Machining Piston to lower compression?

[Jörgen Karlsson]

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
> >
> >
> >