Drag Bike Questions

Discussion in 'Performance Mods' started by theamusingnerd, Aug 9, 2014.

  1. theamusingnerd

    theamusingnerd New Member

    Hello! I am building a drag bike. So far, I have a Dax RT carb, I am putting a tuned pipe on it and a Puch 70cc head. Do you think a 50cc head would be better? My only concern is that in Phoenix, we can only get up to 91 octane gas. Also, what size rear sprocket should I go for? I can't decide between a 40, 39, and a 36. I need the best compromise between acceleration and speed.

  2. butre

    butre Well-Known Member

    You can get some avgas, 100 octane stuff is about $6 a gallon and contains no ethanol. It's leaded, but the lead content is lower than typical leaded street gas
  3. theamusingnerd

    theamusingnerd New Member

    Would you go with a 70cc or a 50cc head?
  4. butre

    butre Well-Known Member

    I'd stick with the 70cc until you're sure you've got the ignition timing and fuel octane right.
    theamusingnerd likes this.
  5. theamusingnerd

    theamusingnerd New Member

    Then what would you reccomend for the rear sprocket?
  6. butre

    butre Well-Known Member

    depends on what power the engine is making. I like using 40 tooth sprockets personally.
  7. HeadSmess

    HeadSmess Well-Known Member

    major lack of info!

    what engine are you going to run? primary redux ratios if it isnt a standard HT? what size tyres are you running?

    port work, what have you done, and what RPM have you tuned it for? is the pipe also tuned for the same rpm? are you geared to allow it to reach such an rpm?

    yarda yarda... shifting or single speed?

    wouldnt worry about compression ratios and octane if its only doing short runs, if it detonates...who cares? pistons are cheap.

    reed valve or piston ported? what size intake? does the carb match the intake? a bigger carb on the standard intake is still restricted by the intake.

    standard HT id go 39 or 40, if its on 26" wheels...
  8. theamusingnerd

    theamusingnerd New Member

    Standard HT engine. Port matched. Extended intake. It will have a bicycle motor works expansion chamber and a Puch 70cc hi hi compression head. Dax RT Carb. Single speed. Also, I am planning on driving too, as I will have to get it to and from the track and everything.
    Last edited: Aug 9, 2014
  9. jaguar

    jaguar Well-Known Member

    Maybe you should return to a short intake tract since the extended one focuses power at lower rpm instead of at high rpm like a race bike needs.
  10. theamusingnerd

    theamusingnerd New Member

    Although I need good acceleration as I am drag racing. I also need to clear the frame. If you think it would help a lot, I can make it work.
  11. jaguar

    jaguar Well-Known Member

    It all depends on your ability to slip the clutch just right so you can maintain high rpm from the start to the finish.
    I think you should shorten the intake for the best high rpm power.
  12. theamusingnerd

    theamusingnerd New Member

    OK. I will work on that. I need to get it shortened, but still have the carb fit my frame. Thanks!
  13. HeadSmess

    HeadSmess Well-Known Member

    port matching... do it yourself or just buy it and have no real idea if its actually done properly?

    cutting the piston skirt some to increase intake duration, which will affect the length of intake required...

    direct quote. i really do love mr jennings and his book that im not going to link to here...try google ;)

    "My friends at
    McCulloch, who acquainted me with the stub-exhaust testing technique, tell of tests they
    have performed with an engine having an intake period of 120-degrees, and with pipes
    ranging from 5- to 9 1⁄2-inches in length they were able to move the power peak anywhere
    from 3000 rpm to 8000 rpm. The 9 1⁄2-inch length gave substantially the same power at
    3000 rpm and 4000 rpm; at 5000 rpm, an 8 1⁄2-inch was best; at 6000 rpm and 7000 rpm
    the same power could be had with either a 9 1⁄2- or 6 1⁄2-inch length; and the same was true
    at 8000 rpm, but with lengths of 5 1⁄2and 8-inches. From these tests, it was determined
    that the best compromise length for the entire range 3000-8000 rpm was 8-inches; a 5 1⁄2-
    inch length was slightly better for the range 6000-8000 rpm. It should be understood that
    these lengths only apply to this specific example, and would vary with any changes in
    intake timing, crankcase volume and intake tract diameter.
    They are given here only to
    illustrate that: A) it is possible to make a given intake timing work over a wide speed
    range by varying intake tract length; and B) that the same length may produce more than
    one peak, or conversely that more than one length may be effective at any given power

    read that part B again.

    B) that the same length may produce more than
    one peak, or conversely that more than one length may be effective at any given power

    did you check the port timings with a degree wheel, and is the exhaust pipe going to suit those timings?

    cleaned any of the casting flash from inside the ports, mainly the transfers?

    will you try the "stub exhaust" tuning trick for determining exactly what length intake will suit your particular style?

    dont just swallow everything youre told. if you REALLY want to make a drag bike... do some research, get your hands dirty, and over all, experiment! basically...NO ONE has the answers you seek, only YOU can find them, via EXPERIENCE ;)

    and who exactly are you racing with, and where? ask some of those people...they already have some experience, sometimes theyll let go of some hard won secrets ;)

    oh yeah, and have fun :) be prepared to kill a few engines, waste a few dollars... still a lot cheaper than real dragbikes! and safer, too!
  14. theamusingnerd

    theamusingnerd New Member

    Thanks! I did the port matching myself. I am racing against a few of my friends who are building some bikes. We are racing on a friend's dad's empty lot.
  15. HeadSmess

    HeadSmess Well-Known Member

    "The 9 1⁄2-inch length gave substantially the same power at
    3000 rpm and 4000 rpm;

    at 6000 rpm and 7000 rpm
    the same power could be had with either a 9 1⁄2- or 6 1⁄2-inch length;"

    ;) notice the same length can give BOTH low end and top end ;) mainly due to harmonics and resonance.

    think of a twostroke engine as a flute...a saxaphone...each and every part serves a purpose (even if all those lil levers do look downright confusing! iunno. i cant play one!)

    they seriously are a musical instrument :jester:

    heres the link to above mentioned book. read many many times!


    but remember its 41 years old... its not THE final word, but still...its a good read, and explains just about every aspect you need to know.

    dont worry about heads or sprocket ratios just yet... save the money for tools... dentist drills on ebay are excellent for porting ;)

    wish there were some people around here that i could race... only ever seen two other people while riding in five or more years!
    Last edited by a moderator: Dec 15, 2015