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Decking the head to increase compression

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  • Decking the head to increase compression

    So it seems like calculating the compression ratio on a 2 stroke is sort of a black art. I’m planning on decking the cylinder on my G62 to increase compression. Rather than try to calculate the ratio, can I just take 2 or 3mm off the height? and check the squish at the top? How much squish do I actually need?
    *******Pedding since '02********

  • #2

    It should not be black magic to calculate compression ratio. It is a well defined term. Take [volume at BDC) / (volume at TDC)] = compression ratio. Decking the cylinder will increase compression, but that will only be because you are decreasing the squish, so it won't be a huge improvement. To really make a difference in compression, you will need a different dome.

    If you want to deck the cylinder, I think that the smallest squish you can go for is about 0.6mm. Depending on where the 60 starts at with the squish, you can only cut so much out before you will be ramming the piston into the dome. Keep in mind that as things heat up, the rod will stretch, as well as the piston. So running a super small squish runs the risk of ramming the piston into the dome at temperature.

    I think that just running a thinner copper gasket will be better than decking the cylinder. You probably only have about that much clearance anyway. Decking 2-3mm is probably too much, I would guess that you could cut out a max of 0.25-0.5mm.
    Instagram: @base_cat

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    • #3
      Originally posted by Basement Cat View Post

      It should not be black magic to calculate compression ratio. It is a well defined term. Take [volume at BDC) / (volume at TDC)] = compression ratio. Decking the cylinder will increase compression, but that will only be because you are decreasing the squish, so it won't be a huge improvement. To really make a difference in compression, you will need a different dome.

      If you want to deck the cylinder, I think that the smallest squish you can go for is about 0.6mm. Depending on where the 60 starts at with the squish, you can only cut so much out before you will be ramming the piston into the dome. Keep in mind that as things heat up, the rod will stretch, as well as the piston. So running a super small squish runs the risk of ramming the piston into the dome at temperature.

      I think that just running a thinner copper gasket will be better than decking the cylinder. You probably only have about that much clearance anyway. Decking 2-3mm is probably too much, I would guess that you could cut out a max of 0.25-0.5mm.
      Thank you. I ended up pulling the gasket entirely. Squish looked like I could’ve taken out a little more.

      I was was eager to get this motor together, My plan was to get it running for a baseline, measure port timing and pull it back apart for porting.
      *******Pedding since '02********

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