Modified Mustangs & FordsHow To Engine
Pistons Turn Your Engine's Linear Motion Into Rotary Motion
Pistons are an engine-building element we like to brag about, but you won't catch many of us boasting about having cast pistons. Cast pistons are for wimps. Many of us like to talk up hypereutectic pistons, which are certainly better than cast pistons. But, it is surely the manly men out there who like bragging about having forged aluminum pistons. Having forged pistons means you spent the bucks necessary to get them inside your engine build. Spinning an engine with forged aluminum pistons means you can spin it higher and nastier than most of the folks on your block.
What's the real truth behind piston selection? Will cast pistons survive a 7,000-rpm blast? Can you use hypereutectic pistons in supercharged or nitrous applications? At what point must you bite the bullet and opt for forged pistons? High-dome or dished? Big valve reliefs or small ones? Wide rings or narrow ones? Chrome moly or cast iron? Lots of questions--with an even greater number of answers.
The piston's primary responsibility is to take thermal energy created by the ignition of fuel and air, and transform it into linear motion. Linear motion acts on the crankshaft journal and becomes rotary motion--power that does the work for us. The piston has one of the toughest jobs in your engine--it connects thermal energy with your engine's mechanicals. It is at the leading edge of the power picture.
To understand the heat energy that works on your Ford's pistons, you need to understand physics. One pound of gasoline has 20,000 btus (British thermal units). One gallon of gasoline weighs approximately 6.5 pounds. That means we're looking at approximately 130,000 btus in a gallon of gasoline. More simply put, that's 2,545 btus in one hour from a pound of gasoline. That's a lot of thermal energy working on the pistons. This means we need to give proper piston selection a lot of thought.
Piston technology has come a long way. Computer-aided design and CNC machining technology has made it possible to make custom pistons for just about any application you can think of. With this technology has come lighter pistons with less skirt that offer less friction, but this is only the beginning.
Die-cast pistons are made by pouring molten aluminum into a mold. Then, the piston is machined into a finished product. It doesn't get any more involved than that when you're talking cast pistons. Look at a cast piston's surface, and you can see the rough die-cast aluminum.
Hypereutectic sounds impressive, doesn't it? But hypereutectic pistons are little more than a die-cast slug with a high silicone content. This makes the surfaces harder and shinier. It also changes the expansion properties, allowing you to run tighter piston-to-cylinder-wall clearances. You can run a hypereutectic piston a lot harder than you can a cast unit.
Forged pistons are more involved, and, certainly, more expensive to make. Instead of a simple mold, we need a giant press, which rams the aluminum into a complex mold under high pressure. Machining forged pistons is no small feat. It is both time consuming and expensive. The forged piston advantage is greater strength, harder surfaces, more predictable expansion properties, and virtually no porosity .
Another advantage to forged pistons is the ability to make them lighter and with less skirt. We can do this because forged pistons are stronger. We can machine more meat out of them without suffering structural losses.