compression increase via piston

Discussion in 'Performance Mods' started by jaguar, Apr 29, 2012.

  1. jaguar

    jaguar Well-Known Member

    There is this piston from www.treatland.tv that is 1mm taller from the wrist pin than the standard 48cc piston. That is probably the perfect distance to bring the compression to around 130 or 140 psi and allow you to keep the head gasket. And it is cheaper than buying a high compression head. That added height would change the port timing though which would run terrible unless you used a rotary tool to change the port heights first. Here are my recommendations:
    Exhaust port
    26.3mm from top of cylinder (for 107 degrees ATDC which gives the best all around power with the standard exhaust pipe)
    Transfer ports
    30.7mm from top of cylinder (for 123 degrees ATDC which gives 1 degree more blowdown time than stock)
    Intake port
    56.7mm from top of cylinder to bottom of port (which gives a perfect 62 degrees BTDC timing)
    Piston skirt
    45.1mm from top of piston edge to bottom middle of intake skirt. (the exhaust side skirt needs to be reduced to 46mm to keep it from hitting the crank wheels)

    from treatland.tv
    (FOR MORE COMPRESSION)
    honda hobbit stock 40mm piston $30
    50mm from piston edge to bottom of piston
    24mm from wrist pin to bottom of piston
    26mm from wrist pin to top edge of piston
    10mm wrist pin diameter
    1.5mm ring vertical thickness
     

  2. Fabian

    Fabian Well-Known Member

    I like your technical information Jaguar.

    Other members may find it very useful and a method to shortcut their own development time.
     
  3. jaguar

    jaguar Well-Known Member

    That is the whole idea. No need to reinvent the wheel. Testing and developing has been my hobby the past year and I just hope others take advantage of the tricks of the trade that I have presented.
     
  4. jaguar

    jaguar Well-Known Member

    Also, an additional head gasket will have to be added, or you can use a dremel to remove metal at the head where the top edge of the higher piston would otherwise hit.
     
  5. happycheapskate

    happycheapskate Active Member

    If you add a gasket, or remove metal from the head, will the piston swap still increase compression?
     
  6. Fabian

    Fabian Well-Known Member

    Jaguar, do you know of any alternative cylinder and piston combinations in a larger bore size than 47mm which will fit on the crankcase of the Chinese 2-stroke bicycle engine.
    A genuine 80cc of swept cylinder volume would be desirable.
    A 50mm or better still a 52mm piston is a tempting thought.
     
  7. jaguar

    jaguar Well-Known Member

    Normal compression is about 100 and I think it increases about 25psi with every .5mm vertical space decrease, so adding a .6mm gasket should give about 120psi with the 1mm higher piston. Using a rotary tool on the outer edges of the head, to make space for the piston, would probably allow around 145psi which would be my preference. Just make sure you buy a new wrist pin bearing because the original chinese one can't take the increased pressure, especially if you still have a standard CDI which fires too early at high rpm which causes too much combustion pressure and causes failure of that bearing.
    I am sure there are other top ends that can fit but you'll have to start a thread to see what others have used and how much of a match/mismatch it is for the cylinder studs and transfer ports. I've only worked with 48cc and 55cc but JNMotors is sending me a 66cc engine to modify for racing. One problem with thinking about using a larger diameter piston/cylinder is that the engine is already a bit out of proportion from the ideal square design which would be equal stroke and bore. It has 38mm stroke and 47mm bore. Also the connecting rod is longer than the ideal 2x stroke which reduces torque. But I guarantee you this 66cc can be made into a fire-breathing monster without enlarging the bore. I will do the same mods to it that I've done to my 55cc which is almost strong enough to do wheelies without pulling on the bars.
     
    Last edited: Sep 14, 2012
  8. Fabian

    Fabian Well-Known Member

    My requirements are different in that i want significantly more low rpm torque.

    Do you know of any alternative cylinder and piston combinations in a larger bore size than 47mm which will fit on the crankcase of the Chinese 2-stroke bicycle engine?
     
  9. jaguar

    jaguar Well-Known Member

    My question to you would be to find out what you have done to maximize the power of the engine you already have.
    There's so much you can do to surpass the power you'd get from having 2mm more bore.
    Best things for low rpm torque: higher compression, better carb (Dellorto), intake and exhaust extension, torque pipe (which you can approximate by adding a baffle extension to one of the aftermarket expansion chambers), and using JBWeld and a rotary tool to redirect the transfer flow more rearward (which makes a big difference over the sucky stock arrangement).
     
  10. Fabian

    Fabian Well-Known Member

    Do you know of any alternative cylinders with a 52mm bore size and matching piston with a 10mm wrist pin bore size to suit the standard connecting rod?
     
  11. jaguar

    jaguar Well-Known Member

    No I don't. When you go very much over the standard size then you also have to check the cylinder stud spacing to make sure its the same. Whenever I look for an alternative piston I go to treatland.tv or scooterpartsexpress.com
    You can look there for a complete top end that would work. But take this warning: Those top ends are almost always for engines with transmissions which means they are ported to be peaky, not torquey like a one speed needs.
     
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