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Exhaust influence on performance Q

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by Marmitemonkey, Jan 21, 2008.

  1. Now I know that after market exhausts make a bike sound much cooler and sexy, but I've read a couple of threads in which it is mentioned that changing the exhaust can increase performance.

    Can someone explain how this is so please?

    Never really had an interest in how these things work until I got the new bike.

    Oh yeah, and simple-ish lingo would be cool if poss. I'm still learning to talk the torque. :)


  2. Less restrictive exhaust = air goes out faster = you can put more air in faster, so you can also add more fuel, and get a bigger bang = more power.

    edit: For the SV, the exhaust is about the sound. If the exhaust took away 2hp, I'd still put it on due to sounding so good.
  3. In simple terms, the engine must expend a certain amount of horsepower to push the spent exhaust gases out of its cylinders and through the exhaust system and muffler.

    If the exhaust system forces the gases to pass through lots of small holes or round sharp, right angled bends or to expand and contract several times, it will take more power to force the gases through than pushing them through a short, smooth straight pipe.

    There are also effects whereby pipe dimensions (particularly lengths of sections and volumes of muffler chambers) can be used to create a suction effect at the cylinder at certain engine speeds and assist the engine in expelling the gases. A system designed properly to take advantage of this will generally outperform a short, smooth, straight pipe (which is why the straight throughs on most Harleys are rather pointless).

    Trouble is, most of the restrictions mentioned a couple of paragraphs ago have a beneficial effect on muffling, and indeed, in combination, are how most mufflers actually work. Unless you've got unlimited volume and weight available, it's hard to produce an exhaust system that muffles well and gives good power. On a bike we are both weight and space restricted and so pipe design is always a compromise between noise and power.

    It's not always a bad compromise though. A mate has a seriously breathed on Harley Sportster. It's quieter than 95% of WA Harleys and yet will leave them for dead over the quarter mile :twisted: .
  4. It's only a couple of percent improvement.

    A staintune with the baffles intact will still feel more responsive than the average, restrictive, standard pong box. (Pong box really implies a 2 stroke expansion chamber, but I just wanted to use the expression tonite. So there.)

    In conjunction with a less restrictive airbox and some EFI mapping to compensate, you can have a noticeably, even excitingly more responsive engine.

    Do it.


    Trevor G

    PS PatB explained it well enough. Ask if you don't understand.

    The intake air flow is also just as important as the exhaust.
  5. I hate to go against what other people have said, but PatB has missed a few points.
    The suction effect mentioned, happens in the exhaust headers. Due to the extensive research done by 'decent' bike manufacturers, there is little room for improvement on the headers. The pipe itself is on no real concern, hence why exhausts are mounted in all sorts of different locations.
    I will agree that a less restrictive pipe will give more power, but so will just removing the pipe altogether (not the headers). If you decide to modify the pipe only, you can change the length all you want, but (i agree) less bends is better. The muffler itself is only there for two reasons, to reduce sound and emissions.
    If you do modify any exhaust component, you will need to modify the fuel/air mixture. This is more due to the fact that the intake needs to keep up with the exhaust in effect.
    The only difference from what I have said is for two-strokes, which rely on an expansion chamber to get their power. But all said and done, why would you try and modify a two-stroke?
  6. Thanks for the info.

    Once I can afford one I'll probably get a new exhaust.

    Just for the sexy sound. :grin: