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996
Head Repair
Repair and Re-Work of a
37/31 Valve Conversion
A
customer from abroad approached me needing some detonation damage
repaired on his ported elsewhere 996 big valve heads. At the same time
as the repair I was asked to give them a look over/flow test and see if
any improvements could be made.
Looking
at the heads all that had been done was a fitting of the larger valves
and a mirror polishing of the ports and combustion chamber. After my
first initial flow and velocity test it was revealed improvements could
be made.
The
first modification was correcting the valve seat angles that were cut
into the combustion chamber way too deeply. Too deep and the shrouding
kills the low and mid lift flow respectively. I applied a shallower top
cut then set about re- blending the combustion chamber around the seats
to remove the intrusive material. A flow test revealed a significant
gain had followed. The race valves previously fitted had taken quite a
hammering. .There was extensive pitting on the valves that could not be
restored so some of my own designed 37/31 valves (as pictured below)
were in order. The valves use a different clearance, are wasted, very
light and have a different shape aiding airflow.
I
changed the port size and shape to optimise it to work with 748r cams. For longevity I set about installing some of my
own designed colsibro race valve guides. The colsibro race guides are a
little shorter than stock, very wear resistant and are actually a little
undersized. This means the guides can be fitted reamed and micro honed
to the correct size of the valve making them a nice snug fit unlike the
rather loose Ducati OE items. A loose or worn out guide amongst passing
oil into the combustion chamber can destroy a new valve job in a short
period of time due to a symptom called valve walking.
The
next batch of modifications were a re-contouring of the inlet port short
side radius pictured below. The port radius shape and width is critical
for high valve lift flow and making sustained high rpm power. This was
re-shaped and re-checked on the flow bench several times.
Inlet
port short turn pictured left.
The
temporarily roughed in, inlet and exhaust seat cuts were finally
precision cut using the PEG FM1 to some different seat profiles below.
As the seats had already been cut very deeply before, it was hit and
miss on whether they could be saved. Deep seats end up with shimming
problems. Luckily these were saved by using very large 3.9mm closing
shims supplied by Mike at EMS.
Finally
after welding up the detonation damage, skimming the head and repeating
the operation three times we finally had a satisfying repair and much
improved cylinder head.
The
final flow graph. Blue is a stock 996 , Black is this big valve head
before modifications and red is the same head but repaired and improved
by me.
