Machining Piston to lower compression?

[Peter Berrevoets]

Not knowing what engines he was generalizing about, but it should be noted
that the aidi turbo pistons are  a deep dish type already. The addition of a
thicker head gasket is unlikely to cause the problems that he is talking
about here because the typical north american piston of that vintage was
either marginally dished or was in fact crowned to raise compression.

just my $0.02

Peter Berrevoets
1990 200TQ
Toronto, Canada
http://frontpage.home.net/pjberr/



> >-----Original Message-----
> >From: quattro-admin at audifans.com
> >[mailto:quattro-admin at audifans.com]On
> >Behalf Of Jörgen Karlsson
> >Sent: Sunday, March 04, 2001 9:41 AM
> >To: Rob Andrews; Alexander van Gerbig; Audi List
> >Cc: Audi List
> >Subject: RE: Machining Piston to lower compression?
> >
> >
> >
> >
> >Corky Bell are of course right, but his 'will NOT work'
> >yelling is not a
> >good idea. There are far to many applications where thicker
> >gaskets work
> >well, this is not only with low power engines but it is
> >successfully used on
> >engines that produce close to 200hp/litre.
> >
> >With the boost levels we are talking about when
> >turbocharging a N/A five
> >cylinder I don't think that there will be any need for a compression
> >reduction anyway. But if it detonates, a thicker gasket is
> >worth a try.
> >
> >I haven't seen any good results with double gaskets, they
> >tend to fail.
> >
> >Jörgen
> >
> >
> >
> >
> >
> >
> >
> >> I quote from Maximum Boost...
> >> " A variety of methods exist to change a compression ratio.
> >> Almost all are
> >> UNacceptable.  The crux of the matter is upsetting the
> >"squish volume"
> >> around the rim of the chamber.  A chamber is designed so that the
> >> charge is
> >> pushed towards its center as the piston achieves top dead
> >center.  The is
> >> perhaps the strongest deterrent to detonation designed
> >into the system, as
> >> it tends to either eliminate end gas or keep charge
> >turbulence high.  This
> >> squish volume is a rim about .3 to .4" wide around the
> >chamber, and approx
> >> .04 thick-a big washer-shaped volume between the piston
> >and the head.
> >> Consider "squish volume" sacred and do not tamper.  It is
> >> possible to err so
> >> badly in removing the squish that a resulting 7-1 compression
> >> ratio may ping
> >> worse than a 9-1 with proper squish.  Clearly then,
> >choices for reducing
> >> compression ratio are limited to opening up selected parts of the
> >> head side
> >> of the chamber, installing a new piston with a dish in the
> >center, or
> >> remachining the original piston to create a dish.  It is
> >perhaps a little
> >> risky to undertake remachining a combustion chamber,
> >because the thickness
> >> of the material is usually unknown.  Furthermore, chamber shapes
> >> are closely
> >> controlled features of most modern engines.  If the
> >chamber must be recut,
> >> ultrasonic inspection can determine the thickness of the material.
> >> Commercial inspection service companies frequently offer
> >this service.  An
> >> entirely new piston, with the required dish that maintains
> >the squish
> >> volume, is a proper approach.  Machine a dish into the
> >original piston is
> >> sound, provided the top thickness is adequate.  A
> >reasonable rule would
> >> require the top thickness to be at least 6% of the bore.
> >Approaches to
> >> lowering the compression ratio that do NOT work are thicker head
> >> gaskets and
> >> shorter connecting rods."
> >> The reason these companies offer it because they don't
> >know better I
> >> presume.
> >> Why do you think APR has a Stage IV for the 1.8 with new
> >pistons?  They
> >> actually have engineers that design this product
> >extensively.  You don't
> >> increase gasket thickness to get a lower CR.  Shame on those
> >> other companies
> >> for making others think so :)
> >>
> >>
> >> > -----Original Message-----
> >> > From: Alexander van Gerbig [mailto:Audi_80 at email.msn.com]
> >> > Sent: Friday, March 02, 2001 7:23 PM
> >> > To: Rob Andrews
> >> > Subject: Re: Machining Piston to lower compression?
> >> >
> >> >
> >> >     Than how come Callaway, ND, 8vTurbo, and so on offer
> >this to lower
> >> > compression and run higher boost?  I am not being
> >critical, I'm really
> >> > curious.  I will turbocharging my N/A engine shortly, 6psi, and
> >> I will not
> >> > be lowering my compression via a double gasket.
> >> >
> >> > Cheers,
> >> >
> >> > Alexander van Gerbig -- '88 80
> >> >
> >> > The Audi  80 Pages-----------------
> >> > http://surf.to/the80pages.com
> >> >
> >> > North Ferrisburg, VT 05473
> >> >
> >> >
> >> >
> >