[Audi20V_Kruemmer] Re: [s-cars] I don't understand lowering CR for boost

[serge]

Mike, in my simple world the squish area I was referring to is the total
volume of the combustion chamber when the piston is TDC.

Serge



> Sorry Serge, I'm not sure a CR change necessarily has to affect or change
> all or even part of a piston's  "squish" area(s), at least as I understand
> "squish."  I believe squish is also sometimes referred to as "quench" as
> well.  
> 
> See, for example the KB Piston discussion at
> http://www.beckracing.com/page05.htm ".... Excessive cylinder pressure will
> encourage engine-destroying detonation and no piston is immune to its'
> effects. An important first step is to set the assembled quench (a.k.a.
> "squish") distance to .040". The quench distance is the compressed thickness
> of the head gasket plus the deck clearance (the distance your piston is down
> in the bore). If your piston height (not dome height) is above the block
> deck, subtract the overage from the gasket thickness to get a true assembled
> quench distance. The quench area is the flat part of the piston that would
> contact a similar flat area on the cylinder head if you had .000" assembled
> quench height. In a running engine, the .040" quench decreases to a close
> collision between the piston and cylinder head. The shock wave from the
> close collision drives air at high velocity through the combustion chamber.
> This movement tends to cool hot spots, averages the chamber temperature,
> reduces detonation and increases power. ...."
> 
> Elsewhere comments are made to the effect that the "0.04" is something of a
> critical distance - any greater space than that and you don't get the "clap"
> needed to produce both the pressure wave & displacement needed to provide
> "quench."  As you may know, the stock AAN piston is relatively flat but
> there is a "slightly" recessed circle in the piston crown.  I don't have a
> piston handy, and I've never measured how deep the recessed circle is.  I
> kind of now wish I had a piston handy to try to determine the diameter and
> depth of that recess to guestimate what, if any effect the dish might have
> vis-à-vis KB Piston's "suggested" required no more than 0.040" gap.
> 
> Sounds to me like squish & quench are at least as important in creating a
> "slap" or "clap" which, as a  pressure wave, is as important as physically
> moving the combustion mixture in terms of assisting, forming and shaping the
> burn.  
> 
> One could, I assume, still retain this feature, the clap, while still
> lowering a cylinders final compression ratio by, just as one example,
> leaving a ring around the outer edge of the piston while still hollowing out
> a dish in the center.
> 
> Mike
> 
> 
>> -----Original Message-----
>> From: serge 
> 
> ...... 
> 
>> 
>> I guess I'm saying the same thing as Rich WRT squish area.
>> Easier to get a more complete burn in a slightly more open
>> squish (noun).
>> 
>> 
>> Serge Filanovsky
>> 
>> 95 S6 Avant
>> 
> 
> 
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