HT Exhaust Mod Feedback

Discussion in 'Performance Mods' started by AussieSteve, Nov 25, 2009.

  1. AussieSteve

    AussieSteve Active Member

    I've been reading through old threads on the usual HT muffler mods, (holes drilled in outlet pipe or pipe cut off altogether), but the results seem inconclusive.
    Some say that they had a dramatic improvement, others little or none, while others said there was initially no change, but following up by jetting richer improved performance.
    In all cases, of course, noise increases.

    Are these mods more likely to increase top or bottom end power, if any increase at all?

    If there is an improvement, is it worth cutting the outlet pipe short, then stuffing the tail of the muffler loosely with stainless steel pot scourers to reduce noise, or is this just likely to bring the restriction back up to where it was originally?

    Any thoughts on this subject?

    ... Steve


    DJEEPER Member

    maybe slightly, but not really worth it. just buy a tuned pipe. besides the slant head mod, the best money i have spent so far.
  3. AussieSteve

    AussieSteve Active Member

    I would, but the tuned pipes available are designed to increase top-end power. My overall aim is to improve bottom to mid-range performance a bit and top-end only very slightly. I really only want slightly less restriction and an easier to clean pipe at this point, without heaps of extra noise.

    Also, here in Australia, the more these bikes look and sound like motorbikes, the more attention the cops will pay. They are technically illegal.
    An expansion chamber is too motorbike-looking for my liking.

    The extra benefit is that I can vary the amount of stainless pot scrubber material to vary back-pressure and increase/decrease noise reduction.

    Sketch attached.
    Incidentally, how many baffle plates are in the stock muffler, if you know? 1 or 2?
    ... Steve


    Attached Files:


    DJEEPER Member

    not true. The SBP muffler is adjustable to get the pressure wave to hit certain resonance frequencies associated with the different RPM ranges in a desired powerband.

    you might wanna look into it :detective:
  5. AussieSteve

    AussieSteve Active Member

    Didn't know that. I stand corrected. I will check them out. If only I can make one 'stealthy'-looking and still get good bottom-end, I'm there.

    Incidentally, with the new billet head I got a good immediate improvement, about 2kph up on top speed and 1 kph down on comfortable bottom speed, but two 3.5mm holes 2" into the stinger from the cap made a world of difference. (One hole right through.)
    And not enough noise increase to be concerned about. I couldn't tell the difference.
    Comfortable slow speed has dropped from ~15kph to ~12kph and top speed has increased from 36kph to 44kph. Acceleration much better too.
    Next the heavier flywheel, then possibly a tuned SBP pipe. (Does heat-resistant paint come in 'invisible'?)
    ... Steve
  6. motorpsycho

    motorpsycho Active Member

    here's what i did...
    i have the spooky tooth expasion chamber which is not tunable like the sbp pipe.
    The spooky tooh chamber is pretty much just a straight pipe with no muffler, but it does make a nice improvement in low - mid-top end power over the stock pipe & muffler.
    The problem is that the spooky tooth pipe is a bit loud...louder than i thought it would be.
    I like loud, but after running it a few times like that, it got kind of annoying.
    so, I took the stock pipe, and cut the muffler end off. I took the muffler apart and got rid of the cateletic looking thing that's inside of it. (it looks like a honeycomb kind of thing) this leaves you with a plate, and a baffle tube with some holes drilled through it. I drilled a few holes in the plate to improve flow (this did not affect noise very much).
    Then, i cut the end of the muffler tube at a 45 degree angle. I re-installed the end cap (which has the plate and the baffle tube welded to it) but i pushed the end cap up inside the muffler tube and held it in place with 2 rivits. when you look at my muffler now, it looks like a big wide open straight pipe...but the baffle and end cap are hidden up inside of it. i also removed the small pipe that stuck out of the end of the muffler cap.
    I attached the muffler to the expansion chamber by drilling 3 holes and using rivits to hols it on. I sealed the joint between the expansion chamber and the pipe with some exhaust sealer (it's like silicone, but made for sealing exhaust systems).
    This increased my bottom end torque quite a bit and didn't seem to take away from the top end at all. it still has a better sound than the stock pipe & muffler, and it seems to flow a lot better. the spooky tooth pipe gives it a nice raspy sound, and the muffler quiets it down some. But i do like a motor that sounds healthy with some noise...and this definitly did the trick. it also looks cool too.
    I think it's more of a resonator now than a muffler, which is what i was looking for.
    I tried messing around with a lawnmower muffler and altho it seemed to do a good job, it didn't fit the look of my bike. A bare steel lawnmower muffler stuck on thr end of a nice chrome expansion chamber, just didn't look good to me. plus, the lawnmower muffler ended up in a spot to where it would spray a mist of oil all over my nice whitewall slick that i have on the back.
    unfortunalty, i did not take any pics of the end cap out of the muffler to show the plate and the tube with the holes in it.
    I got rid of this peice inside the stock muffler

    the end result of how it looks. yes, my crank chain rubs on the pipe a little, but i rarely peddle my bike so it's no big deal. the pipe also looks very close to the rear tire, but there is a good 1/2" gap between the pipe & the tire.
    Last edited: Nov 27, 2009
  7. AussieSteve

    AussieSteve Active Member

    Overall, you've increased your pipe length now, which probably accounts for the bottom-end torque improvement.
    Incidentally, using the gear ratio calculator for a 26" wheel diameter, the increase from 36kph to 44kph equates to an increase from 5250 RPM to 6420RPM. Not bad for such minor mods.
    I didn't know that corrugated-cardboard-like section was in there. Interesting. I thought there were just one or two baffle plates. Where would it be in my drawings above and how long is it? Is the piece in the pic all that there is of it?

    ... Steve.
  8. motorpsycho

    motorpsycho Active Member

    well i'm not sure if your pipe is EXACTLY like mine or not...but chances are that it is. That corrugated peice is at the top of the muffler (closest to the pipe that comes out of the engine). On my pipe, there was one screw through the side of the muffler body, which held that corrugated peice in there. look at my second pic, and you will see the screw right by the chain. I just put the screw back in with loc-tite to close and seal the hole. The corrugated peice is about 1 inch thick.
    the baffle in my pipe has one plate, and one baffle tube. there are about 8 holes cross drilled into the tube, and the plate is slightly smaller in diameter than the inside of the muffler case.
    here's how i got the muffler all apart...they are a PAIN to try and get apart if you don't do it this way (in my opinion).
    there should be 3 screws on the outside of the muffler body (2 hold the end cap / baffle in, and one holds the corrugated peice in.) remove all 3 screws, and the shake the pipe until you hear the currugated peice moving back & forth. once it's freed up and moving around inside the pipe, hold the pipe upside down (exhaust end pointed at the floor). now, move the pipe up & down so that the corrugated peice acts as a slide hammer, and it will "hammer" the baffle plate / end cap out of the pipe. move it up slow, and then move it down fast to get the corrugated peice to come down with some force against the baffle plate. it took me about 10 shakes like this to get the muffler all apart.
    I have no idea how used your pipe is, so it may have some build up inside of it which may take more effort than it took me. I only ran my stock pipe for about a month, and it had little to no build up inside of it, so it came apart pretty easy. the corrugated peice in the pic. is excatly how it looked when it came out of the pipe. there is very little oil residue / build up on it.

    I'm using a 20" bike with 20" wheels and my top end speed is approx 32 mph.
    i want to go to a 36 tooth sprocket to get a little more top end...and i know that i will loose bottom end by doing this.
    Last edited: Nov 27, 2009
  9. AussieSteve

    AussieSteve Active Member

    Sounds like mine is different to yours. Similar in appearance, except that there are no screws whatsoever on the outside, apart from the single one at the bottom to fasten the end cap.
    Mine appears to be welded together.

    20" wheels and 32mph. 44T sprocket? That's 9700RPM. (Or did you mean kph)

    ... Steve
  10. motorpsycho

    motorpsycho Active Member

    nope...mph. i have the 50 c.c. slant head engine, and it revs like crazy.
    but, it's not over revving (in my opinion) when i hit 32 mph. no, i was not going down hill with a tail wind, his was on flat
    when i hit 32mph, i let off the throttle..i didn't hold it there for a long time. the way i got the 32 mph reading was with a digital speedo that i had hooked up, and it might not be exactly 32 mph is an estimate. I do not fully trust the electronic speedometers. all i know is that i was hauling butt and the speedo was at 32 mph.
    all i have done to my engine is the expansion chamber and a high flow k&n style air filter. i did alot of tweaking to the stock carb, and a lot of playing around with gas/oil ratios. yeah, i have 20" rims, and my rear tire is a 20 x 2.25 slick.
    i can pass my friends 80 c.c. 24" schwinn and pull away from him when we ride together.
    the motor i have runs extremely well and it makes a ton of power for only being 50 c.c.'s.
    i doubt that it's hitting the 9700 rpms that you got on the calculator. my rear sprocket isn't a 44's a 41 tooth.
    i would not be surprised if my 32 mph speed was more like 25 -27 mph due to a possibly inaccurate speedometer. the speedo only worked for a day anyway.
    Last edited: Nov 27, 2009
  11. AussieSteve

    AussieSteve Active Member

    41T, that makes a big difference. Let me do my calc - 9050RPM at 32mph w/ 20" wheel and 41T rear sprocket. Still doing pretty well.
    Mind you, the 48cc engines rev much higher than the 66's, from what I've read. Also, on my 66cc, just the RSE head and 2 x 3.5mm holes on the inside of the tail-pipe/stinger added 1000 RPM to the previous best.
    These RPMs are only for test. There's no way any engine likes WOT on a regular basis. Best to cruise at about the pace where acceleration begins to drop off. (On mine, max 44kph, cruise at 35-37kph at most.)
    A couple of points on the electronic bike computer/speedos:-
    1. If they're set up well and not getting false extra pulses or missing some, they should be very accurate. On each turn of the wheel, the pulse tells the bike computer to add x cm to the total, for odometer. It then calculates speed by time taken between pulses. No reason why this shouldn't be accurate, assuming:-

    2. The sensor is placed close enough to the magnet so that the reed switch closes cleanly every time and:-

    3. The exact diameter of your wheel (under normal load) is entered into the computer. Don't just use the generic settings. They're way off, even on better (Cateye) speedos. Remember too that as tyre pressure varies, so will your speedo readout, so do the test again if you change your tyre pressure.

    Best is to place the valve closest to the ground and draw a chalk line on the ground.
    Next, sit on the bike in your usual position, full weight on the seat and roll forward one full turn of the wheel. Mark the spot. Measure and enter this into the computer as circumference, or divide by pi for diameter. (You'll be amazed at the difference between measured wheel circumferences and the charts supplied with most, if not all, bike computers. Incidentally, for those thinking of fitting one, avoid the cheapies - many hate motorised bikes and won't work. Also some are bad on bumps - the sensor shakes too hard and the reed switch opens and closes quickly, producing a higher speed readout. My 200W electric bike has one of these. I clocked 87kph a couple of weeks ago, against a headwind. (My 2-stroke bike has a Cateye speedo - works perfectly everywhere.)
    Wow, didn't mean to write a book.
    ... Steve
  12. happycheapskate

    happycheapskate Active Member

    I have a grubee 48cc skyhawk and I'm looking into a bolt on box muffler for it, weedeater or lawn mower style. I hope to find something with common facings and just bolt it onto the engine. I don't want pipes and all hanging off the bike. Any info, please pm or mohdoggies ya
  13. happycheapskate

    happycheapskate Active Member

    Thread for boxy muffler started here: