are experiencing is the low frequency sound. My assumption was that restricting the orifice in the middle of the exhaust to smaller hole would eliminate the low frequency sound due to wavelength. Or I might be significantly off. – Alexus Aug 25 '16 at 15:23
  • As for the turbocharger. I have dual map set up that I can switch between low and high boost. I am not planing to run this valve at WOT, because that's just bad for the turbo. My idea was to run it for just highway cruise where max boost I get is 5 PSI(29 psi is max o my tune). – Alexus Aug 25 '16 at 15:24
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    If you were to drill butterfly you might have a problem with the centre spindle? Wouldn't it be better to set the "closed" position to be partially open? – Steve Matthews Aug 26 '16 at 7:28

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