"Fiat found combustion chamber
shape is critical for engine
response. The first European
1290 cc 128 heads had a flat area
opposite the spark plug. All
USA versions have the combustion
chamber shape shown - the recessed
area in the head (right side
of drawing) brought three more DIN
horsepower with the same
compression ratio (to 73 DIN or 83
SAE with 1290 cc engine).
This doesn't compare with the power
gained by using the early
1116 cc head to gain compression.
The early head doesn't have
the recessed area which is milled
out of the casting for current
engines and fits all the SOHC 1290
and 1498 cc engines.
Although the early head doesn't have
as efficient a combustion
chamber, you more than make up for
it with the increased
compression ratio."