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

[Richard Tanimura]

Mike,

You have a remarkable list of references. You can throw more references out
there than I can keep up with. But the discussion is damned educational and
just plain fun. There is more here than I ever dreamed - which is the fun
part.

Just looking a the different piston configurations doesn't make me any wiser
about squish/quench. Intuitively, I would think that you would want the
squish around the perimeter of the piston sending the shock wave towards the
center. That could be completely wrong but is just a gut feeling. Without
much reflection, I would just prefer to see the center of pressure directly
above the center line of the rod. But like I say, that isn't even half
thought through. I think this would argue for something like a dished out
piston. I think a lot of people in Europe are simply lower CR using slightly
thicker head gaskets.

I am going to go into lurk for a few days to digest this stuff. Uhh, but
before I do, you wouldn't happen to have any references on the failure modes
of piston rods would you?

Rich


-----Original Message-----
From: MLP [mailto:mlped at qwest.net]
Sent: den 11 mars 2004 22:17
To: Audi20V_Kruemmer at yahoogroups.com; s-car-list at audifans.com;
CaptMagu at aol.com; richard at tanimuras.com
Subject: RE: [Audi20V_Kruemmer] Re: [s-cars] I don't understand lowering
CR for boost


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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