Welcome to Netrider ... Connecting Riders!

Interested in talking motorbikes with a terrific community of riders?
Signup (it's quick and free) to join the discussions and access the full suite of tools and information that Netrider has to offer.

Exhaust system science/design

Discussion in 'Technical and Troubleshooting Torque' started by Farab, Aug 3, 2009.

  1. Does anyone here have any basic info on the science (design) behind exhaust systems and mufflers (cans)?
    Interested in learning about his. Thanks!

  2. there used to be a site that you could input where your peak power was in the rev range, tube diameter etc and it gave you the ideal header length for either power of toque.

    I think it disappeared a while ago.

    I'd be interested in the maths and theory behind it if anyone knows.
  3. Depends on a lot of things, such as engine layout, firing order, what RPM you want peak power at, restrictions such as mufflers, etc.

    Say you have an inline four. You want to tune the manifold so that all the exhaust pulses arrive at roughly the same point in a neat order. This will depend hugely on the firing order of your system, as the distance from each cylinder to the collector will be different.

    As each "pulse" travels down the exhaust it will create a low pressure zone behind it that, tuned carefully, you can use to help scavenge the next cylinder like its pulling the exhaust out. Of course, the gap between pulses changes with rpm, so there will a specific RPM at which it is operating the most effeciently.

    Fun part. Resonances related to internal volume of the exhaust, volume of the pulses (whcich changes with rpm/fuel load) and materials/weight of the exhaust all combine to affect that rpm, and sometimes can give you multiple points, or combine to give you a horribly destructive system.

    Now lets say you have a V4, now you have two options. A loooong tuned length 4-1 type setup, or dual 2-1 exhausts. The 2-1 would have less resistance, at the expense of scavenging power and added weight. Same issues with a V8.

    Throw a turbo into the mix and it becomes even more complicated.'

    That's a basic, basic intro to it all as far as I understand it. The more complex science well.....aerodynamics etc is a multi billion dollar, Degree, masters, etc type field.
  4. Then theres the economic side.

    If you are not going for outright power so much as emissions and noise, then the intake comes into play as well. You can bleed intake air pass the engine to purify the exhaust, have multiple catalytic convertors with built in electric heaters to get them up to operating temp quickly. Resonator chambers on the intake to prevent destructive resonance along the intake path. Glass fiber packed mufflers or variable length exhaust headers (ie, flaps open at different rpm effectively changing the exhaust length/restriction. Multiple power peaks).

    Then throw in a dirty, dirty fuel like diesel and it all steps up another notch.
  5. Scientific design of exhaust and intake systems.
    by Philip H. Smith C.Eng., M.I.Mech.E., M.S.A.E.
    in collaboration with John C. Morrison B.Sc., Ph.D., M.I.Mech.E.

    It gives a thorough and lucid account of the physical principles governing the behaviour of gases as they pass through the engine and the theories and practical research methods used in evolving more effecient induction manifolds and exhaust systems both with and without silencers.

    In short it tells you, theoretically, how to build a performance exhaust. In practice:
    1.) it's difficult to get anyone to build the thing properly...and I mean properly (get the correct tube size, mandrel bends, exact primary lengths for each port etc etc)
    2.) once you've built it won't give you the maximum power or performance either. You still have to put it on an engine dyno, test it, modify it, change it, remake it to suit the unique characteristics of your engine (port shape, flow, can lift duration, bore size etc etc etc .....................and then you'll find that you haven't gained that much over some stupid exhaust sytem you buy off the shelf.

    I know a factory backed guy that built 10 different exhaust systems. From stock he gained about 10 HP, improved torque over a wider range smoother throttle reponse etc etc From an off the shelf exhaust he gained 5 HP.

    Sure it was nice but he spent approx 10 grand. :LOL: :LOL: :LOL: