Compression ratios and head shaving etc

["John Fielding" <johnf@xxxxxxxxxxxxxxx>]

Hi All,

The recent discussions on CR of Motronic engines etc raises an interesting point which
from the comments I have seen so far most people are not aware of.

First off the Motronic engines had a 10:1 CR and not 10.4:1, these I think are the Borgo
after market pistons.

Secondly, if you increase the bore diametre by, say, 0.040" it will increase the CR more
than if you shaved 0.040" off the head. .  Do the maths and you will see what I mean.
The PI factor in the piston area formula is the clue as the combustion chamber is a fixed
volume.

When oversize pistons are made for a particular
engine the piston manufacturers decrease the piston crown dimension to reduce the CR and
keep it within the stock specification.  If you swap pistons from another make, the Mazda
or BMW pistons are a popular choice here, with the corresponding bore diametre adjustment,
you can end up with a higher CR. Because of this adjustment in the piston crown dimension
you do not see an increase in CR as you would expect.  Alfa pistons are not normally
available in oversize as they have liners supplied as part of the piston sets and it is
not recommended to remove any significant metal from the liners as they become too thin
for the pressures involved.  A friend who did this had all his liners split from top to
bottom!

The Alfetta inspection manual gives three sizes for pistons, but these are simply
production tolerances. The minimum size is 83.935mm and the maximum is 83.965mm,
 i.e. a tolerance of 0.03mm total = 0.0012" not exactly oversize types!

On my first Alfetta 2L race engine we fitted Mazda pistons with Renault 5 rings (a long
story!) and shaved a lot off the head.  We ended up with 12:1 CR which with a fairly wild
cam with 13mm lift and twin Weber 40's yielded an output of 140kW as against 106kW for a
stock 2L engine.  This was using 102 octane race fuel.  Insufficient ignition advance
prevented any further increase.  The tuner had another Alfa 2L engine which had shown
156kW on the dyno with a  similar setup.

However, 13mm lift cams gave the valve springs a severe workout and
broken springs were a common problem.  Coil binding due to valve acceleration on
opening was gotten around by sacrificing some lift by allowing more clearance, made it
noisy but this was a pure race engine anyway.  Valve head to piston crown clearance was
tight, only 1.5mm static and additional valve pockets where needed to provide this safe
clearance.  At 8000 rpm the calculated clearance with a hot engine dropped to half this
figure.
No room for mistakes when degreeing the cams!

Head gasket sealing also can be a moot point when
the CR is pushed too far.  The bottom end of the 2L engine is well able to handle the
higher loads, although the 1800cc engine would be a better choice because  the shorter
stroke yields higher rpm at lower piston speeds.  Taking the 2L & 1800cc engines side by
side the 1800cc engine should make more power every time.

Lightening flywheels is another point of contention.  The biggest "flywheel" in the
Alfetta is the driveshaft, there is little one can do to lighten this.  However, a word of
caution on the clutch flywheel.  The balancing shop I use warned me about attempting to
lighten the clutch flywheel.  He showed me pictures of several exploded flywheels when
owners decided to try their hand at carving excess metal off these.  The damage this
causes to the transaxle has to be seen to appreciate the dangers involved.  It is also
essential to have the clutch flywheel and the pressure plate balanced as a unit every time
you change the clutch disk or have the contact surface trued up.  The friction plate is
not required as it never occupies the same place in use and they are balanced to a very
high degree in manufacture.  My clutch flywheel assembly was spun up to 10,000rpm when
balanced on the machine! The balancing guy told me that every Alfa clutch flywheel he had
had through his shop was out of balance, some to an alarming degree.

The engine flywheel can be lightened with due care.  I reduced the stock flywheel down
from 17kg to 6.5kg by a combination of milling and turning.  The flywheel and crank were
then balanced as a set.  The cast iron used is a very high quality grade.  After
lightening only one small 6mm drill hole was required to bring it back to perfect
balance, the crank required no additional balancing.  This says a lot for the Alfa design
engineers who designed the flywheel and crank assembly.

The front pulley is so small and light it isn't really necessary to include this part.
The driveshaft is essential to have balanced as a separate item.  Very few balancing shops
can undertake this work as the jointed construction makes it impossible to hold it on the
conventional balancer.  My friend made a special fixture and can balance to within
1/4gram.  As the driveshaft rotates at engine speed any out of balance here can be quite
scary!

John
Durban
South Africa
Alfetta 1.8L turbo
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