Scott sez:
>(and I've never read a really good explanation of *why* forged pistons
>require additional clearance)
If my memories of materials class are not faulty, forgings have higher
coefficients of thermal expansion than castings for two reasons:
- forged aluminum is denser than aluminum cast at atmospheric pressure.
Eg., for pistons of a given diameter at a given reference "cold" temperature,
a forged piston will have more aluminum atoms in its head than a cast piston.
When heated, each aluminum atom tries to occupy the same amount of space
regardless of whether it happens to be in a casting or a forging, so the
forging will end up expanding more because it has more atoms to contribute to
the expansion.
- the forging process flows the material out of its relaxed as-cast
grain state. When heated, the stresses left in the matrix conspire to
distort the piston in some direction, although this direction is rather hard
to predict.
Since forged pistons expand more in the range of T,surr to T,max,engine, they
must be smaller at T,surr in order to prevent seizing at T,max,engine.
The benefits of forgings (increased hardness and toughness) over castings are
also due to their prestressed grain structure, so you just have to decide
what you want - an engine that runs quietly and cleanly from startup, one
that will live at 14:1 CR and 10,000RPM, or some compromise.
This is my opinion, and I am open to hard evidence one way or the other. I
run castings in street cars, forgings in my autocrosser, and dream of
cylinders without friction.
Ernest Davis
wily1@aol.com
10/8/93, 18:38
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