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Custom exhaust for a WRX powered kit car

  1. Aug 13, 2012 #1
    Hello all,
    I am nearly finished with my AA, and next year I will start school at VRI at WWU. I cant wait!!!

    Until then I am working on a private project. A 1970's Kit car called a Sterling. Due to expense I cannot make this car a mid engine, so I am staying with a rear engine configuration, but replacing the stock 1600cc VW motor with a Turbocharged WRX engine.

    Trick is the exhaust will be extremely short, maybe 6 feet in total length.

    What is really bothering me about this exhaust isnt building it, but rather tuning the sound. WRX engines come in a unequal length header(low and rumbly) and an equal length header (higher pitch and raspy).

    Is there a way to adjust an exhaust note to be more of an exotic car tone or pitch? Isnt it merely adapting the frequencies of the sounds? Couldnt a muffler be made to absorb certain frequencies, and basically ignore others? If only I could make this car sound like a lamborghini, Id be in heaven.

    If a V12 has a given exhaust note due to internal moving parts, and frequency of exhaust firing pulses, couldnt a 4 cylinder exhaust be adapted to mimic this? Even if kind of closer?

    I do not want a fart box honda civic sound coming out of this car.

    Any thoughts or ideas?

    Just because it hasnt been done, doesnt mean it "cant be done"
     
    Last edited: Aug 13, 2012
  2. jcsd
  3. Aug 14, 2012 #2
    My thoughts on this are driven around a reflective muffler design, that stacks frequency. So that the output frequency is higher than the input via internal reflection.
    Amplitude of the sound wave would be lost in the reflection which is fine, I am after tuning the sound, not volume.
    If the design of the exhaust components would increase the frequency of the output sound, it would be equivalent to that of having more exhaust gas impulses. Depending on the design, this could double or even triple the frequency simulating a V8, or V12.
    Given the amplitude would be diminished (call it frictional loss from the reflection) the muffler would quiet the exhaust, while increasing its more exotic note.
    Most muffler technology is built around sound deadening, not frequencies. Reflective mufflers and resonators use this design to cancel out the frequencies to reduce noise. But properly adjusted, I feel the theory is sound in that the sound could be tuned in way not done before that Im aware of.
     
    Last edited: Aug 14, 2012
  4. Aug 14, 2012 #3
    The general idea is sound but you'll never get a boxer engine to sound like anything other than a boxer engine i'm afraid. It has a very characteristic (and cool, for a 4 pot) sound, and i'm not sure why you want to change it.

    Imagine this more as playing with an instrument, you can play around with the sounds it makes, and make it sound similar to other instruments in its 'class'. But you can't make it sound fundamentally different.


    The fundamental sound comes from firing overlap, which generally relates to what the crank looks like.

    So an I4 sounds similar to a flat plane V8, as they share a common crank type.
    An I6 has the same characteristics as a V12.

    As a boxer engine has a rather unique crank/piston layout, its produces a unique sound. Even though they both have 4 cylinders, an I4 and a flat 4 sound nothing alike.
     
  5. Aug 14, 2012 #4
    Too true. There is a huge difference between using equal length headers, and unequal length headers. Additional tweaks can be had by omitting the resonator.
    However, Im putting this in a car that looks like a cross between a Lamborghini miura and a Ford GT. So getting the exhaust note both original and exotic sounding is one of my priorities. Its a complete package setup. If it looks like a million bucks, but sound like a nickel, guess what? LOL
    Just like any cars design, if every part of the car sound, feels, and looks like a million buck, then the illusion of it being that is much more complete. The failure of most kit cars isnt their design, as much as its the builders attention to detail.

    I aware of the crank design. And why they sound like they do. But Im trying to find out "the possibilities". Maybe setup some experiments and test out various shapes and lengths to see what works. Short of installing a speaker under the car to make it sound better. LOL

    The drone from a WRX is an icinic sound, just like the sound from an old VW beetle. Its an instantly recognizable sound, or tone. I want to avoid that kind of recognizable sound, and make the exhaust ADD to the believability of the new redesign.
     
    Last edited: Aug 14, 2012
  6. Nov 13, 2013 #5
    I have a similar question. I'm fabricating headers for subarus and rather than trying to change what the engine sound comes out as, I would like to ensure that it does not have a "tinny" sound to it.

    I'm using .065" tubing for naturally aspirated applications. What could deepen the tone of the header? just a random thought but would welding some bar to the outside of it help deepen the tone? Thicker steel tubing?

    Any pointers would help.

    Question 2:

    The unequal length headers obviously have the rumble from the unequal sides.

    How would the rumble change as a function of delta length (difference in lengths of each side), as a sinusoidal function or otherwise?

    Any info would be greatly appreciated, very glad I found a physics perspective on automotive engineering

    Pete
     
  7. Nov 18, 2013 #6
    The turbo has removed the raspiness you seek.
     
  8. Jan 4, 2015 #7
    I had a corvair with large diameter exhaust . It sounded exotic and turned heads every where.
    5.0 mustangs used an "x" pipe or a similar design, "H" pipe.
    The people that build dune buggies use Subaru motors, they should be able to guide you.
     
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