Machining Piston to lower compression?

[Rob Andrews]

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-----------------
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>
> North Ferrisburg, VT 05473
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>