RE: 4 cylinder head question

["Peter Webb" <webb.p@xxxxxxxxx>]

John and Digest;

Taken from the John Kartalamakis book on CR:

V = Chamber Volume
T = Head Thickness difference

CR=(392.5+V)/V for the 1600
CR=(444.75+V)/V for the 1750
CR=(490.5+V)/V for the 2000

and

T = V / 54.76

Where T is the amount the head thickness is decreased to decrease V and
54.76 being the constant ratio.

Using the example formula from the book and my own round numbers:

Where stock V = 60cc and CR = 9.1:1

Desired CR = 11:1

11 = (490.5+V)/V
V = 490.5 / 11
V = 44.59

So:

Stock V = 60 minus V = 44.59 = 15.05 cc decrease in chamber volume for 11:1
CR.

T = 15.05 / 54.76

T - 2.7mm off the head thickness.

Hope this helps you figure how much your head is cut.  You mentioned 50cc in
your post.  It just so happens the illustration in the book uses a V of
50.05 to achieve a CR of 10.8:1 and a T being 1.8mm.   So in all likelyhood,
your head is cut somewhere around 1.8 - 1.9mm

-Peter


-----Original Message-----
From: owner-alfa@domain.elided [mailto:owner-alfa@domain.elided]On Behalf Of
John Fielding
Sent: Monday, June 09, 2003 2:00 PM
To: alfa-digest@domain.elided
Subject: 4 cylinder head question


Hi All,

I would answer this question myself if I had a head handy, but!

The head in question is either a 2L or 1750 engine.

Can someone who has a head off the engine measure the combustion chamber
depth and
diametre?  The measurement can be either in mm or inches, I can convert into
the form I
require.

I am having an offline correspondence with another digestee and trying to
figure out the ratio of metal removed to CR increase.  It seems the Alfa
hemispherical
head is anything but.  The combustion chamber bowl depth is much less than a
true
hemisphere and hence the CR increase for a particular amount of metal
removed is highly
non-linear.  From data I have from official Alfa documents the combustion
chamber volume
is 61,3cc for the 1750 and 2L engines, but the actual volume is closer to
50cc, which
means the chamber is a truncated hemisphere.  How much it is truncated is
hard to tell
without an accurate measurement of the depth and diametre.

TIA

John
Durban
South Africa
Alfetta 1.8L turbo - resting at present :(
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