need compression reduction info

[jkarlsso@metabolex.com (Jonas K.)]

On 1/11/02 8:38 AM, "Anthony Pelletier" <vwnuts@yahoo.com> wrote:

> I was wondering if
> anyone knew what it would take to drop the compression
> of a 1.8 16v from the stock 10:1 to 9:1?

Tony, 

The only reason I can think of to drop the compression is for turbo purposes
(maybe NOS?). 

If you are installing a turbo system, I HIGHLY recommend buying Corky Bell's
book "Maximum Boost". It details everything you want to know, including how
to calculate compression ratio ((Total volume at BDC)/(total volume at
TDC)). 

This also covers how to properly drop compression to accommodate a turbo.
But until you get the book, here's what he says (paraphrased)

----
Modern motors use a squish area on the piston to swirl around the air/fuel
mixture, thus reducing detonation. This Squish area must be preserved in
order to keep the turbo motor from knocking. Basically the squish area is
the outer1-2mm of the piston face.

Steps of reducing compression ratio that preserve the squish area are:
removing metal from the head side of the combustion chamber; using valves
that are more concave; removing metal from the piston face (not touching the
outer rim); using pistons that create a larger combustion chamber (factory
or custom). Anytime metal is removed one must make sure that structural
integrity is not compromised.

Similarly, ways of reducing compression that are NOT valid are, using
shorter rods, or using a thicker headgasket.

Tampering with the squish area can be so bad that the car will knock more
with the lower compression that it would have if everything was left alone.
-----

Something to think about is that since the 16v motor has more volume in the
head than the 8v, one can use 10:1 8v pistons to get at a fairly low
compression ratio. I think something like 7.5:1 or so. Shave some off the
head and you can get it back up some. These will retain the squish area
AFAIK. To get to a 9:1, the only way I can think of is to buy custom
pistons. A worthwhile investment if you're serious.

Putting in a 2.0l motor won't change the compression much (if at all).

HTH
Jonas