You can see in this shot that the Ross pistons are flat top with the valve reliefs. This changes the compression ratio to 9.35:1. The RS piston top is dished and yields an 8.5:1 compression ratio.
Talking with both George @ Ross and the guys down at the machine shop, both of them are pretty sure that the RS pistons are forged. The pistons didn't have the casting markings on the side like the Impulse XS and GSi pistons did.
To them, that was impressive that a car from '91 was so built up properly. All I could say was; "yea, I know. . ." ;-)
They probably said to themselves "he is a smug little bastard!"
What is the difference between a forged piston and a cast piston?
A cast piston is liquid molten aluminum poured or injected into a mold. A forged piston is made from a solid ingot and pressed or extruded into shape before machining.
Forging is a manufacturing process where metal is pressed, pounded or squeezed under great pressure into high strength parts known as forgings. This is usually done by heating the metal, but some forgings are produced without heating. 
Generally, forged components are shaped by either a hammer or a press. Forging on the hammer is carried out in a succession of die impressions using repeated blows. In a press, the component is usually hit only once in each die impression.
The Forging Process
The three basic types of aluminum alloy forgings are: open-die forgings, closed-die forgings, and rolled rings.
In open die forging, the work component is not completely confined as it is being shaped by the dies. This process is commonly associated with large parts such as shafts, sleeves, and disks, but the part’s weight can range from 5 to 500,000 lbs.
Most open die forgings are produced on flat sides. Round swaging dies and V dies are also used in pairs or with a flat die.
As the forging workpiece is hammered or pressed, it is repeatedly manipulated between the dies until it reaches final forged dimensions. Because the process is inexact and requires a skilled forging operator, substantial workpiece stock allowances are retained to accommodate forging irregularities. The forged part is rough machined and then finished machined to final dimensions.
In open die forging, metals are worked above their recrystallization temperatures. Since the process requires repeated changes in workpiece positioning, the workpiece cools during open die forging below its hot-working or recrystallization temperature. It then must be reheated before forging can continue.
Impression die forging is the most basic example of impression die forging and accounts for the majority of forging production. In impression die forging, two dies are brought together and the workpiece undergoes plastic deformation until its enlarged sides touch the die side walls.
Some material flows outside the die impression, forming flash. The flash cools rapidly and presents increased resistance to deformation, effectively becoming part of the tool. This builds pressure inside the bulk of the workpiece, aiding material flow into unfilled impressions.
Ring rolling has evolved from an art into a strictly controlled engineering process. In the ring rolling process, a preform is heated to forging temperature and placed over the internal roll of the rolling machine. Pressure is applied to the wall by the main roll as the ring rotates. The cross-sectional area is reduced as the inner and outer diameters are expanded.
Rings can be rolled into numerous sizes, ranging from rollerbearing sleeves to rings of 25 ft in diameter with face heights of more than 80 in.
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