9A block from Audi 88

[brett at ataxxia.com (Brett Van Sprewenburg)]

On Nov 28, 2006, at 7:41 AM, Dan Bubb wrote:

> My understanding is that the 9A had a slightly higher CR than the PL,
> like 10.5 instead of 10 (maybe simply due to the increased
> displacement), although I've seen other data stating the 9A was also
> 10:1.


This was my understanding as well.  I believe the increased  
compression on the 2L 9a motor was due to increased bore, as well as  
the pistons being slightly differently "styled".  In the 1.8L PL  
motor, the pistons have a full angular cut across them as a valve  
relief, while the 9a pistons have a pair of dishes.  This difference  
should decrease the volume inside the combustion chamber, all else  
being equal, leading to a slight increase in compression.

Interestingly, the numbers out of the 2 Bentley manuals I have which  
have the 9A list it as 10:1, but I believe the owners manual for my  
last B3 Passat stated 10.8:1.

Given that increased bore leads to increased compression, and the  
slight decrease in chamber volume, all else held equal between the PL  
and 9A, I'd have to say the 9A has a greater compression ratio and  
10.5:1 or 10.8:1 is not out of the question.

> But I don't believe the combustion chamber volume is different between
> the 9A and PL heads, so swapping heads would not change CR.
> OTOH putting an 8V head on a 16V block will really raise the CR since
> combustion chamber volume is only ~30cc instead of 45cc or whatever
> the 16V head is.
> Dan
>
>


==Brett