Running without back-pressure?

Discussion in '2-Stroke Engines' started by cmb271, May 16, 2016.

  1. cmb271

    cmb271 Member

    Okay, I haven't posted much in a while because after loosing the key that holds a sprocket onto the 7G transmission of my 49cc 4 stroke and not having the means to replace the transmission I lost interest in investing time into my motorized bicycle. Now that my car is currently sitting in my yard after being hit by a pick-up truck from the side I've decided to look back into the motorized bicycle way of life.

    Instead of going with the 4 stroke again I've decided to just buy a 2 stroke kit and a new bicycle since the old one has seen better days. I haven't order either yet as I'm waiting for my paycheck on Wednesday so until then I'm just in planning mode on what I'm going to do. I'm going to try and do it right this time, being given a clean slab allows me to look at what made my previous motorized bicycle builds fail and how to avoid them.

    I still have the parts to my old 2 stroke build and among the parts is my old straight pipe exhaust. The muffler portion of it broke at the weld leaving me with a straight pipe. I enjoyed the noise it made and was wondering on the new build if it'll cause any damage to the engine if I ran it as a straight pipe exhaust compared to using the default muffler that will come with the kit.

  2. butre

    butre Well-Known Member

    if you're not going to use an expansion chamber then back pressure is unimportant. the only thing no muffler will hurt is your ears, and maybe your wallet if a cop is having a bad day and wants to ticket you for it.
  3. jaguar

    jaguar Well-Known Member

    there is no power advantage to using a straight pipe without muffler so just use the new one.
  4. cmb271

    cmb271 Member

    I'm honestly not worried about performance enhancement I'm more worried about reliability, I don't know what the pros and cons of running a 2 stroke without a muffler.
  5. butre

    butre Well-Known Member

    again the only thing you're guaranteed to hurt is your ears.
  6. Frankfort MB's

    Frankfort MB's Well-Known Member

    Just take the cap off the stock exhaust and there you go! A race bike!! Garrenteed 20HP gain!!

    Nah just noise;)
  7. HeadSmess

    HeadSmess Well-Known Member

    noise noise noise noise and more noise@extremely loud levels dot com

    its just about bearable if you dont have the outlet pointing in your face or straight at the ground.

    drill a hell of a lot of holes in the last four inches, wrap it in fibreglass insulation, and wire a drink can with ends removed around fibreglass... mmm, dodgy swiss muffler :)
  8. KCvale

    KCvale Motorized Bicycle Vendor

    Well, 2-strokes have no valves. They do intake and exhaust at the same time.
    No exhaust pipe, or a strait open one, simply robs you of power and makes a really big mess.

    All the unburnt fuel/oil mix goes out the tailpipe.

    45u likes this.
  9. 45u

    45u Active Member

    This the truth and nothing but the truth. Pipes have a LOT TO DO with how well a 2 cycle runs.
    Last edited: May 19, 2016
  10. HeadSmess

    HeadSmess Well-Known Member

    agreed agreed, if you want PERFORMANCE from a two stroke, you get a well designed tuned pipe/expansion chamber. its simply a supercharger for 2 strokes.

    if you only want to reduce noise, you stick on the stock standard exhaust as equipped.

    if you couldnt care less and just want to get home, or around for a few days until parts arrive... remember that virtually no 2 stroke outboard engine runs a muffler...AT ALL! they actually run with negative back pressure, as the propwash is "sucking" on the standard exhaust outlet, ie, the middle of the prop.

    no engine will be damaged by running with no exhaust system (look at the old aircraft...they had little 6 inch pipes only to clear the fuselage) AS LONG AS you tune the carb to suit. they just maynt produce all the power they can....
  11. KCvale

    KCvale Motorized Bicycle Vendor

    I like to refer to them as a Reverse Super Charger, but the same meaning ;-}

    Yep. and maybe a short muffler on the end of that.

    Yep, you need to adjust the carb to let more fuel go right through and out the exhaust pipe.
    It the case of outboard motors, they just dump that unburnt gas/oil mix right into the water.
    Back in your time reference days gas had Lead in it too, yummy in the water supply, think Flint.

    Of course you can get the best of both worlds...
    An expansion chamber 'reverse super charger' AND an excellent muffler on the back end like this.


    A tuned X pipe with an auto Glass Pack welded on the end.

    Man I miss that bike, it was one of my first 1/2 dozen builds and my personal ride for more than year.
    Fast (3-speed 66cc shifter), Quite (X-pipe with muffler) and Comfortable, (Front and Seat Post shocks).

    Sorry, I digress, just a fond memory moment, I built that in 2010.

    I am on the 4-stroke band wagon these days, they have valves so inherently more efficient and quieter to begin with, and much easier to quite down on the exhaust side without a performance loss on these small engines, which quite frankly, isn't much to start with so you take advantage of everything they have and that you can enhance to fit your need.

    On a 2-stroke, X-pipes rule, watch any motocross event, not a SINGE BIKE runs a straight pipe.
    45u likes this.
  12. 45u

    45u Active Member

    This is the pipe I will be running on build 2. Good chance I will have to put a silencer on it.

  13. butre

    butre Well-Known Member

    the loudness on those pipes isn't too bad, you may not need a silencer. especially since you're used to working with harleys
  14. 45u

    45u Active Member

    LOL My Harley for 116,000 plus miles has had good mufflers as I like to hear my stereo, CB and if I have a passenger the intercom. LOL but yes I do work on ones that are loud but not like years ago as with out any back pressure on Harley s they will not perform well on the street only WOT. I plain on trying it before I do anything to it.