It’s commonplace these days to read a classified ad of a restomod or a heavily tuned vehicle and see that some engine parts have been replaced by forged internals. But why is that better?
Donut Media has sought to find out with a new investigation into the different metallurgical properties of cast, billet, and forged pistons that involves Mrs. Frizzle’s Floridian brother and the Hydraulic Press Channel.
It is, first and foremost, important to understand the different processes behind the names. Casting a piston simply means melting the metal (actually a number of metals mixed together to make an alloy), pouring it into a mold, and letting it set.
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It’s a simple concept and it’s cheap because it’s easy to do over and over again. Casting is therefore popular with OEMs because it means they can churn out a lot of pieces. It also has some design benefits, allowing the maker a lot of freedom to determine the recipe of the alloy and the shape of the part.
Since you’re pouring a metal broth into a fancy ice cube tray, though, there’s a certain lack of control over how the metal sets up. Looking at the crystals inside a cast piston, there’s little uniformity and sometimes air pockets and foreign substances can be left over from the casting process. That means that a cast piston is weaker than a forged or billet one even if they’re made of the exact same substance.
A billet piston, meanwhile, is formed from a larger piece of metal that is then cut down to size. The manufacturing process means that the crystals inside the metal are tightly packed and of a uniform size. The act of cutting parts, though, can lead to a weak point where the grain of the metal is severed.
Finally, a forged piston starts off much like a billet piston, but rather than cutting it down to size, the billet is heated, placed in a forge, and pressed into shape with thousands of pounds of force. That means that the crystals form a grain that is tight and perfectly aligned.
With the perfect alignment of the grain, there are fewer failure points, which means a stronger material and the ability to make certain areas sturdier. In a nutshell, it all leads to a piston that’s able to take more of the engine’s explosive abuse.