49cc HS exhaust length too short??

Discussion in '4-Stroke Engines' started by Aaron Brown, Dec 29, 2016.

  1. Aaron Brown

    Aaron Brown New Member

    I have a poo poo pipe on with no expansion or muffler part. So the straight pipe is about 18" total and tucked nicely.

    Too short? Do I absolutely need at least 30"? All power ranges seem okay but maybe longer would help? Thanks!
     

    <
  2. Old Bob

    Old Bob Member

    Too short for the limited rpm MB engines run.There are two design considerations. 1 is the speed of sound, it relates to the pressure pulse traveling up and down the pipe.2 is the speed of the exhaust gas.The pipe needs to be long enough to prevent atmospheric pressure from flooding pipe before exhaust valve shuts, and the length long enough for pulse to assist intake during cam overlap period.pipe diameter needs consideration also.
    Any of David Vizards books or Phillip H Smith book will help with the formula.
     
  3. jaguar

    jaguar Well-Known Member

    30" is more than enough.
    anyway the effect of an immediate change in cross sectional area from header size to open air does very little in the way of making a healthy long suction pulse.
    also on piston port intakes (no reed valve) the suction pulse sucks onto the closed system of the crankcase and so does little good. With piston port intakes the only thing of any real benefit is the return baffle wave (a pressure wave) from a normal expansion chamber which increases the dynamic compression and reduces loss of intake charge out the exhaust port..
    Please click onto my signature link to read about expansion chambers.
     
  4. Old Bob

    Old Bob Member

    This is a four stroke conversion, take your nonsense some where else.
     
  5. jaguar

    jaguar Well-Known Member

    same applies to 4 strokes. the rpm range of effect of an abruptly ending pipe is too small.
    4 stroke manufacturers don't design the pipe lengths to be "tuned" to the engine.
    racing 4 strokes do try to take advantage of that effect by tuning the length so the reverse suction pulse arrives back at the exhaust valve just before it closes at the engines top rpm.
    but a high rpm racing 4 stroke is a far cry from these sh*t engines made for bicycles.
     
  6. jaguar

    jaguar Well-Known Member

    from the net:
    "it would not provide as much of a boost as a tuned pipe on a two stroke but done properly it will increase the rpm 200-400 on the top end."
     
  7. Old Bob

    Old Bob Member

    Go educate yourself on the subject matter, headers do add power if made to the proper length.
     
  8. jaguar

    jaguar Well-Known Member

    OK then educate me since you think you know more than me.
    Tell me something I don't already know.
     
  9. Old Bob

    Old Bob Member

    Go buy the books I mentioned above.

    "from the net:
    "it would not provide as much of a boost as a tuned pipe on a two stroke but done properly it will increase the rpm 200-400 on the top end."

    I forgot they can't put it on the net if its not true.:rolleyes:
     
  10. jaguar

    jaguar Well-Known Member

    you know Bob, I don't mind people showing me I don't know everything but what I do mind is people being sarcastic and insulting without any good reason.

    these forums exist for the knowledgable to help the not-so. No ego or adversarial attitude is necessary.
     
  11. jaguar

    jaguar Well-Known Member

    only if there is "overlap" of the durations of exhaust and intake opening does the suction wave aid in engine power (by improving delivery ratio).
    I highly doubt his poo-poo engine has any overlap.
     
  12. Steve Best

    Steve Best Well-Known Member

    Good advice Old Bob.
    I used to have several of Philip H Smith's books that sadly I loaned and never came back.
    I built a 6 cyl header following his formula, for a Ford 250 six in a 62 Comet. Worked wonderful.
    Low 14 sec 6 cyl auto complete with a single barrel carb. A Friday night money maker.

    Exhaust tuning has a significant effect on 4 strokes, including length tuning.
    Cone collectors and Brooklands style mufflers have the same range widening effect as tuned 2 stroke pipes.
    Vizard didn't write anything about small gas engines did he? but Smith's stuff could apply to anything.

    The header on this car was built according to Smith's books:
    [​IMG]
    Ford Maverick with 351C 4bbl. Hand made equal length headers and out the back duals.
    Philip Smith's theories work!

    Steve
     
  13. jaguar

    jaguar Well-Known Member

    wow. a blanket endorsement without any care as to whether or not the exhaust and intake durations overlap. wow.
     
  14. Steve Best

    Steve Best Well-Known Member

    Absolutely a blanket endorsement of Philip Smith's books!
    A lot more "heady" and harder to read than Gordon Jennings.
    I suspect he might have been some of Jennings source material.

    I have "hot-rodded" most engines I have ever owned, including my lawn mowers and rototillers.
    A 12"-18" piece of straight pipe generally increased the torque of any of them.
    The rototiller got an even longer pipe (24"-30"???) which helped its torque considerably.
    No math theory went into this, I just kept adding iron pipe and couplers until it worked the best.
    With the pipe aimed at the ground no muffler was needed. Reasonably quiet (as a muffled Briggs&Stratton).

    The Briggs definitely has overlap, it is only the rare blower or turbo 4 stroke that doesn't.
    Port momentum (any pipe length +6") helps any 4 stroke engine with overlap.
    Longer lengths help to provide a suction as it opens, usually on a harmonic because we cannot make the pipe long enough for the low rpm of the Briggs.
     
  15. Old Bob

    Old Bob Member

    Vizards books give you intake and exhaust formulas that apply to any size 4 stroke.
    Jag, go back to the 2 smoke forum, leave us alone here you don't know what you are talking about.
     
    MotorBicycleRacing likes this.
  16. Steve Best

    Steve Best Well-Known Member

    Most of Vizard's books I read were vehicle or sport specific, so I don't recall pouring over formulas like I did with Smith. Vizard is an easier read, for sure.
     
  17. gary55

    gary55 Well-Known Member

    I don't know if this is the ideal setup. The head pipe is 7/8 ID and 15" long. The straight pipe is 1 7/16" id 28" long. I'm no authority on this, but i am guessing it works buy the exhaust gases creating a draw on the head tube when they enter the larger straight pipe aiding the intake during the valve overlap. I guess that the head pipe being less restrictive than stock would aid in the scavenging as well. Bikeberry pipe.
    [​IMG]
     
  18. Old Bob

    Old Bob Member

    Steve, David Vizards How To Build Horsepower 2nd Edition

    has formulas that apply to any engine ,although somewhat basic, do the job. The important part is he explains the principles.
    You can tune the header to improve power in any rpm range you desire, not just the top end.
     
  19. CrazyDan

    CrazyDan Active Member

    Think I read somewhere that the perfect length is 28.5" with a 7/8" id.
     
  20. Aaron Brown

    Aaron Brown New Member

    Thanks all, I'll educate myself with those books when I get more time.

    That kinda rings a bell Dan, thanks. Getting some 7/8ID flex sounds like a good buy for most builds.

    Until then, would hole drilling into the poo poo baffle (side?) improve flow and top end performance enough to matter? Or would that tend to defeat the purpose or make worse? Assuming it didn't look like swiss cheese
     
Loading...