Squish!

Discussion in '96-04 - 2V Specific' started by 96blak54, Jun 29, 2014.

  1. 96blak54

    96blak54 Legend

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    Lets talk about why "squish" has a significance to the out put of an engine. Squish improves the stability of combustion.

    Its known to achieve more compression is to simply reduce the combustion chamber. This is done by block and head decking, piston height, rod length, head gasket thickness and so on. Modifying the chamber is another way, but lets keep this simple. Comression is essentially what we power hungry Americans are chacing after wether it be by N/A or forced induction. But an inherent flaw gets created in are search for more compression. ..pre-ignition!

    Pre-ignition refers to an undesired combustion before a desired spark location of the piston. In other words pre-ignition is the fuel self combusting at the wrong time.

    A static compression ratio is the mathematical given space of total compression area. This area is made up of the top of piston at the top of its bore, crushed gasket thickness, and head chamber. Try to imagine an engine cut away view.

    Now...a static ratio of 13:1 with a squish distance .050" is not the same as 13:1 squish distance .020". Squish distance refers to the distance amount between the piston crown and head surface. The .050" will be more prone to pre-ignite than .020".

    Lets talk about what happens in the chamber. The intake valve opens, the mixture rushes in as the piston travels down creating a tornado vortex swirl keeping the mixture homogeneous or uniform throughout. As the piston travels up its compression stroke, that mixture gets moved around again untill the piston starts to slow down as its nearing top dead center and the mixture slows as well but being compressed. A mixture not in motion will self combust and if there is room between the piston and head pre-ignition will be more probable. But in a last attempt to get that mixture moving at the pistons slowest rate, ...we reduce pre-combustion by squishing the mixture by using the moving pistons crown pressing up to the heads flat surface area of the comustion chamber. This causes the mixture to suddenly move before it has a chance to become stagnant or still while being compressed. Still motion gas being compressed pre-ignites. This squished mixture rushes into the only open available area, the chamber thus creating a tighter package to also improve flame travel time.

    If the more space between the head and piston ATDC the more lame the cumbustion becomes. The least is the opposite to the point of the piston kissing the head thus stabilizing the combustion to withstand higher compression.
     
  2. 96blak54

    96blak54 Legend

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    Food for thought? Todays 4v 4cyl engines run 10.5:1 on 87octane
     
  3. 03DSGGT

    03DSGGT SN95 Supporting Member

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    :thumbsup: Great info. I vote this to be a sticky!
     
  4. 96blak54

    96blak54 Legend

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    We all know how much compression can be had with a pi swap and stock cams. Neighborhood of 10.5:1+..? By adding aftermarket cams the compression ratio will drop. By adding longtubes or a diffrent intake manifold ...again the compression ratio is altered greatly but the challenge is figuring out how to mathematically put a number to it so that we can keep are compression up. This is called Dynamic compression.
     
  5. 03DSGGT

    03DSGGT SN95 Supporting Member

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    So how does an intake manifold and longtubes alter compression ratio? Ive never heard this before and Im very interested to learn. I always heard that by running high compression, you can run larger cams to bleed off compression and make it pump gas friendly. Also, when I was planning my other build, I was mentioning running about 12:1 on pump gas, and a few guys were saying that its a waste to run that high of compression on pump gas because you have to back the timing off so much it will be sluggish. So in your opinion, where is the sweet spot for compression ratio on a 4.6 running on 93 on the street? Assuming that the motor has an aftermarket intake, longtubes, zero decked block, and say my size cams which are 238*/244* duration?
     
  6. 96blak54

    96blak54 Legend

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    Maybe I should have stated "able to run higher compression with long tubes" instead of alter the actual compression.

    Alot of heat is generated in the bore at high rpm's. This heat kills the ability to run higher compression. Lets say a compression ratio of 11:1 and its pinging in the upper rpms. Instead of uping the octane, change your stock log exhaust manifolds out for long tubes.

    Yes more compression can be had with cams but to narrow down how close you can get it, we need to know the actual intake closing degree after the piston is on its way up into conpression stroke. Advertised duration and the degree from the manufacturer specs is a joke. We need to know the actual intake valve closing and not the .050"lift degree closing point. One would be very surprised how much .049" valve lift can flow....intake or exhaust.
    What heads are you going with?
     
  7. 03DSGGT

    03DSGGT SN95 Supporting Member

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    Im going to stick with my Trick Flow 38's. Right now Im at about 10.3:1 compression with the stock intake and shortblock. My original plan was to go around 12:1 on my Teksid shortblock with the same heads and cams that I have now, the block zero decked, and a Street Burner intake manifold with the Cobra throttle body setup. When I had mentioned this in the past, some guys said to knock the compression down to 11 to 11.5:1 so the timing could be maxed out for the 93. However, my car has a slight stumble between 2-3K when under load, and my tuner said that it was because of the big overlap in the cams and my compression being low. He said that he would run atleast 11.5:1 with these cams and that the extra compression would help to light off the cams a little better in that rpm range. SO, basically from what I have been told by others, I figured that 11.5:1 would be a good middle ground because its high enough for the cams, but its not too high to where they have to bump the timing down to run on 93....
     
  8. 96blak54

    96blak54 Legend

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    What cams? Can you link me the specs?
     
  9. 03DSGGT

    03DSGGT SN95 Supporting Member

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  10. 96blak54

    96blak54 Legend

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    What tranny and rear gear ratio? Call cushman up and ask what the ABDC intake valve closing is? Did you say u are using Trickflow intake ?
     
  11. 96blak54

    96blak54 Legend

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    Pm sent
     
  12. 03DSGGT

    03DSGGT SN95 Supporting Member

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    Pm received. I'm still running the PI intake currently but want to switch to the TFS Street Burner on the Teksid. Trans is a 3650 and rear gear is 4.30...
     
  13. 96blak54

    96blak54 Legend

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    Ok good , but we gotta know what the ABDC intake valve close timing is before we can really squeeze the life out it.
     
  14. 96blak54

    96blak54 Legend

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    Also your still in the .030" range for squish. Decking the block more or taller pistons will get you there
     
  15. 03DSGGT

    03DSGGT SN95 Supporting Member

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    Ok so Im going to ask Jim what the After Top Dead Center closing of the intake valve is?
     
  16. 96blak54

    96blak54 Legend

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    Correct
     
  17. 96blak54

    96blak54 Legend

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    I can explain that one if you like?
     
  18. 96blak54

    96blak54 Legend

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    A pi headed teksid with monster cams at .020 squish can run 93octane with a static 13.5:1
     
  19. 03DSGGT

    03DSGGT SN95 Supporting Member

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    Yes please explain. Im very interested!
     
  20. 96blak54

    96blak54 Legend

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    When the piston reaches the bottom of the intake stroke, the intake valve will still be open much after the piston is traveling back up on the compression stroke. This dwelled valve opening reduces dynamic compression while allowing the incoming rush to charge in. At high rpm's, the rush of air is unimaginable through the intake port and is creating a super charged affect while the piston is at its slowest speeds (TDC, BDC). This air movement could be characterised as laser sharp. The movement of air from intake port entry is out of control but when traveling through the port the air is being shaped into a mass. As the piston slows down towards the bottom, that mass is still charging through. Alot more power can be had at low valve lift because of scavenging or charge from the air. Ive stated "valve opened" which could be .049" valve open or closed, an area the cam manufacturer likes to keep a secret and normally the manufacturer specs. I never state max valve lift which usually follows the piston down and the valve starts to close ABDC keeping that charge pressure. All this reduces the dynamic compression.