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Exhaust theories

Discussion in 'Members' Lounge' started by Kanned, Dec 15, 2007.

  1. Kanned

    Kanned Beast

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    Exhaust Theory

    "Again, you may hear a few SPMT's tell you that "Borla mufflers make horsepower!" Or "An engine needs some backpressure to run properly!" Nonsense. A muffler can no more "make" horsepower than Wile E. Coyote can catch roadrunners. Any technician with any dyno experience will tell you that the best mufflers are no mufflers at all!"

    It was a fairly interesting read, but I'm confused by the above quote. Are you telling me any and every mention of back-pressure's role was/is false? Maybe I'm just a noob :shrug:
     
  2. totalburnout

    totalburnout Well-Known Member VIP

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    Back pressure depends on the application.

    The way I've always been told is for a naturally aspirated application you want some back pressure, so you want to be sure you don't go with too huge of exhaust piping.

    With forced induction, I think the general consensus is that you want as free flowing exhaust as possible, to make the most power.
     
  3. nootrac22

    nootrac22 Well-Known Member VIP

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    it makes sence to me, but i am a noob
     
  4. Kanned

    Kanned Beast

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    I have pretty much been told the same thing myself, but having none at all while stock seems ridiculous. With no forced induction I have a 2.5"(I believe) straight pipe with one elbow and one resonator going back to a 4" tip and I think it feels like it's lacking back pressure.
     
  5. c97x

    c97x sorry boss, its turbo

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    well here's the thing, on n/a motors too big of piping will cause a drop in exhaust velocity, which hurts the scavaging effect. a good tuned exhuast sytem will be like a vaccum sucking air out of the combustion chamber during overlap period, thus allowing even more fresh air and gas into the chambers. but a turbo car since the turbine is in the way of the exhaust stream back pressure is doubled(i believe) so bassicly the fatter the exhuast the better to reduce backpressure.
     
  6. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    Backpressure is an oversimplification. You want the best laminar flow through your exhaust system with minimum energy loss and minimum friction- but you don't want to go too big that your exhaust is turbulent at low engine speed, and you don't want to be so small that your exhaust can't get through at high engine speed.

    Easy.
     
  7. Citizen_Insane

    Citizen_Insane Senior Member

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    :withstupid: when you study fluid mechanics you learn things like this :) (or should that be a :( ).

    I really don't care what some guy that never went to college and runs some garage tuning shop says, the mechanics of it dictate otherwise. The other thing to take into consideration is what kind of load and RPM the engine normally sees. If you want more mid range torque (like for an AutoX car), go a tiny bit smaller. If you want maximum peak power (like for drag racing) you want the largest pipe that will allow fully laminar flow so that you get lots of flow @ high RPM high load when the engine is producing the most exhaust.

    I'm with you on the no muffler thing tho. The fewer the bends and other restrictions, the more power. I still highly advocate for CATs, 2hp is worth it not to put all that crap into the atmosphere.
     
    Last edited: Dec 15, 2007
  8. INJEN78

    INJEN78 HS LEGEND

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    i was always told you need back pressure soo you dont burn up your valves...i dont know what to beleieve:confused:
     
  9. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    :werd: it's a :( and :werd: :werd: :werd:

    So I shall run variable diameter exhaust from the downpipe all the way back. ;)

    :p
     
  10. Dustin_m

    Dustin_m Active Member

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    Cal, what did you study in school?
     
  11. totalburnout

    totalburnout Well-Known Member VIP

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    He's Asian, so everything.;):ph34r:
     
  12. Dustin_m

    Dustin_m Active Member

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    lol
    I've always wondered as he seems like one of the few people on here that has some education in physics and stuff.
    I'm excited and scared as shit to take be taking those classes like fluid mechanics and stuff pretty soon.
     
  13. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    Mechanical Engineering with a heavy focus on thermodynamics and fluid dynamics.
     
  14. Airjockie

    Airjockie Watanabe Whore!!!

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    /waits for Celerity to mention Supertrapp here....
     
  15. K2e2vin

    K2e2vin Senior Member

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    That's what I'm going into!

    :lol:, Supertrapp. I remember seeing those for bikes, then they came out with ones for cars. How do those sound?

    You don't want any back pressure in the system, but you also don't want it too large as stated before it'll be turbulent and hinder scavenging effects.

    As for the turbo thing, I always imagined that you can still take advantage of correct size piping after the turbine.
     
  16. Kanned

    Kanned Beast

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    Oh, totally.
     
  17. E_SolSi

    E_SolSi Member of the 20 nut club Moderator VIP

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    fuckin engineers
     
  18. Kanned

    Kanned Beast

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    :werd:

    Thanks for all the feedback, it's exactly what I was looking for even if I can't understand it all. I'm not as confused anymore =).
     
  19. Celerity

    Celerity Well-Known Member

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    I can actually contribute something tech to a thread. That hasn't happened in at least 3 months.

    My CRX runs a "tuned" exhaust. I tuned it, and I tuned it like a pipe organ.

    The exhaust is mostly stock diameter pieces with smaller or larger pieces welded in wherever the oscilloscope told me to. At the end, I used the Supertrapp 4" unit, but I had to modify the pulse to occupy wavelengths no longer than 1/8 inch . I got down to about 1/4 inch, so I can run the trapp with either 6 or 12 discs with good result.

    6 discs takes me around town, 2000-4000 rpms, and the 12 takes me to autocross or highway flings at 4000 rpm to 6500 rpm.

    For driving impressions, you can ask Turbomirage - who somehow managed to blow the trapp clean off the car. IT moves like a turbo at those RPMs.
     
  20. TurboMirage

    TurboMirage YEEAAAHHH VIP

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    definately a BIG difference between the trapp and no trapp. the car was a fucking rocket WITH it AND a blown headgasket :)
     
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