In my experience over the past 55 years there is a big
difference in piston engine smoothness. A high strung
aluminum block V8 can feel like a hand grenade ready to blow at
6000 RPM and full chat. The LT1-LT6 aluminum block engine in the
Corvette is a good example. The Rolls Royce V8 on the other
hand has an extra thick aluminum block and is very smooth
to say the least. The old cast iron small block Chevy V8
was very smooth compared to the LT1-LT6. Also the small
4L Jaguar aluminum block V8 with a cast iron lower girdle
around the crankshaft is also much much smoother at 6000 RPM and
full chat than the LT1.

In the case of the TSIO 550 at 310 HP it is probably one of
those highly stressed engines where things like cylinders and
crankshafts are moving around.
Basically it is the same block more or less as my 230 HP
O-470 which is not that bad despite the EGT and CHT being
all over the block. I noticed a difference when my O-470 engine
went from 7.5:1 to 10:1 compression ratio.

IMHO The bottom line is block stiffness. Cast iron has a higher modulus
of elasticity that aluminum. Aluminum must be twice or three
times as thick to equal cast iron stiffness and damping.

Paul Lamar

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