need compression reduction info

[jdbubb@ix.netcom.com (Dan Bubb)]

In an effort to clarify things...
I think in Allyn's response he is refering to "squish area", but is talking
about combustion chamber volume.
So, to define things:
Squish area is the part of a combustion chamber where the piston and head
are very close together at TDC. When the piston approaches TDC this area
closes up quickly forcing the air fuel mixture to squirt out of this area
into the major volume of the combustion chamber causing a lot of turbulence
which promotes rapid, efficient burning. On the 16V this is at the front of
the combustion chamber where both the head and piston are flat and parallel
to each other. On the 8V this is at the rear of the chamber. A conventional
4V/cyl engine (where the intake and exhaust valves are approximately
symetrical around the cylinder axis) has a squish area at the front and back
of the chamber and can also have small squish areas on the sides. It all
tends to centralize the mixture causing rapid burning and more power.
Setting up the piston to head clearance so the piston almost touches the
head at maximum RPM (when the loads and deflections are greatest) can make a
big difference in power.
Dan

----- Original Message -----
From: Allyn <amalventano1@home.com>
To: Jonas K. <jkarlsso@metabolex.com>; Anthony Pelletier <vwnuts@yahoo.com>;
scirocco list <scirocco-l@scirocco.org>
Sent: Friday, January 11, 2002 5:07 PM
Subject: Re: need compression reduction info


> The 2.0l bottom/top 16v combo was designed around the same cr as a 1.8
> combo, but the 2.0 has more squish area in the head than the 1.8 (yet both
> have same total squish area).
> Thus using 1.8 head on 2.0 block gives a bit more cr.
> Al
>
> ----- Original Message -----
> From: "Jonas K." <jkarlsso@metabolex.com>
> To: "Anthony Pelletier" <vwnuts@yahoo.com>; "scirocco list"
> <scirocco-l@scirocco.org>
> Sent: Friday, January 11, 2002 4:28 PM
> Subject: Re: need compression reduction info
>
>
> > On 1/11/02 8:38 AM, "Anthony Pelletier" <vwnuts@yahoo.com> wrote:
> >
> > > I was wondering if
> > > anyone knew what it would take to drop the compression
> > > of a 1.8 16v from the stock 10:1 to 9:1?
> >
> > Tony,
> >
> > The only reason I can think of to drop the compression is for turbo
> purposes
> > (maybe NOS?).
> >
> > If you are installing a turbo system, I HIGHLY recommend buying Corky
> Bell's
> > book "Maximum Boost". It details everything you want to know, including
> how
> > to calculate compression ratio ((Total volume at BDC)/(total volume at
> > TDC)).
> >
> > This also covers how to properly drop compression to accommodate a
turbo.
> > But until you get the book, here's what he says (paraphrased)
> >
> > ----
> > Modern motors use a squish area on the piston to swirl around the
air/fuel
> > mixture, thus reducing detonation. This Squish area must be preserved in
> > order to keep the turbo motor from knocking. Basically the squish area
is
> > the outer1-2mm of the piston face.
> >
> > Steps of reducing compression ratio that preserve the squish area are:
> > removing metal from the head side of the combustion chamber; using
valves
> > that are more concave; removing metal from the piston face (not touching
> the
> > outer rim); using pistons that create a larger combustion chamber
(factory
> > or custom). Anytime metal is removed one must make sure that structural
> > integrity is not compromised.
> >
> > Similarly, ways of reducing compression that are NOT valid are, using
> > shorter rods, or using a thicker headgasket.
> >
> > Tampering with the squish area can be so bad that the car will knock
more
> > with the lower compression that it would have if everything was left
> alone.
> > -----
> >
> > Something to think about is that since the 16v motor has more volume in
> the
> > head than the 8v, one can use 10:1 8v pistons to get at a fairly low
> > compression ratio. I think something like 7.5:1 or so. Shave some off
the
> > head and you can get it back up some. These will retain the squish area
> > AFAIK. To get to a 9:1, the only way I can think of is to buy custom
> > pistons. A worthwhile investment if you're serious.
> >
> > Putting in a 2.0l motor won't change the compression much (if at all).
> >
> > HTH
> > Jonas
> >
> >
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>
>
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