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Discussion Nitrous Oxide

Discussion in 'The Pub' started by Frankster, Jun 3, 2021.

  1. jmw76

    jmw76 Well-Known Member Premium Member

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    You need to also think about valve float and bounce.
    I know the valves on these little machines are small, so do not have much mass, but also the springs are pretty light.
    The critical point is at overlap of the intake and exhaust valves just prior to the induction stroke. This is when piston to valve contact can occur. Any issues with the valve timing (valve not being able to faithfully follow the cam profile) at high rpms could result in contact.
     
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  2. Frankster

    Frankster "Play stupid games, win stupid prizes" Staff Member Premium Member Ride and Events Crew

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    Okay, I have now figured out the actual part/area I should be measuring. Please correct me if I'm wrong. I was measuring the area above the valves when I should have been measuring the raised area next to that.

    Actual Squish area.jpg
    So, after redoing the squish test three times, I have readings of 0.8mm to 1.1mm squish area.

    Actual Squish area 2.jpg

    I did the test across all four cylinders. It's hard to get an accurate reading even with vernier calipers. So @gregt, does this mean these engines are built this way or has someone already skimmed this engine? Can I take any more meat out of it?
     
  3. jmw76

    jmw76 Well-Known Member Premium Member

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    I wouldn't look at taking any more off the block (assuming you measured with all gaskets in place. Head and base). My guess is it was probably built that way. To get anywhere near decent CRs on these engines, you do not have a lot of chamber volume to play with. 6.25cc = 11:1. Allowing for the head gasket, the piston crown needs to be a pretty close fit with the chamber.
    You might still be able to take metal off the head to increase the CR. This will reduce the chamber volume, but will not change the clearance between the flat area on top of the piston and the flat face of the cylinder head.
    You need to measure other areas within the chamber to make sure there is enough clearance to the piston.
     
  4. gregt

    gregt Well-Known Member Premium Member

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    If you can get them all to .8mm, that's fine. Plasticine the whole piston crown and by sectioning the high spots you'll find just how close the piston gets to the chamber roof. If you've got 1mm or more at the tightest point, yes, you can take material off the head. Go back and have a look at my thread on the 150 FZR and see what figures I ended up with - and the CR. Then realise those were heavier pistons.
    Valve springs. You could measure the fitted height and the cam lift.Fitted height less cam lift is the compressed height. You need to ensure that after shimming, the spring is still at least 2mm away from coil bind. I shimmed the FZR springs 1mm from memory. Didn't measure the increase of seat poundage sorry.
    The critical valve to piston is the exhaust. The inlet is opening at TDC overlap and is working against the spring. It opens when the piston is rising.
    The exhaust is closing past TDC overlap. If it's floating off the cam at this point - it's only the spring making it follow the cam now - it will tag the falling piston.
     
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