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  1. #1
    Old Ski Insanity w/rpm500's Avatar
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    Counterweight shape on 2 Stroke ?

    Hi guys! working on the cases today on the 500 ..got to thinking, What has anyone done to modify the counterweight on a two stroke in reguards to using it to boost flow into the ports..Anyone have thoughts??

    Back in the CAR CRAFT days, we cleaned the flash off the 4 strokes to aid in its rotating thru oil drain back and cleaned other surfaces such as casting marks to aid its flinging oil off..
    ( not much of a difference, but it felt good anyway).
    With out altering the counterweigts mass, anyone ever hear of ANYTHING to enable it to 'pump' better?
    My thought would be IDEALLY to have counterweight with a scoop shape on the leading edge and a taper on the back like a symetrical air foil with a rough surface to aid in fuel seperation...thoughts??


  2. #2
    Stop staring at my tits NJJer's Avatar
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    Not being any expert on this, I think a full circle crank would be best as it would exhibit less turbulence in the crankcase.

    On our race car engines at work, cranks are knife edge. Than again, these are 2 different animals.

  3. #3
    Old Ski Insanity w/rpm500's Avatar
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    full circle would keep the charge channeled in the right direction, but think the shape of our counterweights (in general) are this way to give the added ability of sweeping the charge into the intake ports.. I was thinking of this today while modifying the intake ports at the sleeve bottoms..those I may have gotten carried away in opening that area to much and will have fill back some with epoxie!!

  4. #4
    Moderator OsideBill's Avatar
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    Here is some pics of a billet crank from a old school lamey 6mm stroked 1200 Hybrid motor. Round was the choice he made back then, good call.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  5. #5
    Old Ski Insanity w/rpm500's Avatar
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    OS,BILL Yes i see the containment those do for the fuel chargeall the way around (prob not as easy to get oil onto those bearings unless they are sealed units)..
    in the last picture the weight is cut back accept for the outer edge (to counter balance the rod and pistion)...do you think that this would have a sweeping effect on the charge into the port?, and if so, could we improve it with some sort of surface modification?

  6. #6
    Moderator OsideBill's Avatar
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    Quote Originally Posted by w/rpm500 View Post
    in the last picture the weight is cut back accept for the outer edge (to counter balance the rod and pistion)...do you think that this would have a sweeping effect on the charge into the port?, and if so, could we improve it with some sort of surface modification?
    I believe that is the theory to this, as far as modifying the outer diameter I would be afraid of drag. You have to figure if I enhance the "pump" I have to have HP to drive it. The cases this went into had to be trough cut in the center, this is where the real action takes place. You would think adding a wiper to the connecting rod would make the most improvement for increased charge and velocity.

  7. #7
    PolarisNut's Avatar
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    I've seen a few motors where guys dimpled the outer circumference of the counterweights...had the appearance of a golf ball. The theory here was that it would...well hell, I have no idea what they were thinking, haha...I'll have to see if i can find that write up again. It was either on MacDizzy, Grabowski's, or some sled site. There isn't a whole lot you can do to the counter weights, without changing their weight/balance.

  8. #8
    reducing turbulance in a 4 stroke is a good thing due to the amount of oil that flies around in them. Some of that oil is from crank shaft windage that picks it up out of the pan. The majority is from the rods flinging it from the rods being pressurized. On a 2 stroke I don`t know if reducing turbulance would be a good thing. Considering thats the only thing that lubes the bearings. If it would be possible to eliminate alot of turbulance since oil is heavier than air the oil could fall out of suspension and just pile up in the bottom of the casses. Thats one of the reasons they don`t really like to polish the intake tract.The rough surface creates turbulance to keep fuel in the air. Granted the air circulating through the bottom end for the next stroke should keep it plenty lubed. sometimes You have to ask is all the work you put into something like that going to be worth the effort and trade off. I could unerstand it for an all out race machine that will be torn down on a regular bassis. But if its a playtoy is a 1/4 to maybe 2 hp on a small 2 stroke worth possibly losing reliability? Don`t get me wrong I am not trying to bash or anything I love building hp on different machines and even thinking outside of the norm to try and find it. It`s good to consider all aspects if your considering doing the changes.

  9. #9
    Old Ski Insanity w/rpm500's Avatar
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    if you dippled a ball (golf) it forces it to spin.. this inturn lets it fly further and streighter than an a ball not dimpled it directs high pressure air infront of the ball to flow to a lower pressure area .. nearest i could see would be to have the crank heavy shot peined to rough surface .. this would remove minimal to no weight or rolling a knurl to the outer surface.. my thoughts are if this were oil it was running thru then you would want it machined smooth which is what most cranks outer faces are...has any one seen this done any other findings .. mcdizzy has great reading but i havent run accross it myself..

    True The gains of this maybe minimal... thou we have seen when it all adds up..... it may be the difference in kicking your buddys smile off his face or loosing yours this next spiring.. just thoughts for you guys soon to be in winter...lol

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