Here we have a baseline cast...
Here we have a baseline cast replacement piston for a 383. These pistons are cheap, but consider the drawbacks of no valve relief, a compression height putting them .080 inch down the hole at TDC, their hefty mass, not to mention Russian Roulette with the material at high rpm-and they really aren't a bargain in a high-output engine.
It goes up, compresses a violent explosion, and goes down. A piston's job is easy enough to understand, but if you examine what it's required to do, you may be amazed it works at all. Think about it-at 6,500 rpm, a piston moves up and down the bore 108 times a second, coming to a complete stop twice on each trip. A piston in a 440 engine at that rpm reaches a speed of 6,627 feet per minute by the time the crank turns to 76 degrees past TDC. That's 0 to 75 mph in just 1.66 inches of travel off TDC; compare that to your 60-foot time. With more rpm or stroke, the numbers get meaner, so it pays to be serious about piston selection if you're serious about reliable performance.
Cast vs. Forged
One of the first distinctions in the way a piston is manufactured is whether it's cast or forged. We've all heard these terms applied to pistons, but what's the difference? Actually, the difference is in the way the piston blank is created. In the casting process, molten aluminum is formed in a mold to the general shape of the piston. Casting has several advantages from a manufacturing point of view. Close tolerances can be held in relation to the piston's finished quality. This minimizes the final machining costs and allows for a lighter piston to be produced with less effort than forging. Cast pistons offer excellent wear and thermal characteristics, allowing for long ring land and skirt life as well as tight running clearances for quiet operation. The main drawback is that cast aluminum is limited in terms of ductility. In other words, an over-stressed cast piston will suddenly and destructively break when it fails.
Stock and some cast replacement...
Stock and some cast replacement pistons feature steel struts cast into the pin area to reduce heat expansion.
Forgings, on the other hand, have metallurgical characteristics different from cast aluminum pistons. Blanks for forged pistons are created from a slug of solid aluminum under extreme pressure in a forging press. The aluminum is pressed into heavy forging dyes to arrive at the rough shape of a piston, forming the piston blank. Unlike a cast piston, a forged piece results in a denser, more ductile material. A piston-forging die must be designed so that the two halves of the die can be separated and the piston blank removed. This precludes the possibility of designing undercuts in the raw forging. As a result, a forged piston blank needs considerable machining to create the modern lightweight forged racing piston we are accustomed to seeing. A forged piston is inherently stronger and more forgiving when its limits are exceeded, typically distorting rather than grenading.
The working characteristics of both forged and cast pistons vary considerably with the specific material and processes involved in their manufacture. Not all cast pistons are created equal, and the same can be said of forged pistons. Generally, the manufacturer considers the intended application of the piston when the design specifications are determined. It pays to follow their guidelines when deciding whether a certain type of piston is suitable for a specific application.
Cast Piston Types and Materials
OE manufacturers fitted their production-based engines with cast pistons-except for a few special high-performance or factory race applications. The economic savings of tooling for large quantity part runs, the desirable characteristics of tight skirt clearances for quiet operation, and the inherent wear resistance of a silicone aluminum alloy fit the bill for OE manufacturers. OE Mopar engines were typically equipped with cast pistons with steel struts cast into the pin area to limit expansion, for very tight clearances. OE Mopar pistons were relatively heavy but adequately over-designed to be durable in performance application. How far can stock Mopar pistons be pushed? Like anything else, they are great until the day they break, and the timing of the break depends on the luck of the draw. A sensible limit would be to keep output under 1 hp per cubic inch, and rpm under 6,000. We know guys have gotten away with much more, and we have pushed these limits dramatically ourselves, but it's a gamble.