Re: Compression decreasing block on MC turbo engines...

[DeWitt Harrison <de@aztek-eng.com>]

On Tue, 11 Nov 1997 10:12:55 +0200, Gerard wrote:
>[ ... ]
>On the MC turbo engines Audi decreased the compression ratio from
>9.2:1 (??) to 7.8:1 by using a spacer between the engine block and the
>head. On the S4 motor the compression is 9.2:1 (if I recall correctly
>from the 1996 vehicle spec books most mechanics have). The S4 still
>runs the 5-cylinder motor. Is this spacer the only difference between
>the motors on these MC engines and the S4?
>[ ... ]

I can't speak thoroughly to the differences between the MC and the 3B.
However, there are no "spacers" involved in either case.  (Where did that
come from?)  Quite a bit of CR adjustment in the 10 V motors has been
done by varying the volume of the relief in the piston tops.  This volume
and that of the head chamber are approximately equal at 30 cc each. A
change of 5 cc here or there makes for big differences. 5 cc is roughly
the volume contribution of the gasket, btw.

Head spacers in the form of thicker or multiple head gaskets are pretty
much in the domain of sleazy N/A to turbo conversions which need to
drop the stock CR due to overall sleazy design and execution. This
measure usually screws up the combustion dynamics by enlarging the
critical "squish" space between piston top and head around the piston's
circumference. (Thanks, Corky.) Additionally, double gasketed heads
can't stand much boost.

The later MCs changed CR to from 7.8 to 8.4.  I'm sure this is just
a piston swap. In principle, you should be able to get to 9.3. This
wouldn't get you 20 V, cross flow or dual knock sensors though.

DeWitt Harrison     de@aztek-eng.com
Boulder, CO
88 5kcstq