Muffler Size = More Power?

Discussion in 'Performance Mods' started by gordonwv, May 11, 2012.

  1. gordonwv

    gordonwv New Member

    Smaller Muffler = More Power OR Taller Muffler = More Power?

  2. Fabian

    Fabian Well-Known Member

    If you want more power and are not concerned by any extra noise, an expansion chamber will be the best option for a 2-stroke engine.
  3. motorpsycho

    motorpsycho Active Member

    A muffler will sometimes add low end torque beleive it or not, because of backpressure.
    A muffler can also kill top end performace because of the restriction.
    an expansion chamber with a high flowing stinger type muffler or baffle is the way to go for more power.
    it will, however, make more noise that a stock exhaust.

    when it comes to mufflers, bigger is not always better. if you have a high horsepower engine, then it may require a bigger muffler so it can breathe. I'm talking HIGH horsepower here.
    on a stock h.t. 2 stroke, a bigger muffler won't get you anything.
    BUT, anything is better than the stock muffler because it's very restrictive.
    the stock muffler's baffle plate can be modified for better flow, which MIGHT give you a little more power, but it will increase the noise.
    but really, if you're looking to make some more power with a 2 stroke, the best bolt on power adder is a tuned expansion chamber.
    they will add a supercharged effect and will give you a nice power band (depending on how it's tuned).
    there's more to an expansion chamber than meets the eye, and they are more than just a louder than stock exhaust.
    I personally don't mind a loud exhaust, as long as it isn't obnoxiously loud to the point to where it sounds like there's no exhaust pipe at all.
    Last edited: May 11, 2012
  4. A bigger muffler will help to a certain extent. An engine is like a vacumm (I'm sure you've heard all this before) The more air you get through it, the better. Now, larger exhaust will help it breathe more, but some backpreasure IS needed. So you don't want a 3in dia exhaust tube, it's doing more harm than good. Now, less bends and curves will help alot, less restrictions. But you'll only get as much air out of an engine as you can get in. So now you have to have an intake with less restrictions, or maybe a scoop of some sort, or low restriction air filter. But when you add more air, you'll have to adjust the fuel/ air mixture. I don't know if all this will effect a 50cc motor or not, but I have a homemade super charger on my 150cc 4 stroke and a 1.5 dia exhaust and it goes when it's turned on! Now, I did adjust the fuel a little to compensate for the extra air, so when it's off it runs a little rich. So yes, a larger exhaust will help, but too large will hurt. However backpressure in 2 strokes are more important than in 4 strokes
  5. jaguar

    jaguar Well-Known Member

    When it comes to a straight pipe & muffler there are three things to consider:
    1. the length of the straight pipe. its length determines when the negative pressure wave comes back to the engine. the right length is in tune with the engine. somewhere around 16" is right for these engines.
    2. the restrictiveness of the muffler. more restriction lessens the noise. it would have to be heavily restricted to cause more backpressure than the stinger diameter.
    3. outlet (stinger) inner diameter. The stinger acts as a bleed off valve for the pipe. It controls the amount of average pressure in the pipe, which a certain amount is needed to counter the loss of intake charge out of the cylinder and into the pipe. My experiments show that a certain stinger restriction is needed for peak power at peak rpms, and that a little bit more restriction aids low rpm power and detracts from upper rpm power. For a muffled straight exhaust the only thing countering the loss of intake charge out the exhaust port is the back pressure caused by the stinger. An ideal stinger would be one that is more restrictive below that 14% point. Something like an electronically controlled valve at the end of the baffle would work. Engine designers make the stinger diameter and length ideal for the intended rpm range of maximum use. So a motocross bike would have a less restrictive stinger/silencer than an enduro bike. The correct diameter is from 58% of the correctly calculated header diameter (not the presently existing diameter) for torquey power, to 62% for high rpm power. Calculated correct header diameter = 2 x sqr root of ((.00085 x cc x rpm)/3.14). A 66cc engine running at 5500 rpm max would need a 19.8mm inner diameter header. So 58% of the 19.8mm header diameter would be 11.5mm. After a few test runs you should check the underside of the piston to make sure it isn't black which would indicate oil burning from too high a piston temperature from too much pipe backpressure. If so then double check the jetting making sure it isn't too lean and then make the stinger a larger diameter if need be.

    The stock exhaust has a 20mm header diameter and 12mm stinger diameter. That is correct for the 66cc engine but not for the 48cc. It needs a 17mm header and 10mm stinger.
  6. dodge dude94

    dodge dude94 Member

    Where is the power band of a short expansion chamber exhaust?
    I would imagine it would be up high, right?
  7. jaguar

    jaguar Well-Known Member

    yes, the shorter the distance from piston to beginning of baffle cone, the more it is in tune with higher rpm.
    Why? Because the higher the rpm, the shorter the time from exhaust opening to transfer closing. the piston is traveling faster.
    The time from exhaust opening to the return of the wave bouncing back from the baffle cone remains the same all through the whole rev range if the exhaust temperature stays the same.
    Those of you with short headers where most of the pipe is in front of the frame aren·t getting any benefit from the return baffle wave. You would have even more torque if you just cut that section off, clamped a coke can (open ended with holes in outlet side) on that has some of those metal kitchen scrubbers in it to deaden sound.
  8. dodge dude94

    dodge dude94 Member

    So, if I had a short expansion chamber, I am getting more HP in the high RPM region, but I am keeping the same low end I had? Or is that decreasing?
  9. jaguar

    jaguar Well-Known Member

    Too short a distance and you lose all advantage of the baffles return wave and are left with only the disadvantage of it. It only exists to boost the cylinder pressure and push back lost fuel mixture into the cylinder at the last top 1000 rpm. At lower than that top 1000 rpm it is a disadvantage because it is contrary to the suction wave from the diffuser. The first expansion chambers were only header and diffuser but they were louder than s-it and were mostly used for racing.
    Today I analyzed the baffle angle of the SBP and JNM pipe and both had an angle (from center) of 12 degrees. That is a steep angle only useful for racing. Even motocross pipes only have a 10 degree angle. All you people with a pocketbike pipe (they all are) need to cut the baffle in half and weld in an extender of 4". You will report back raving about the result, I guarantee it. The low end power will come alive.
  10. dodge dude94

    dodge dude94 Member

  11. jaguar

    jaguar Well-Known Member

    Not quite right. The pipe length has to be "in tune" with the engine porting at peak rpm. Shorter than the right tuned length and you have lost all benefit from the baffle cone. Longer than the right tuned length and you may have lost the ability to achieve the normal peak rpm but gain more power at the lower rpm range at which the pipe is now tuned to. Either way you lose if you are too **** lazy to experiment to find the right length. I already did a lot of testing and have suggestions on my site for lengths to start out with and fine tune it from there. click on my signature link. Those ultra long pipes with mufflers are of some benefit to low down power but contrary to top end speed.