Calculating the compression ratio of an ABY

[GintsK]

Ouch! it is discovery for me! I never put engines together, just tune them.

I read your investigation with care, because I whole life believed to 9.3.

One detail is bit unclear for me: piston protrudes out of block surface, right? Where you take it this volume in to account?
Does it 0.5mm from here?:

and the squish in front of these which is only 0.5mm above deck

It gives me 2.6cc smaller chamber.
So chamber volume 44.7+8.18+3.75-2.6=54.03cc
It gives CR advertised by manufacturer: [54.03+445.2]/54.03=9.24

Am I wrong?

Gints

[gnutz2]

I see whhat you did.

(3.14 x 81 x 81 x 0.5) divide by 4000 = 2.58cc

But if you look at a piston very very carefully only the squish in front of the inlet valves comes above the deck height by .5mm, it is actually tapers from .5mm to level with the deck, the squish behind the inlet valves is level with deck, Audi put some very clever design into these pistons.

I'll get a picture of a piston to show.

I went over and over the calcs but got the same anwser.

[JP]

Keeping the stock squish if you can is important, however can be worked around in combustion chamber design. I personally do not like the idea of leaving the crown so incredibly thin on the thrust side though. That's waiting to do some damage if you are trying to push things to the limit. You can easily gain 2cc in the combustion chamber alone by carefully reworking it without touching the piston.

[gnutz2]

I'm not using these pistons any more, i have some 82.5 wossner pistons now but i see where your coming from.

[petro238]

intresting topic!
...I have one question: what would you consider to be a correct CR and a correct squish height?
I suppose these values change according to the type of performance you are looking for...

[GazzaG60]

i di a more ghetto style on mine.

i had the full surface take down about 1mm. it doesn't leave the piston ideal granted but it worked fine for me. no issues at all. i ran up to 2 bar there or there abouts on it for 2 years ish.