User Tag List

+ Reply to Thread
Results 1 to 11 of 11

Thread: Compression vs Squish

  1. #1
    Tyre Kicker

    Join Date
    Dec 2009
    Location
    Pwllheli, Cymru
    Posts
    44
    Thanks
    1
    Thanked 0 Times in 0 Posts

    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

  2. #2
    I support TS Decade Plus User
    Turbosport Subscriber
    rallyrob's Avatar

    Join Date
    Apr 2010
    Location
    Middlesex
    Age
    52
    Posts
    2,147
    Thanks
    128
    Thanked 150 Times in 144 Posts

    Re: Compression vs Squish

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

  3. #3
    Tyre Kicker

    Join Date
    Dec 2009
    Location
    Pwllheli, Cymru
    Posts
    44
    Thanks
    1
    Thanked 0 Times in 0 Posts

    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

  4. #4
    Racer

    Join Date
    May 2012
    Location
    Camberley, Surrey
    Posts
    2,483
    Thanks
    3
    Thanked 227 Times in 227 Posts

    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.

  5. #5
    TURBOSPORT SPONSOR Turbosport Subscriber
    Turbosport Moderator
    Turbosport Administrator
    Graham's Avatar
    My Race Car
    My 1st Project
    Join Date
    Feb 2006
    Location
    Ely, Cambs
    Age
    53
    Posts
    24,145
    Thanks
    249
    Thanked 1,993 Times in 1,863 Posts

    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

  6. #6
    Racer

    Join Date
    May 2012
    Location
    Camberley, Surrey
    Posts
    2,483
    Thanks
    3
    Thanked 227 Times in 227 Posts

    Re: Compression vs Squish

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

  7. #7
    Tyre Kicker

    Join Date
    Dec 2009
    Location
    Pwllheli, Cymru
    Posts
    44
    Thanks
    1
    Thanked 0 Times in 0 Posts

    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

  8. #8
    Tyre Kicker 3.5cologne's Avatar

    Join Date
    Jun 2017
    Location
    New Zealand
    Posts
    40
    Thanks
    0
    Thanked 2 Times in 2 Posts

    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.

  9. #9
    Tyre Kicker

    Join Date
    Dec 2009
    Location
    Pwllheli, Cymru
    Posts
    44
    Thanks
    1
    Thanked 0 Times in 0 Posts

    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

  10. #10
    Spanner Monkey

    Join Date
    Oct 2008
    Location
    italy
    Age
    53
    Posts
    487
    Thanks
    20
    Thanked 40 Times in 38 Posts

    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.

  11. #11
    TURBOSPORT SPONSOR Turbosport Subscriber
    Turbosport Moderator
    Turbosport Administrator
    Graham's Avatar
    My Race Car
    My 1st Project
    Join Date
    Feb 2006
    Location
    Ely, Cambs
    Age
    53
    Posts
    24,145
    Thanks
    249
    Thanked 1,993 Times in 1,863 Posts

    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

+ Reply to Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts