Default 4 stroke muffler outlet too small? Whats inside?

Discussion in '4-Stroke Engines' started by go you good thing, Jul 15, 2008.

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  1. Hi,

    I have just received my Grubee Skyhawk 2 kit with the standard pipe and muffler for my 4 stroke.

    The muffler looks similar to the one in the 2 stroke kits even to the small exhaust outlet.

    The outlet looks alot smaller compared to the stock Honda GXH50 box.

    The stock inlet and outlet on the Honda muffler are about the same size giving it a balanced flow, I guess.

    Has anyone tried to make the outlet bigger in the kit pipe and if so does it improve performance?

    I'm not actually, at this stage, trying to improve performance beyond standard but trying to keep it as standard.

    Also their is no longer a screw on cap at the base of the kit muffler so I can't see what type of baffling is there. Is it like the 2 stroke inside? If so I would maybe have to take out some of the internals also.



  2. likearock111

    likearock111 Guest

    I think you've got the wrong forum, bud. You might have better luck in "4-Stroke Engines." Good luck!
  3. Yes

    OOps. You are spot on. It was late when I did this one so I will contact a mod to move it.


  4. 4 stroke aftermarket pipe

    Hello, yes it is wrong! I dunno if what they were smoking was not agreeing with them, OR if they cannot tell the diference between a 2-stroke and a 4-stroke?

    The exit (stinger?) of your 4-stroke should be at least as big as your exhaust valve, and that one certainly is NOT.

    I will be addressing this soon myself.

  5. inside

    Maybe they had their mouth over the end of the exhaust pipe drawing back the fumes for a buzz! and found the hole was better off being smaller.

    I was thinking that someone may have put one of those longer pipes (poo poo I think they call them) on and was able to cut up their stock pipe for research.

  6. Irish John

    Irish John Guest

    Someone here said to drill an adjacent hole 5/16" just above the pea-shooter outlet and so I did. It certainly improves the throttle response and the 4-stroke burble on my full length motorbike-type exhaust but there is no noticeable increase in power.
  7. Irish John, do you remember who said that they had drilled the hole?

    I would like to know whats inside the muffler as it may have the same setup as the two stroke and have similar small size holes inside.

    Making a bigger hole should surely improve power and speed.

    That is making me think that something else in there is stopping the flow.

  8. Irish John

    Irish John Guest

    I can't remember where I read it but it was on this forum. I tried it and I liked the result. Nicer noise, bit better throttle response and no extra power.
    I can tell you exactly what is inside the silencer - nothing at all. My long silencer comes off and I can see down it and there's nothing there. The smaller one is the same otherwise we'd be able to undo the end. I don't think you can improve the power and speed of a Honda 4-stroke cos it's pretty highly tuned to start with. It's too small to start making a bigger piston head and making high lift cams etc. Too hard and not worth the effort. It is not as easy to soop up like a HT engine and I'll bet if you could do it as a special project for free at a Technical College then you could probably increase the revs to about 10,000 rpm but the engine would break cos it's not designed for those limits.
  9. bubbatgs

    bubbatgs Member

    Muffler disection!

    I have cut into my stock pipe!!! I will post some photos soon. for the explaination!
    The back 5 inches of the pipe is made to be a sparkarrester! There is a 1" input pipe with eight 5/16s holes drilled into it. (not deburred) The exhaust then traveles through the open area into the tailpipe.(the one you can see at the end of the muffler) This pipe has 6 5/16s holes drilled into it . this is how the manufacturer has created this!
    I purchased the long delux pipe. as soon as I seen the inside of the muffler. I took a long 3/8s drill bit and drilled right through the plate in the end of the out put. I added a little bit of bottom end to the preformance. I don't believe I lost anything on the top end either! The sound only increased by about 7db.:jester:
    I will post photos as soon as I can take some.
  10. Old Bob

    Old Bob Member

    It depends on which Honda. The GXH50 is rated for 7800 from the factory, its no wise to increase rpm above this, how ever power gains can be had in the lower rpm range.
    The GX35 and GX31 are fairly easy to hotrod, I've been making cams for both since for many years, increasing compresion is fairly easy with either engine, depending on the cam you boost power anywhere in the rpm range and especially in the 12,000 -13,000 rpm range.
  11. bubbatgs

    bubbatgs Member

    The inside of the muffler!

    Here are a couple of pictures of the (stock) muffler that I, cut in two!

    Attached Files:

  12. GearNut

    GearNut Active Member

    Looks like a simple muffler design with no concern towards performance. Only designed for db reduction. Not too suprising either......
  13. loquin

    loquin Active Member

    It's really a hybrid three-stage silencer.
    • Stage one is the 6 holes in the side of the inlet pipe in the open portion of the muffler. This is a side resonant silencer, and is designed to absorb sound at a specific frequency.
    • Stage two is the open chamber portion, and is a type of diffusing silencer. It's designed to only pass those frequencies which resonate in the chamber, and is a type of band pass filter. These two stages are combined into one open chamber, rather than being separate, so, this is what makes this a hybrid silencer. It's probably less effective than adding a separate, divided chamber, though. (The 'step' in cross sectional area of the chamber where the inlet pipe ends acts as a type of wall, however, and reflects some of the sound energy back, and acts like a type of wall...)
    • Stage 3 is the portion with the holes, and it's not a spark arrester. This is a side resonant silencer stage, and it is intended to absorb sound at a specific frequency, such as the such as at the fundamental exhaust pulse frequency of the engine. As such, it is a type of notch filter. Now, the total area of the 8 holes is apx. 0.61 square inches, and the area of the 1 inch pipe that they're in is apx. 0.79 square inches. As the total area of the side holes gets closer to, and then exceeds the area of the pipe that they're in, it's performance shifts away from the side resonant silencer towards a diffusing silencer.

    Each 3 dB represents a doubling of the sound energy, so your 7 db increase is more than 4 times the sound energy.

    The fact that the holes aren't deburred is usually intentional. The sharp edges/protrusions add turbulence, and act to shift some of the sound energy up in frequency. (you'll get some 'whistling' as the exhaust pulses move past the sharp edges) But, higher frequency sound is more easily reduced by low-pass filters, so, these burrs make the later stages a little more effective at attenuating the sound.
    Last edited: Nov 24, 2009
  14. Old Bob

    Old Bob Member

    pst wanna know a secret, the best muffler for both noise reduction and perfomance, its the Briggs round tube 4 hp muffler, weld it on the end of a 17-22" long header.

    Its quieter than a "gutted" can muffler and flows very well.They are cheap too, I pay about 2 bucks for them.
  15. professor

    professor Active Member

    Good info here.
    Loquin, I'm impressed with your specific knowledge!
    And Bob- thanks for the reminder of how good those little muffs are.
  16. ibdennyak

    ibdennyak Guest

    Loquin, thanks for the specific info. I've been trying to design a muffler for a generator, and your knowledge just explained a lot of things.

    and Old Bob, that helps too. Gonna try that on one of my bikes. Ummm, just how old are you anyway?:jester:
  17. loquin

    loquin Active Member

    I bought a chapter of an engineering textbook online; the book deals with two-stroke engine design, The chapter I bought, pertains to silencing. It was a tough read (since ibdennyak: it's been many moons since I took any engineering classes, or calculus classes,) but I've summarized/condensed it to an article which I'll be posting, along with a program to calculate the silencing characteristics of various muffler configurations. (the original was published by the textbook author in GWbasic; I'm in the process of converting it to Visual Basic) Stay tuned.

    Old Bob: The bulk of the silencing from the extended pipe is because the length of pipe forms a tuned chamber, which will pass sounds with frequencies which resonate at the pipe length... When you play the trombone, and you lengthen the tube, the notes produced are lower. The same effect is in play with the relatively long chunk of pipe. Then, the muffler at the end silences these frequencies some, as well as further reducing the sounds not passed well by the pipe. As you accelerate, and the RPM's climb, you be getting peaks in the sound at some RPMs, then it will drop off, and an octave higher, the sound will peak again... The trick is in picking a pipe length which doesn't resonate at the speed where you want to cruise.
  18. ibdennyak

    ibdennyak Guest

    Hey thanks Lou, especially for the *condensing/summarizing* part. I did fool around with an article concerning 2 stroke ultralites in Scandinavian countries that are flat out banned from flying unless they are under a certain db level. I did have minor success at sound deadening, but the darn muffler ended up weighing almost as much as me. :ack2:
    Last edited by a moderator: Nov 26, 2009
  19. Old Bob

    Old Bob Member

    I'm well aware of that, I have a dog eared copy of Phillip Smiths in my library, the only book I don't have is Gordon Blairs 4 stroke
    book, I have everything else including Blairs 2 stroke software and excellent books on two strokes along with wave tuning software for engine development and a muffler program.

    With that said, the length of pipe silences only from the volume of the pipe not the length. The length of the pipe will effect the pulse wave activity and the inertia effects.
    You could silence an engine with an empty can large enough.
    You do actually want the pipe tuned to produce peak torque at cruise speed,IE resonanting, to gain max fuel economy, peak torque generally coincides with the lowest BSFC.
    There really is no trick in picking pipe lengths, they can be calculated conveinently for a starting point thats very close to optimum.
    The 4 hp briggs muggler reduces the sounds pressure level of the Honda exhaust note whether its mounted on the head or any length of pipe.
  20. loquin

    loquin Active Member

    Gotta disagree about the silencing not depending upon the length of the pipe/chamber, and only the volume. It's just not so.

    The harmonic frequencies of sound inside a three-dimensional object are definitely related to the length of the object, AND are related to the diameter of the pipe (and to the height and depth of a rectangular cross section)

    These are sounds with wavelengths 2 times the lengths, 1 times the length, 1/2 the length, 1/3 times the length ... 1/N times the length and diameter (or width, and height) of the chamber.

    In fact, in loudspeaker design, it's very important to avoid having the length of the sides be related, else the resonant frequencies can reinforce each other and contribute to bad sounding speakers. It's actually a good idea to have the internal length, width and height of the speaker enclosure be related by the golden ratio (apx. 1.618:1) to minimize reinforcement of resonant frequencies and their harmonics.