maximum RPM/speed

Discussion in 'Performance Mods' started by jaguar, Oct 7, 2014.

  1. jaguar

    jaguar Well-Known Member

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    How to maximize RPM/speed

    Both the 48cc and 66cc (80cc) engine have around 141 degrees exhaust port open duration which is enough for a maximum RPM of 7000. But these engines only turn up to 5500. Why?
    1) ignition timing is too advanced at high RPM. This basically serves as an RPM limiter. Some motocross bikes with digital CDIs stop retarding the timing at about 500 RPM before the max RPM the engineers want the engine to turn at. This causes the timing to be too advanced which causes the peak cylinder pressure to occur too early to do much good at contributing to engine torque.
    2) engine compression is too low. All performance engines have at least 130 psi cranking pressure to maximize engine performance. The Grubee engine only has around 90 psi.
    3) the cheap carburetor that comes with these engines does a weak job of atomizing the fuel. That causes a slower rate of combustion which is terrible for high RPM power. It is also basically non-adjustable except for the needle height which affects mostly mid range power. Without the ability to adjust the mixture ratio at wide open throttle settings then it is hard to have the mixture ratio optimal for optimal engine power at high RPM.
    4) the crank flywheels aren't balanced adequately for the connecting rod and piston weight. The balance holes need to be enlarged with a carbide bit in a drill press. But drilling out the heavy piston wrist pin can be enough to get rid of the worse part of the stock vibration. That, and using a replacement CDI that has ignition timing at high RPM much more retarded than what the stock CDI has.

    So if you want a 27% increase in RPM/speed (from 28mph to 36mph) I am here to tell you that it isn't necessary to change the port durations which is a scary task to most people. Just fix these four points and your bike will run stronger, faster, and with less vibration.
     

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  2. Fabian

    Fabian Well-Known Member

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    Does it? and what other carburettors are we comparing it to?


    So you are inferring that the violent mixture motion through the crankcase and transfer ports doesn't play any part in homogenising air and fuel droplets, and the majority of homogenisation happens in the carburettor venturi?



    Exactly how do you come to this conclusion when the NT carburettor (with a removable main jet for air/fuel adjustment) is a virtual copy of the Dellorts SHA.



    As has been discussed many times before, a single cylinder engine cannot be balanced.
    You will find the engine balance factor (as standard) is set for lower rpms, and for good reason.
     
  3. Zen builder

    Zen builder Member

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    Jag, would you drill out the balance holes, and the wrist pin, or one or the other?
     
  4. Purple Haze

    Purple Haze Active Member

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    The problem has always been that the crank is balanced for a 40mm piston setup (49cc). Getting the weight down on the 47mm (66cc) theoretically balances the engine, but as Fabian has stated, no single cylinder engine can be balanced for all rpm. In practice I have found that judicious use of a Dremel to remove material from the inside of the piston, inside of the pin, and grinding and polishing the top half of the rod will make a huge difference in midrange smoothness, without pulling the crank out of the case.
     
  5. JozsefJordan

    JozsefJordan New Member

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    5500 RPM, huh? I'm using the stock CDI, but 5500 RPM means 53km/h for me, which is a cruising speed for me. My max. was 70km/h at 7500 RPM still with the stock CDI. So what are we talking about? Without any load it can rev up to 10 000 RPM.
     
  6. gary55

    gary55 Well-Known Member

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    This post is 3.5 years old. I don't think they are still talking about it.
     
  7. Frankenstein

    Frankenstein Well-Known Member

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    Let's talk about this great post. I like the picture of the cheetah the best.
     
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  8. Stoneman

    Stoneman Active Member

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    7.5mm carbide drill bit image.jpg
     
  9. Frankenstein

    Frankenstein Well-Known Member

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    For making 9mm tapped threads? Sometines I drill holes in my neighbor's wood deck and a blame it on the wood bees. Eventually one of us will get smarter and start charging for the service. Anyone have a good recipe for boiled chipmunk?
     
  10. Stoneman

    Stoneman Active Member

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    Never tried wood bees.
    Suppose it’s an option if that’s all you got.
     
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  11. gary55

    gary55 Well-Known Member

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    2 cups boiling water 1 red jimmy.
     
  12. FurryOnTheInside

    FurryOnTheInside Well-Known Member

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    We're talking about how to begin the process of tuning the engine not by leaping straight to the exhaust port and raising it. :)

    There's good reason to begin with getting the most out of every stroke. Making good torque is essential for the bike to get the speed so you might actually get 1.5 times the RPM or whatever you dream of. There's enough torque to gain by tuning (such as retarding ignition at high RPM) that you could run a smaller rear sprocket and raise the top speed RPM too. :)

    If you can do 43mph at only 7500 then you likely already don't have a stock 2014 engine. It clearly makes more torque than the stock 2014 engines that are commonly available in South America and the rest of the world, so tell us how your engine makes its torque! Is it a newer version? Grubee? Zeda? Did you tune it to make the best torque? What did you do? :)
     
    Last edited: Feb 10, 2018
  13. FurryOnTheInside

    FurryOnTheInside Well-Known Member

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    Very nice but you also have the drill press to put it in!

    Is no one selling drilled out wrist pins? Would be a bit cheaper than a drill press lol.

    Not to worry though, if anyone doesn't have the machine shop in their garage; It is possible, IME, to get the vibration so low it isn't a concern at all at any speed, and is totally comfortable at cruising speeds.

    I retarded the timing (AT ALL RPM) by making an offset magnet key, and I drilled balance holes in the crank without removing it (and with a common cordless drill!), and I lightened the piston and upper part of the connecting rod, but I didn't have to drill out the wrist pin or split the case. So if you have limited space or resources you can still do enough to make a big difference, even without any seriously large, heavy or expensive tools.
    Hopefully I can also have total comfort and better torque at high RPM with a better CDI. ;););)
     
  14. Stoneman

    Stoneman Active Member

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    Would a lathe work? I have one of those kicking around.
     
  15. FurryOnTheInside

    FurryOnTheInside Well-Known Member

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    I should think a lathe is preferable but there's only so many members have either so I'm just pointing out that the less well equipped members and lurkers can still get good results by doing all the other stuff I mentioned which doesn't require any larger tools. Although it would be better and easier to lighten the wrist pin, if you can. :)
     
  16. Stoneman

    Stoneman Active Member

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    Yes defiantly, I agree
    They can be pricey so if you buy hoping just to use a hand drill I wouldn’t attempt it and may be an expensive lesson
    Even a bench drill will be tricky without a good vise.
    As Jag mentioned it’s probaly easier to go to a machine shop

    I had the idea of drilling them out and selling them, see how it goes

    I’ve been very lazy in doing it all though haha
     
  17. FurryOnTheInside

    FurryOnTheInside Well-Known Member

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    I wouldn't attempt it with a hand drill either! There's absolutely no chance of getting the hole centred all the way through the pin, and it is a very hard steel which will need to be cooled and lubed with cutting fluid.

    {{ Mainly aimed at future lurkers:
    It's okay to use a hand drill and silicone lube aerosol spray to drill balance holes in the crank because they don't require such precision and I think it's just a mild steel.
    Still, use a 3/16" carbide bit for the pilot holes before using the 3/8" carbide bit to finish the radially drilled balance holes (about 15mm deep seems to do it). }}

    If you find it's economical to drill out individual pins to sell that would save a lot of work lightening the piston and rod so they should be popular (if people have sense). I just expect it to take too much time and carbide bits to be viable. If the pins were manufactured with thinner wall in the first place it might be a better way.
     
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