Intake mod

Discussion in '2-Stroke Engines' started by hurricane, Oct 14, 2010.

  1. hurricane

    hurricane Member

    Ok so i did not want to spend the cash to order one since I can make one myself. It took me about 3 hours to build 3 of them since I figured i had all my equipment set up I might as well make one for each bike.

    Well after making them and adding one to 2 different bikes , one is 80cc and the other is 49cc(respectively) I took both for a ride, man was I disappointed.:no:

    On my 80cc engine it made no difference at all, and on my grubee with the new style carb it leaked so much fuel from the bowl that I had to remove the short intake and readd the old one in.( I think if my carb was level this would not have happened )

    So after all this work and effort Ive come to the conclusion that short straight intake tubes are not worth using , Time to experiment with longer ones and see if that will make any noticeable improvements.

    Can anyone tell me if I did it wrong , or is longer intake tubes better ?

    Attached Files:

  2. motorpsycho

    motorpsycho Active Member

    on a car engine anyway, short intake runners (tubes) are good for low end torque.
    long intake runners (tubes) are more for top end power.
    I'm not sure how this all plays on with 2 strokes tho.
    my 80 c.c. 2 stroke has the stock intake manifold, but my 49 c.c. 2 stroke doesn't have an intake tube at all. the carb is bolted right to the cylinder.
    both of my engines run great.
  3. HeadSmess

    HeadSmess Well-Known Member

    you have to think of a two stroke engine as a nasty sounding musical instrument. a flute, possibly... (i cant play em, so i dont like em... :p )


    so the engine is tuned and balanced to run at a certain rpm. so you design your TUNED pipe for that specific rpm. then your intake length is also TUNED to the same rpm.

    are you sure you have the longer/shorter intakes round the right way psycho?

    cus ive always thought it was long pipes for low rpm and short for high rpm.

    the idea is that when the engine is at the desired rpm, these pipes become resonant.

    standing waves. pressure waves, rarefaction and compression.... so as the ports open, a pulse is created. travels up the pipe, reverts, and reaches the port again, just as the poirt opens. this means the air pressure is higher, and the air just wants to flow into the now open port. this means more air per "gulp"

    at the wrong rpm though...the pulse has already hit the port(or hasnt even gotten there yet...), and is travelling back up the pipe again. so when the port does open, the air suddenly has to try and reverse direction to get in there... so theres less air.

    the length and diameter of pipe also affects factors such as laminar flow, fluid friction, turbulence... once again why i thought its the short pipes for high rpm... less pipe, less friction, more flow...

    ummmm, hurricane, these intake manifolds of yours... my only suggestion is...port matching? im fairly sure the engine side is squarish...not perfectly round. also the angle. i know the standard pipe is pretty rough, but it does match the angle of the intake port...

    now... a piston ported twostroke prefers long intakes.basically the induction port opens as piston rises, drawing air in... but at tdc, piston starts descending...expelling air. the only thing preventing this reversion is the momentum of the air already travelling in... a column of air in the intake manifold.

    once again...what reed valves are for.
    Last edited: Oct 14, 2010
  4. hurricane

    hurricane Member

    I just know they did not work for me , I copied the same idea as manic mechanic intake mods looked like
  5. motorpsycho

    motorpsycho Active Member

    well, this is how i think of it having a background in drag racing (talking v-8 engines here tho)
    look at a pro stock drag car's intake manifold. it is a tall. long runner tunnel ram intake, which is good for making high horsepower at high rpms. (4000, - 10,000 rpms) these are bad intake manifolds for street use because most street engines will never get to the rpms that these intakes are made to "work" in. one of these intakes n the street will not allow the engine to be driveable at 3000-4000 rpms.

    now, look at a standard hi-perf. street engine manifold. you will see short intake runners, that are curved in some cases. these intakes are designed for lower rpm use, (2000 - 7000 rpms) which is where the peak power is made, and where most street engines are rpm wise most of the time.
    I am just applying the same principle to a 2 stroke, but i may be wrong.

    now if you look at exhaust, that is the opposite (from what i know).
    if you run an engine with headers that have short length collectors, and no pipe or mufflers, you will get more top end power.
    if you want to increase your bottom end power, you make the collectors longer by adding lengths of pipe to them.(still with no mufflers).
    more back pressure in the exhaust will give you more bottom end torque. less back pressure will give you higher rpms, and more power in those higher rpms.
    Last edited: Oct 15, 2010