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Exhaust acoustics

  1. Jul 30, 2014 #1
    I am working on a custom car project, and will be using a Subaru WRX 2.0 liter turbo charged engine.

    I did a search under the question I am looking for an answer to, but only found one thread that was closed. I hope you all might be able to educate me, on what my possible options are.

    I would ideally like to find a way in a very short exhaust system to modify the exhausts pulses, and tone.

    I am working on testing the feasibility of a bypass system like used in the Astin Martin Vanquish. A throttle reactive bypass system that allows the car to be civil under normal use, but emits a nice growl when things get fun.

    However, I would like ideally to remove the trademark Subaru STI Drone associated with most performance subarus, and lean more toward something that sounds more exotic, and less of a pulse to it. I do like the bass level on the exhaust though.

    I know I cant get something for nothing, and my options are limited. Would there be anything that could modify the acoustics of the exhaust that I could do? Maybe changing diameters, etc.
    Im even willing to make my own muffler/resonator etc. I'd just like to learn a little more of what I can tweak to make this car sound a little better.
    I have access to Catia V5R to run some simulations. If it might help let me know. Id like to be as educated as I can while approaching the design of this exhaust for the est possible results.

    Thanks in advance.
    Last edited: Jul 30, 2014
  2. jcsd
  3. Jul 31, 2014 #2


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    Have you considered an audio system with strategically placed speakers?
    Linked to the engine RPM and throttle it can do precisely whatever you want.
  4. Jul 31, 2014 #3
    It sounds like you're trying to make a tuba sound like a piano. That pulse sound is due to the H configuration that allows for even spacing/time between each cylinder firing. You could pipe the cylinders together that are next to eachother in the firing order to create sort of a left-right-left-right stereophonic sound but that wont affect the pulse drone you've mentioned. Exotic is such a broad word because we could be from different continents. I assume you're talking about the trebly sound associated with high revving v-12 and v-6 engines?
  5. Jul 31, 2014 #4
    From a technical point of view you trying to do is alter 'sound quality', and pick out engine orders in a way that sounds good.

    Realistically, without extensively measuring stuff, you aren't going to be able to make 'educated' alterations as you don't know exactly what effect each bit. Typically this type of this is simulated with a 1D gas dynamic model.

    The first step would be to get a spectral analysis of the noise. Then at least you know where you are starting from. you can then identify what the drone is, what orders are prominent.

    For just tuning, I'd suggest looking up forums that people have swapped out for aftermarket. Then look at how the design differs.

    EDIT: Does it have equal length or uneven headers?
  6. Jul 31, 2014 #5
    Its an OEM turbo so the headers are I think are equal length. Which I know gives a higher pitch than the unequal length ones do.
    I guess a better way to say this might be, Im looking for a Refined sound. Not a fart box muffler sound. The bypass system is an idea, to give it a touch of raw only once in awhile when the throttle is really pressed down.
  7. Aug 4, 2014 #6
    Certainly the STi OEM headers are unequal length, not sure about the WRX. It's the unequal length headers that give a lot of the characteristic "burble". Really after that the sound is only controlled by a centre resonator (stops drone at highway speeds) and a rear silencer. The stock STi (go and listen to it at your nearest Subaru dealer) is pretty refined. You might just be able to bypass the centre resonator and/or rear silencer under hard acceleration to get what you want.
  8. Aug 14, 2014 #7
    The Impreza's I have heard with equal length manifolds sound much more exotic and are not easily recognisable, I'd suggest you infact have the stock unequal length units and a swap to the equals would go a long way towards your goal.
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