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Thread: Compression vs Squish

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    Compression vs Squish

    Just sizing up a potential build for a smallblock vauxhall with various bits I've acquired for previous engines etc.

    It'd be a 1.8 8v on an 81mm bore and 88.2mm stroke, however, the problem I'm currently facing is that to get down to the correct compression ratio (just under 12.0:1), I'm looking at having the piston approx 1mm down the bore and a 2.26mm cometic gasket, so a total deck/squish clearance of 3.25mm there or thereabouts.
    So, my question is, is it detrimental to run at that large a clearance, irrespective of having the correct compression ratio? Or is it "less than ideal" but tolerable in the real world? The car will be on mapped ignition if that helps things.

    Thanks

    Peredur

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    Re: Compression vs Squish

    Crumbs is the head flat, if not then it must have very small combustion chambers?

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    Re: Compression vs Squish

    Head I've got is off a 1400, only keen to use it because it's been ported and has +1mm valves. Chamber vol is approx 21.8cc

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    Re: Compression vs Squish

    I'd hog out the head! Correct squish allied with a slightly lower CR will be better than a high CR and substandard squish which will be detonation prone. A 1.0mm squish is safe, 0.8mm if you want to push the limits.

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    Re: Compression vs Squish

    Quote Originally Posted by katana View Post
    I'd hog out the head! Correct squish allied with a slightly lower CR will be better than a high CR and substandard squish which will be detonation prone. A 1.0mm squish is safe, 0.8mm if you want to push the limits.
    man speaks the truth

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    Re: Compression vs Squish

    Quote Originally Posted by Graham View Post
    man speaks the truth
    Thanks - I try to learn from my peers!

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    Re: Compression vs Squish

    Thanks folks. Thought that would be the case but good to sound it out.
    Will go back to Plan A of a 1300 motor and work on an 1800 later on with the larger 1.6 gsi head.

    Thanks

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    Re: Compression vs Squish

    Google " David Vizard" and try and find his articles on the development of the Hillman Avenger he raced many decades ago. IIRC that engine in the Avenger had the same sort of configuration where it had no squish.

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    Re: Compression vs Squish

    Secondary question then off the back of your answers:
    Would there be a significant improvement in power/torque if I were to tighten the clearance from 1.3mm to 0.75-0.80mm? Reason I ask is if I run with zero deck height, I can use an oem reinz gasket which slightly overhangs the bore (much like a 2.1 pinto build) and would give 1.3mm squish i.e. the gasket thickness, or alternatively if I tighten the squish by pushing the pistons up to 0.5mm proud of the deck I'd need a cometic gasket which has a larger bore

    Thanks

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    Re: Compression vs Squish

    it depends on how much the piston rocks at tdc (therefore its clearence in the bore, the skirt lenght...) keep it in mind.

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    Re: Compression vs Squish

    its not so much piston rock you need to cater for, its rod stretch, the more you rev the engine the longer the rods get, made worse by heavy pistons

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