How quiet can you make a 2 stroke?

Discussion in 'Thumpers' started by svejkovat, Apr 15, 2010.

  1. svejkovat

    svejkovat Been here awhile

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    Performance is absolutely not an issue. A one cylinder water cooled KX80 Kawasaki dirt bike motor being used in a fun scooter project. The 2 stroke it's replacing is rated at 2.5hp. The KX80 weighs the same (actually a few oz less!) and is rated at 16hp.

    I'd be very happy with 8hp if I could make it stealth quiet. Seems like it would be possible, especially being water cooled, with a standard (non expansion) enduro muffler and a little more attention paid to silencing the intake. Would I have to do any retuning to the engine?

    Anyone ever tried to calm a two stroke beast? Just how quiet can you get these things? I had a Yamaha 80cc "Mini Trail" when I was a kid. That's how quiet I'd like this to be. Obviously I want more than 2.5 hp out of it or it's not worth the effort. An enduro pipe would probably spread out the power band a little bit as well, no?
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  2. KLXplode

    KLXplode Desert Hunter

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    Step one: Take off muffler
    Step 2: Sell twostroke
    Step 3: Buy a fourstroke:evil :deal
    #2
  3. svejkovat

    svejkovat Been here awhile

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    ????????

    I googled it as well. No one is really interested in losing any performance for the sake of noise reduction, so 100 percent of online info focuses on tweaking the silencer, or wrapping the expansion chamber with insulation in order to get a db or two below max limits for sanctioned events. Helps me none at all.

    I guess I'll have to just start experimenting. I hate to do it blind. If I didn't have this particular motor I would not even think to do this, so selling it and buying a four stroke is not an option. It's a '96 model, and selling it to get a watercooled 80cc four stroke would put me 500 dollars in the wrong direction. In fact, an 80cc fourstroke racing bike is still going to be waaaaaaay too loud for my aims and I'd be in the same spot of trying to repipe it.

    Anyone know of a more suitable forum where I might pose this question?
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  4. KLXplode

    KLXplode Desert Hunter

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    Leave it on here for a few days you will get some responses. It just takes time, like you said mess around with different silencers, mufflers, and insulation. :beer . Keep looking.
    #4
  5. Fubars

    Fubars What would Scoobydo?

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    You could try a Kawasaki KDX200 stock muffler. They're kinda heavy, though.
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  6. HeadTrauma

    HeadTrauma ¯\_(ツ)_/¯

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    If you're not adverse to doing some exhaust fabrication, a non-expansion exhaust from a engine roughly as quiet as you're looking for would probably work. Potential sources could be outboard boat engines, lawnmowers, etc. You could even use multiple mufflers from a smaller quiet engine to get adequate flow. Yes, you would probably have to address the engine's jetting.

    That wouldn't help at all. Four-strokes seem to have more energy in the low-frequency ranges in the exhaust that are harder to muffle and carry further....hence the term "thumper."
    #6
  7. Flatulator

    Flatulator Banned

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    If this scooter has plenty of room for a big airbox and big pipe ... you could make it VERY quiet ... without even losing much performance at all

    ... even more so if the pipe and airbox could get hidden inside a plastic compartment ... (don't know what scoot you're talking about)

    example ? ... the KDX200 ... a very quiet bike that is 2.5 times bigger displacement .. so you could make quieter than that with out much trouble

    ... don't worry about starting from scratch ... since virtualluy nobody does what you're trying to do ... your path is unknown (to you)

    in steps >

    cut/rotate/fit/weld the stock KX80 pipe to fit on the bike fairly nice .. dont make the overall volume/length much smaller than stock

    adapt the largest (stock)OEM muff from a 2 stroke that you can find cheap ... (KTM200/250/300 EXC? - all quiet bikes)
    - stock Banshee muffs are quiet too

    consider doing a 180 'return' for the muff ... (so it lays over or under the pipe - with another .. mmm .. 180 (or so) ... for the final exit

    If you want to keep as much of the stock power as possible ... keep the final exhaust exit the same ID as the stock KX80 had ... don't go smaller

    Do an image search for aftermarket moped/scooter pipes to see what I'm talking about (with the 180 returns).

    airbox ? ... make it as big as possible ... do some multiple internal baffling's to prevent 'straight shot' path of air(noise)

    Too much power and noise ? ... add a few inches of overall pipe length if you have room ... make a stop to keep the powervalve (it has one?) from opening

    The KDX200 used a double walled pipe for its first 20 inches to help kill noise

    It could be whisper quiet ... and haul ass
    #7
  8. LILBIT

    LILBIT Ride you must.

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    Ignorance. A two stroke is easier to make stealth. The sound waves do not carry as well as an opened up thumper. Any offroad race that does DB checks can attest.

    A smaller diameter longer muffler with a DB Snorkel. On the bike it will still seem noisy. The pipe(not muffler) and cylinder do that. From 50ft at a reasonable RPM things will be much different.

    I had a KTM 300 with a snorkel and good packing. Riding here at my house trail my wife would have to come outside to hear me unless i was flogging it. Even at RPM it was not annoying.
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  9. ADVCoop

    ADVCoop Been here awhile

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    One reason two strokes can seem loud is the thin pipes. I have had some success with wrapping the pipe with header wrap. Sometimes you have to re-jet and it kind of looks like crap so I haven't done it in awhile. I like how the DBsnorkel works but I'm cheap. I have also welded elbows on the silencer end to direct the sound down, that helps some too.

    My KX125 is quieter stock than my nephews TTR125 with FMF PowerCore 4 exhaust on it that's for sure.
    #9
  10. KLXplode

    KLXplode Desert Hunter

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    I know i know, Im just saying what I would put in it. Im a fan of 4strokes. Im in a family with XR's and I used to have a yz250. I know plenty.
    #10
  11. TNC

    TNC Candyass Camper

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    At our state circuit enduro last month, I'm a check captain for two checks during the race. I got to see about 300 dirt bikes come through our checks...twice. I was amazed at how quiet the KTM200's were. There were stupid quiet. The exhaust note was unbelievably low. I'm assuming these bikes had stock exhausts, because just about every KTM200 that came through was this quiet. I'd go find someone with one of these models and see how KTM made these bikes so quiet. I got see/hear everything from 80cc race bikes to 600+cc bikes, and these particular KTMs were absolutely the quietest.
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  12. plugeye

    plugeye unforgiven

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    please post your results,
    this is real interesting
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  13. gplassm

    gplassm Been here awhile

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    I feel that the two strokes are easier to quiet down than the four strokes. I have an '04 KDX 200, with a stock pipe and silencer, and a Db Snorkel attached to it. As some have mentioned, while sitting on the bike, the bike sounds almost just as loud as it does without the Snorkel, but from 100 ft away the device makes a big difference. I am not able to offer actual sound measurements, but I did ask my wife to stand on the porch and listen as I rode into the woods both with and without the Snorkel attached. She said that the Snorkel made a big difference in sound reduction.
    You have several main areas to look at: The silencer, the pipe itself (wrapping will help), intake (if you can find an airbox that has an intake snorkel, it will make a huge difference), and general vibration and noise containment for the entire engine bay. Stuufing foam into every crevis will help stop random vibrations and frequency resonances form metal parts. If you can "box in" the entire engine compartment with something like ceiling tiles, you will reduce noise emmision significantly. It may help to get the silencer exit as close to the ground as possible, and point it downward.
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  14. 28marcb

    28marcb Been here awhile

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    I can get my two stroke exhaust quiet enough that I can rev my engine with my ear a foot away from it. What becomes the culprit at that point is engine noise from the exhaust manifold area, exhaust springs vibrating and the expansion chamber...

    A db snorkel in combination with a ktm hockeystick or kdx 220/200 stock exhaust can will get your volume down to where you're probably happy, but the engine noise will be much more substantial at that point..The pipe wrap might be a starting point for that..
    #14
  15. svejkovat

    svejkovat Been here awhile

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    Ok, so far so good. From inauspicious beginnings comes a little wellspring of interest. At the risk of killing that interest, I'll explain the aim a little further.


    Here's the patient awaiting transplant..
    [​IMG]

    ... and both motors balanced on a board to compare size/weight...
    [​IMG]
    The KX80 is actually a few ounces lighter and a few mm smaller in all dimensions (the carb m'fold will have to be refabbed to angle away from the frame). The original Puch motor on the right is spec'd at 1.5 hp. The KX80 is spec'd at 18hp. I can afford to trim some performance if need be. Yes, this could gnash the wheel bearings, rip the spokes out, and leave me in the bushes. But I'm going to replace the swingarm and rear wheel with sturdier aluminum alloys from the next 50 to 80cc racing bike that I manage to find parted out for cheap. And just because i have some extra power under the hood doesn't mean I intend to use all of it.

    Much of the purpose of this whole exercise is just the challenge to see if I can do it without any obvious signs that it's a 'hack'. I'm going to paint the new motor black and maintain the original plastic "body panels" as much as possible. The tiny radiator is going to go behind the headllamp and the hoses will be the smallest useable diameter of highly flexible jacketed type I can find and be bundled in with the electrical harness. As a matter of fact if I manage to end up with a suitable non-expansion pipe it will be almost indiistinguishable from the stock Puch moped. Perfect.

    Are you still with me? I'm not crazy. There is a very good reason for this. I love spinning around on this moped. It's so unassuming and old school that I can get away with taking it almost everywhere a bicycle is permitted. Seriously. 99 out of a hundred people, including cops and ladies with babies in strollers, just smile at me and call it good. I can use this in many many more areas than a full fairing modern scooter and I wouldn't want to compromise that aspect of it. I also have a really nice R65 BMW for the real rides. But I find myself taking the Puch out almost as often just because I can go practially anywhere on it without bothering anyone... so the desire to make it whisper quiet.

    I really don't mind if anyone wants to say this is foolishness. This little Puch is such a perfect combination of bicycle, scooter, motorbike for streets, parks, recreation areas, and even minor trails, that I'd never part with it. It's an acquired taste, and I'm a bit stuck on it. If I absolutely must, I kill the engine and pedal, so really nowhere is off limits. I pedaled it out to the end of a lake Michigan state park pier last summer and two park rangers strolled by me without a word.

    But 1.5hp has serious limits. Most uphills are a battle. 23mph max and panting. And the Puch is a one speed. So it goes from clutch slipping "c'mon baby, you can do it!" to hair raising max rpm in the stretch of one city block. I went looking for little performance tweaks, like larger carbs and pipes and bore kits, and then saw this KX80 engine on craigslist for 150 bucks complete. I had a dream! I also have a complete shop and a tig welder, so I don't have to just dream.

    I'm planning on gearing the sprokets tall so that I only use first, second, and third, and never even see fourth, fifth, or sixth, and never exceed 40mph (did you see those front forks?:huh ) I just want easier acceleration and the abiility to keep up with city traffic on hills, and a nice low gear for putzing through back trails when I'd like to go exploring.

    Crazy? I don't know. Does it qualifiy as adventure riding? Absolutely! Some guys took a couple of Puchs identical to the one above the length and back of South America some time back. Personally, I would not have left my R65 at home, but I bet they had one hell of a great adventure!

    Anyone still care to offer some advice?
    #15
  16. tommyg

    tommyg Long timer

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    this could be cool, ill be watching.
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  17. Zecatfish

    Zecatfish XTique Rider

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    Someone use to make a rubber S shaped exhaust thingamajig. It would make a two stroke whisper quiet.
    I tried to google it up, but couldn't find it. :cry
    Maybe you can have better luck.
    It just clamped to the silencer and curved back under it and then one more out. It was similar in shape to the Expansion chamber.
    #17
  18. Fubars

    Fubars What would Scoobydo?

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    Got me wanting to drop a 250 in one of those.......

    No, you're not crazy. Carry on.
    #18
  19. svejkovat

    svejkovat Been here awhile

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    That's the snorkel that was mentioned earlier.
    (Mod delete of bad link)

    It's an interesting concept if you read on the particulars. Emitting a first pulse and then a secondary note following the bends. I think it's a bit of comprimise though for the sake of flexibility. You can use it with your stock, or aftermarket, silencer. And you can just remove it on the trail/track if you want to let'er rip. Since I'm doing something permanent, I'd like to go full on 'stealth'.

    Most bike silencers I've seen are a straight shot of perf pipe through packing. I wonder why not just fab a little bigger silencer and put an S perf pipe of the same ID through it surrounded by a tight packing? Who knows, maybe the KTM200 that TNC mentioned uses something similar. Stuffing a couple of 180 degree returns, like flatulator (fittingly - he's just been waiting for this thread hasn't he?) suggested, inside of the silencer. Maybe even a corkscrew or helix perf tube?

    There's a KTM dealer two miles from my house. I'll stop by tomorrow.
    #19
  20. Krabill

    Krabill Long timer

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    Definitely keep posting progress pictures. I want to see this thing come together.
    #20