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  1. #1
    mrbtd's Avatar
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    Help me understand squish/compression

    I have 2 stock heads cut for 150-lb compression.

    Is it a combination of dome shape and squish that makes the 150lbs ?

    Standing opposite side of the carbs I get
    Old head w/ one layer of head gasket (copper coat added)
    #1 L = .059 R = .056
    #2 L = .059 R = .064
    #3 L = .064 R = .059

    New head (lowell cut) I was only able to do # 3 because I ran out of solder.
    2 Layers of head gasket (middle removed)
    #3 L = .078 R = .078

    I know the gasket is .010 so thats a difference of .009
    I measured with the L parallel to the crank

    When we tighten the squish down to .040 does the head just get thinned out ? At what point does the cylinder need to be decked ?
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  2. #2
    Moderator OsideBill's Avatar
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    Volume is what determines compression. Total CC's

    Squish helps cool your piston and force the charge into the dome. By removing layers of the gasket to tighten squish you are effectively reducing volume and in turn raising your compression. Same goes for decking.

  3. #3
    mrbtd's Avatar
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    Ran across this when trying to get more details - it is good reading.

    http://www.greenhulk.net/forums/show...ghlight=squish

  4. #4
    spud's Avatar
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    One thing you need to be sure of is that you measure the thinnest end of the solder and that the solder is against the cylinder wall, also you need to make sure you are measuring over the piston pin (that keeps any piston rock out of the equation). Your squish numbers look high. .040" is a good squish number for these engines. I would definately start with the head, then remove layers from the gasket.

    How thick is the squish lip on your head? Are you running dual base gaskets under your cylinders?

  5. #5
    mrbtd's Avatar
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    The base gaskets are just the stock set up. As far as I know the cylinders have never been off.

    For the head I took off I will measure the lip - I am not sure what it is for the one I put on.

    I was measuring the solder from the very tip on each piece and I only did one @ a time. I see where doing the L and R side measurements @ the same time would eliminate the rock in the pistons.

  6. #6
    GPRPITMAN's Avatar
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    Quote Originally Posted by spud View Post
    One thing you need to be sure of is that you measure the thinnest end of the solder and that the solder is against the cylinder wall, also you need to make sure you are measuring over the piston pin (that keeps any piston rock out of the equation). Your squish numbers look high. .040" is a good squish number for these engines. I would definately start with the head, then remove layers from the gasket.

    How thick is the squish lip on your head? Are you running dual base gaskets under your cylinders?
    I agree with spud here 40 thou is about right for these motors 78 is really high to me Is your head centerd?

  7. #7
    I like 'e way yew tawlk Ric's Avatar
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    When y'all speak of squish, it sounds like you mean quench at one point, and then gasket crush at another. Yes, the amount of gasket crush will have an effect on the CR. Quench is designed into the head, and is basically a flat spot for the piston to get REAL close to, "squishing" the intake charge, thereby evenly distributing the fuel mixture, along with any heat that could otherwise be tucked into a hotspot in the cylinder, leading to detonation. The end result is the ability to run lower octane gas on higher compression with no power loss amd no ping... to a point, that is.

    Not sure what y'all meant by squish, but there's my $.02.

  8. #8
    Dave Sharp dav_dman's Avatar
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    hard to believe lowell would not have his guy put a 40 thou squish band on that. He's the guy that first told me about tighter squish with the right shaped dome yielding a layer of cooler air for the hotter air to slide out on...something about not as much room for turbulence to get in the way of the exiting gases and heat the entire mix up..
    Last edited by dav_dman; 09-11-2007 at 08:25 PM.

  9. #9
    www.2-Stroke-Porting.com
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    Quote Originally Posted by mrbtd View Post
    I have 2 stock heads cut for 150-lb compression.

    Is it a combination of dome shape and squish that makes the 150lbs ?

    Standing opposite side of the carbs I get
    Old head w/ one layer of head gasket (copper coat added)
    #1 L = .059 R = .056
    #2 L = .059 R = .064
    #3 L = .064 R = .059

    New head (lowell cut) I was only able to do # 3 because I ran out of solder.
    2 Layers of head gasket (middle removed)
    #3 L = .078 R = .078

    I know the gasket is .010 so thats a difference of .009
    I measured with the L parallel to the crank

    When we tighten the squish down to .040 does the head just get thinned out ? At what point does the cylinder need to be decked ?

    All of the CNC heads are cut with a 2-3 thou (0.0025") "step". Step ='s distance from deck down to squish angle.

    We leave a 2-3 thou step for two reasons: easy to measure overall cylinder diameter & the step will press / cut into the head gasket / coating and help seal. Lots of people use the 'no step' head and it works fine. I simply have better success with that small edge in making the seal.

    To answer your question - the the cylinders must be decked to achieve the desired squish of 40 thou. I recommend the cylinder be decked to get the squish clearance right - And use two layers of head gasket.

    I hope this helps,

    Lowell

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