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  • 408
    408
    2 years ago
    Quoting beaglebasher on 15 Aug 2022 09:53 AMedited: 15 Aug 2022 10:01 AM

    I am trying to catch up with this thread.  What  is this "SQUISH" thing?

    Its a term I am not familiar with. I have a fair idea what it is but for the benefit of everybody on the forum " SQUISH "  needs to be explained.  I cant be fucked googling it.


    Here you go Beagle, in basic terms, this is how I understand the 'squish' process and what it does.
    On the intake stroke, the downward travel of the piston creates a vacuum which draws air into the cylinder via the intake valve. As the intake charge goes past the injectors, they are firing a fuel mist into the intake air stream and some mixing of the air and fuel takes place. Mixing also takes place when the fuel is bounced off the walls of the intake manifold, the back of the valve, and the valve stem. The fuel has to undergo a change from liquid to gas.
    The piston and cylinder have a circular cross section while the head has an egg shaped cross section at the cylinder interface. The egg shape accommodates the valve movement of the exhaust valve and the larger intake valve and the circular shape 'reduces' from the sides and the head effectively forms a 'part ceiling' at the sides of the cylinder.
    When the piston travels up on the combustion stroke, some of the air fuel mix is squashed between the piston and the part ceiling of the head and is forced out into the combustion chamber. This turbulence contributes substantially to the complete mixing of the air-fuel mix.
    The 'squish' is the gap between the piston at tdc and the underside of the head.
    If the piston edge at tdc is flush with the top of the cylinder then you have zero deck height and the squish is set by the thickness of the head gasket.
    If the piston at tdc is below the top of the cylinder by a couple of thou, then the squish is increased by that distance.
    If the piston is above the cylinder surface by a couple of thou, then the squish is decreased by that distance.
    Some articles refer to a squish of 1mm (0.040"), many harley performance builds aim for 0.030".
    Together with the pressure and the heat, a good squish and increased turbulence in the combustion chamber provides more complete fuel mixing, better combustion, combustion efficiency, and less carbon build up in the combustion chamber.
    It is also possible to run slightly higher compression with this improved efficiency, which provides more power.
  • 408
    408
    2 years ago
    OK , some pics might help. Found some pics of 110 heads on ebay.
    The squish area is the area between the circular outline of the head gasket and the yellow line.

    110 head.png
    110 head squish band.png
  • beaglebasher
    beaglebasher
    2 years ago
    Quoting beaglebasher on 15 Aug 2022 09:53 AMedited: 15 Aug 2022 10:01 AM

    I am trying to catch up with this thread.  What  is this "SQUISH" thing?

    Its a term I am not familiar with. I have a fair idea what it is but for the benefit of everybody on the forum " SQUISH "  needs to be explained.  I cant be fucked googling it.

    Quoting 408 on 16 Aug 2022 01:32 AM


    Here you go Beagle, in basic terms, this is how I understand the 'squish' process and what it does.
    On the intake stroke, the downward travel of the piston creates a vacuum which draws air into the cylinder via the intake valve. As the intake charge goes past the injectors, they are firing a fuel mist into the intake air stream and some mixing of the air and fuel takes place. Mixing also takes place when the fuel is bounced off the walls of the intake manifold, the back of the valve, and the valve stem. The fuel has to undergo a change from liquid to gas.
    The piston and cylinder have a circular cross section while the head has an egg shaped cross section at the cylinder interface. The egg shape accommodates the valve movement of the exhaust valve and the larger intake valve and the circular shape 'reduces' from the sides and the head effectively forms a 'part ceiling' at the sides of the cylinder.
    When the piston travels up on the combustion stroke, some of the air fuel mix is squashed between the piston and the part ceiling of the head and is forced out into the combustion chamber. This turbulence contributes substantially to the complete mixing of the air-fuel mix.
    The 'squish' is the gap between the piston at tdc and the underside of the head.
    If the piston edge at tdc is flush with the top of the cylinder then you have zero deck height and the squish is set by the thickness of the head gasket.
    If the piston at tdc is below the top of the cylinder by a couple of thou, then the squish is increased by that distance.
    If the piston is above the cylinder surface by a couple of thou, then the squish is decreased by that distance.
    Some articles refer to a squish of 1mm (0.040"), many harley performance builds aim for 0.030".
    Together with the pressure and the heat, a good squish and increased turbulence in the combustion chamber provides more complete fuel mixing, better combustion, combustion efficiency, and less carbon build up in the combustion chamber.
    It is also possible to run slightly higher compression with this improved efficiency, which provides more power.

    My head is fuckin sore
  • Lushy
    Lushy
    2 years ago
    many harley performance builds aim for 0.030".


     These are the ones where the piston starts hitting the head after the build is not brand new. 
  • Hoodeng
    Hoodeng
    2 years ago
    Quoting Lushy on 16 Aug 2022 08:41 AM

    many harley performance builds aim for 0.030".



     These are the ones where the piston starts hitting the head after the build is not brand new. 

    HA HA!!!!!!!!!!

    Tink tink.
  • Lushy
    Lushy
    2 years ago
    Go for your life. I didn't Google any links, just have pulled a few top ends off with witness marks on the head/ piston. I do qualify that by saying I don't know about the EVO clearance, just twin cams. 
  • 408
    408
    2 years ago
    Performance build implies running a good tune, or carbon build-up will close up that gap and lead to piston/head contact.
  • Lushy
    Lushy
    2 years ago
    Mate I spend a fair portion of my life tuning (and fixing)Harley "performance builds" almost no-one checks the squish properly, maybe in the world of the interweb, but not in real life. And I do mean almost no-one.  One after another they come in the door with less than optimal settings or builds. For a STREET bike, not a drag bike or a coffee shop bike, they need to be safe and consistent with a good tune, and a good combo of parts. HP and Tq will also come with those ingredients. I set all the harley stuff at .040" because of the above reasons. If I was building one for myself I might sneak it to .037" but really the difference in real word performance in say a 130hp TC110 with a .040" squish to the same engine with a .035" is almost not measurable. At .030" one is relying on the cylinder growth from hot to cold to keep you clear from contact, you personally may get away with it but the ones I see don't. The current world of M8 web builds is to fit a miracle cam and have mental cylinder pressure, you get staggering torque at WOT at 2000rpm  (where you never get the chance to do that on the road) and a great dyno sheet for the front bar, but in real life( not a four second dyno test with the knock sensor turned off) the poor thing is baking hot after 20 minutes riding and then pulling 5 degrees of ignition out on the knock sensors anyway. Where is the torque/power then?
  • Lushy
    Lushy
    2 years ago
    Wombat, if you have the thing apart and then measure the squish(piston to head clearance), I am suggesting maybe .040" or 1mm is a good option. As Hoody said previous most EVOs are way more than this. Fitting thinner gaskets or light machining will get you a significant gain going from say .065" to .040" . The gain you will get going and extra .005" closer will be negligible. It is a 35+yr old bike not a MotoGP engine.
  • 408
    408
    2 years ago
    Quoting Lushy on 16 Aug 2022 11:19 PM

    Mate I spend a fair portion of my life tuning (and fixing)Harley "performance builds" almost no-one checks the squish properly, maybe in the world of the interweb, but not in real life. And I do mean almost no-one.  One after another they come in the door with less than optimal settings or builds. For a STREET bike, not a drag bike or a coffee shop bike, they need to be safe and consistent with a good tune, and a good combo of parts. HP and Tq will also come with those ingredients. I set all the harley stuff at .040" because of the above reasons. If I was building one for myself I might sneak it to .037" but really the difference in real word performance in say a 130hp TC110 with a .040" squish to the same engine with a .035" is almost not measurable. At .030" one is relying on the cylinder growth from hot to cold to keep you clear from contact, you personally may get away with it but the ones I see don't. The current world of M8 web builds is to fit a miracle cam and have mental cylinder pressure, you get staggering torque at WOT at 2000rpm  (where you never get the chance to do that on the road) and a great dyno sheet for the front bar, but in real life( not a four second dyno test with the knock sensor turned off) the poor thing is baking hot after 20 minutes riding and then pulling 5 degrees of ignition out on the knock sensors anyway. Where is the torque/power then?


    I know what you do for a living, and take on board any advice you give out on this forum.
    I understand why you would set squish at 0.040" for the work in your shop.
    I set mine at 0.030" because I know there are benefits if it is done right, but I measure and check everything I do.
    I tune my bike, for crank to run and idle, as well as cruise, then go to a shop for WOT.
    For me, the proof is in the pudding. I have had no piston to head contact, but I warm the bike up before I take off.
    I have just changed cams and heads and gone again with 0.030".



    uncorrected dyno 2 (1024x735).jpg
  • Hoodeng
    Hoodeng
    2 years ago
    God, this is probably why i didn't reply earlier.
    As Nev pointed out, giving one dimension to build to does not automatically imply it will work in all builds.

    First, squish, also in a different application referred to as quench,, two different things that are also the same in an environment. Squish is making the compressed fuel air charge active in the chamber on compression, as in it gets squeezed across the chamber making it more active and complete in the combustion process the, angle/discharge of squish typically runs towards the spark plug. Squish in a flat .10°, 20° etc,  is measured the same and has the same result, you need pistons that match these angles.

    Quench is a bit different, this was evolved in mass production with the advent of common V8's in the fifties, higher compressions were utilized, what was happening for one definition was that open chamber ran hotter on the piston crown and combustion chamber surface, a contributory development to get temps and dets down was to close the chamber down by filling the off side of the spark plug so that the head and chamber were quite close leading to this area running cooler.

    We want squish, why do we set to .040" by default?, one reason is variation in people measuring it, if you produce accurate measurement we can cut the cylinder to suit, i have received test pieces that are too long, in the wrong place, test incorrectly performed plaits you name it, 10mm x 2mm soft solder placed over the gudgeon at 90° to the pin each side in symmetric and close to the edge in non symmetric chambers, evo's can distort the reading if too thick a solder is used as the piston tilts. .030" is fine if you know the cylinder assy will run clean for its operating life, usually carbon is the culprit of loss of clearance, as i described earlier if the test is inconclusive i would get the person to do the test again, or, machine for extra clearance. .040" again, why? over .040" and the squish effect diminishes exponentially, under .040" the gain for a street motor is minimal, another for .040", when we do the test and see that we need a ???" reduction to achieve the clearance, i cut the cylinder base to suit a .040" head and .020" base, why?, you will get these thickness gaskets standard in any MLS kit unless specified otherwise. IE: anyone can service the engines top end without anything special.

    As for Nev's comment about huge torque very early in the rev range, this phenomena is a constant source of mirth. In the tuning process the test is done in one gear and the engine is run through its typical RPM range, obviously testing from idle is irrelevant,when was the last time you took off on your bike without applying some throttle, some do, I've seen it, they are special. The test rpm is proceeded in what would be a typical usage range, full throttle is applied for the test as what is being tested is the engines ability to make peak torque for each rpm point, the actual peak torque read on the graph is peak cylinder fill of fuel air charge that is being combusted.
    Do we ride like this? no, we are slipping the clutch typically at the late teen rpm point on our way to accelerating the vehicle. Although i have heard guys ride their bikes like this,, i can only imagine the engine is near bursting with this treatment. But, as they say, do what you want with what you got.
    DSC04429.JPG
  • Hoodeng
    Hoodeng
    2 years ago
    This is why i said, God.....
    But,,, a bit of misinterpretation has been cleared, even if not in direct answer to your initial question. hopefully it might help choices in the future.
  • Jay-Dee
    Jay-Dee
    2 years ago
    You mentioned additional mods in the future after fitting a cam now, that's how it started to get off track.

    Once you start talking about additional mods on a forum with enthusiasts, and knowledgeable ones at that who are willing to share the benefit of their expertise, the sky then becomes the limit.
  • tussuck
    tussuck
    2 years ago
    I recon we can milk this thread for another week or so!  lol
  • beaglebasher
    beaglebasher
    2 years ago
    Personally speaking I have been enjoying the technical aspects of this thread. I would be enjoying it even more if my head wasn't bursting !
  • beaglebasher
    beaglebasher
    2 years ago
    Not that I am confused or anything but what is the difference between compression and squish again?
    I always thought that squish was how much the gaskets would be squashed  when you tightened the head bolts.
    I am just a dirty arsed old fitter but. 

  • 408
    408
    2 years ago

    static compression or corrected compression ?
  • beaglebasher
    beaglebasher
    2 years ago
    What is the difference  between static and corrected compression?
    I will drop a couple of panadols and have another think about it.
  • tussuck
    tussuck
    2 years ago
    What blew me away was just how high the compression was when I was running at 10lbs of boost!  Thank god the starting compression is a mighty low 8:1 on Evos... 
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