Holy compression!

Discussion in 'General Questions' started by Wags, Sep 5, 2016.

  1. Wags

    Wags New Member

    Just got done with my every engine performance mod experiment. I took 38mm stroke with zl rod China girl (because that's what I had) and added an arrow reed valve kit with keihin carb, holed piston, third transfer port, port matched intake and exhaust, sickbike expansion chamber (soon to be kx65 expansion chamber), sickbike shifter, jaguar cdi with high output coil, iridium spark plug, the two transfer ports were matched to the cylinder, 6cc fred head and was decked for .70mm squish band. After running a compresion test I found my compression to be 179psi! after dialing in the cdi and carb it runs great and has no power issues. Now that I've given all the necessary information my question is... how do i bring the compresion down without sacrificing my squish band or power? I know I'm putting horrible stress on my bearings and wrist pin and is only a matter of time before they fail. Any suggestions?
     

  2. FurryOnTheInside

    FurryOnTheInside Active Member

    7cc Fred head?
     
  3. FurryOnTheInside

    FurryOnTheInside Active Member

    It is my understanding, just from reading here, and I may be wrong but iirc..
    High compression damages cylinder plating; high RPM, grossly advanced timing and detonation damages the bearings. So high compression only damages bearings IF it contributes to or leads to detonation.
    But you don't want to ruin the cylinder either so it doesn't change anything, you still want to lower the compression.

    Estimating the result of switching to the 7cc head might not be as simple as 179 * 6 / 7 = though, as Steve Best showed us in his thread about his volume measurements of (non Fred) heads with piston in place VS filled level. I certainly would be very interested to hear about your results if you switch to the 7cc Fred head!

    I think Power, Reliability and Affordability are tripartite in engines, just as Lightness, Reliability and Affordability are in cycle components. You can't get more of one value without losing some of one or both of the others.
    At least, from my reading that's how I think it works anyway. :)
     
    Last edited: Sep 5, 2016
  4. Wags

    Wags New Member

    Thank you for your sugestion. You would be correct that the larger cc head would probably be the solution but in this case I am already way over budget. The list of things I have done is just on the engine. That doesn't include seat post mounted gas tank, mounts, sensors, throttle, bike, etc., etc.,etc.. I was hoping for some kind of "trick" that I don't know about yet. Or some kind of home remedy.
     
  5. FurryOnTheInside

    FurryOnTheInside Active Member

    Sounds like modding a spare cylinder so it's ready to use when needed would be the cheapest option.
    You aren't having any problems with the high compression yet, so you can just leave things as they are, enjoy the high power you getting from your high comp' engine, and replace the cheap cylinder when the old one starts to flake. :)
     
  6. Wags

    Wags New Member

    I think I will just enjoy it. The next best thing to ridding is working on these bikes and my tto temp gauge says I only got to 336 max degrees. I've got milling the Boost port in the cylinder down to a science and only an hour and a half out of my life. Ill just keep an eye on it. Ill know more today. I'm going for longer ride.
     
    FurryOnTheInside likes this.
  7. skyash

    skyash Active Member

    A good oil helps cool and lube .
     
  8. Wags

    Wags New Member

    I only use premium gas (93 or higher) and always castor927 synthetic.
     
  9. jaguar

    jaguar Well-Known Member

    If you have a rotary tool with you can remove metal from the cylinder heads "bowl" till the compression comes down.
    It would take a lot of work though. I'd just remove the squish band. Jennings said they are only needed on engines with high compression or that rev over 9000.
    If you do nothing then say bye-bye to your bearings and cylinder plating.
     
  10. Wags

    Wags New Member

    I have a couple different size copper head gaskets. It couldn't hurt to try a larger gap and see what happens. I would rather not cut into the brand new head just yet. Also, I have to comment on your cdi. I have zero, i repeat, zero vibrations. It's like a sewing machine. All I did to the crank was true it, shave a little weight off the upper connecting rod and lightened the piston by drilling holes in it for the reed valve . Still not sure I'm getting the most out of the cdi yet but that will come.
     
  11. Wags

    Wags New Member

    $_1.JPG this one is the next project. Liquid ktm 50
     
  12. Wags

    Wags New Member

    Here's the other thing. The arrow reed valve and all the material I had to remove from my tranfers to get them to match, not to mention the Boost port material that was removed had to be made up in the crank case. I stuffed around the bearings to make up for that loss. Besides the ability to pump more fuel into the combustion chamber, I wonder how much extra compresion comes from that modification? I am not good at trig and most of the formulas I read about that could give me an idea, had to many variables.
    Here's a silly question...how do i know if I have detonation? Besides looking at the edges of the piston. Are there identifying sounds? My spark plug is the perfect light chocolate color.
     
  13. jaguar

    jaguar Well-Known Member

    I once had detonation at 155psi and slightly lean jetting. It happened when carrying around a pal of mine on my 100cc street Suzuki. I didn't hear anything so if there's anything to hear it's very slight. Left a crater in the middle of the piston. So I retarded the ignition and richened the jetting. Eventually I reduced the compression to 135psi because the engine was getting too hot.
    Stuffing the crank a little still isn't going to make up for all the space that the arrow adds (since its set back from the cylinder).
    Wags, what exactly did you do to the crank to true it? What exactly did you do to the conrod?
    Isn't it wonderful to have a smooth ride? No one believes me when I tell them. They have to experience it to believe it.
    There are even people on this forum that fight against any efforts to balance the engine because some professor on a video said it was impossible to completely balance a 2 stroke (which is true). But it is possible to balance it till all you feel is a slight tingling and you can see what's in your mirror.
     
  14. Wags

    Wags New Member

    I took the crank to my local motorcycle and snowmobile mom and pop shop where I know the owner. I asked if he could ballence the crank and he explained exactly how expensive a balancing machine is and it's not something he has to do. But he told me he could make sure the crank shaft was true. He took his time and it is now perfect. as for the conecting rod I took my dremel and smoothed all the flash and rough spots only around the upper rod and around the outside of where the wrist pin goes through making sure not to compromise the structure. It was hard to measure the actual weight removed but after pivoting the upper on my digital scale through the hole process I removed about 1.73 grams. That number is in reference to nothing. Just a number I started with in case I need to do it again or remove more. Dumb luck.
     
  15. Wags

    Wags New Member

    Is it the kx60 or kx85 expantion chamber that it's design is easiest to work with? And what years? I know cr machine sells the kx series header kit but doesnt say witch has the best layout ro start with.
     
  16. Wags

    Wags New Member

  17. jaguar

    jaguar Well-Known Member

    nice work. looks better than mine. :)
     
  18. skyash

    skyash Active Member

    Is that just like the window in my pocket bike piston
     
  19. Steve Best

    Steve Best Active Member

    First of all, Maxima Castor927 is not a synthetic. Exact opposite. It is castor bean oil.
    Excellent stuff. I am using it right now on my motorized 48cc bike.
    I find the more you use of it, the better it works, due to the sealing effect I'd guess. I'm using 24:1.
    Messy black drippings but very little smoke and goes like hell. Better than the Motul 800 synthetic 40:1.
    Cylinder walls are holding perfect. Subjectively less wear evident than the synthetic. Great for sloppy tolerance motors.

    Wags, if you don't believe in the value of squish bands, just keep adding headgaskets to lower the compression pressure. My testing (and I am a die-hard Jennings follower) has shown that squish is needed to keep detonation at bay in these engines, so I like to keep 0.5mm to 1.5mm squish and about 20%-50% area in the head. Chamber shape, squish slope and other things enter into it too. No simple answer. Try adding another or a thicker headgasket and see where it takes you. Otherwise machine the cylinder head chamber bowl out larger.

    Another approach is to raise the cylinder with more base gaskets (and lower the head if need be). Raising the exhaust port will lower the compression pressure somewhat and move the powerband to a higher rpm.

    If you like the way the engine works now, open up the chamber bowl.

    Compression favours low rpm power only (especially on a piped 2 stroke) and throws heat into the piston like crazy. It is hard on chrome, wristpin and big end bearings.
     
    Last edited: Sep 18, 2016
  20. Wags

    Wags New Member

    I have excellent power right now and I'm thinking some it has to be so to the squish band. I'm going to try the next size up copper head gasket I have and see where that takes me. Oh, and I didn't even look at the castor bottle for the word synthetic. That's just what I was told when it was sold to me for high revving engines. Good to know before I looked like an even further jackass.
     
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