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

[Gerard <gerard@poboxes.com>]

> 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.

This job, as far as I know, is the cheap and sleazy way of dropping the
compression ratio when doing turbo conversions. I was just following
this up to find out if Audi ever went this route.

> 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.

What year did they change the CR of the MC?

I suppose if I did hunt down S4 pistons (which seem to be in abundance
at the Audi/Porsche stripping works in Johannesburg (??!!??)) I would
need to gain more air flow by fitting the 20V head as well, right?
Say, if I coughed up the dough and bought the S4 pistons, would they
fit? Isn't the S4 a 2.5 litre motor? I spoke to someone at the stripping
works and he said the job would need new con-rods and crank as well.
True? As for dual knock sensors, I have them already.

Thanks.

G.

PS: Oh well, these silly questions is just me feeling my way around to
    more power. Usually not to be taken seriously. :) But it is better
    to ask and be stupid than to do and die. :)
-- 
"a thousand miles from here, there is another person smiling"
1990 Turbo (200T)
name   : gerard van vught
tel    : +27-57-912 2658 (w) / 082 923 9609 (cell)
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         gerard@acenet.co.za / van_vught@frg.issi.co.za