Piston Fundamentals

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.

Piston Basics
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.

Forged pistons are made from a variety of different aluminum alloys. the two most popular alloys are 4032 and 2618. Federal-Mogul's Speed Pro Division tell us they use an alloy called VMS-45, an alloy similar to 2618. Speed Pro forged pistons include 11-percent silicon for added strength. This additional silicon content also controls expansion properties, which has always been a challenge with forged aluminum pistons.

Probably one of the biggest advantages of forged pistons is durability. Not only do they hold up better than cast or hypereutectic pistons, they fail gradually under severe duty conditions. This buys time if you get into trouble. It prevents complete and total engine failure.

Installing forged pistons in your daily commuter doesn't make much sense unless you intend to throw nitrous or supercharging at them periodically. Using forged pistons in a mild-mannered engine is overkill and economic foolishness. You simply don't need them. And for all the hoopla over forged pistons, they're not always desirable in street engines because they're noisy when cold. They rattle up a storm in a cold engine because they have different expansion properties than cast or hypereutectic pistons. They are loose cold and snug hot.

Piston selection is actually a simple process rooted in how you intend to use the engine. If you're building a daily driver or weekend cruiser, cast pistons will get the job done. If you're a tad nervous about it, consider this--cast pistons were used in 7,000-rpm screamers in SCCA competition in the '60s. Take comfort in knowing they will live nicely in your street-driven restomod.

If you intend to do a little weekend drag racing, hypereutectic pistons will get you through with smooth, quiet performance and reliability, as long as you stay away from nitrous and supercharging. Nitrous and supercharging call for the use of forged pistons without exception, due to the extremes of heat and shock presented by these power adders.

Piston Dynamics
With material issues out of the way, we're ready to look at piston design. There is much more to piston design than we have room to discuss here. Instead, we're going to touch on the basics. Piston design and shape greatly effect how an engine performs. When pistons are too heavy, we lose power. Design in too much skirt, and we lose power through excessive friction. Too little skirt, and the piston becomes unstable. Shoehorn in too much displacement, push the wrist pin into the ring grooves, and you have a formula for piston failure because this exerts too much heat on the pin and boss.

In the dreamy world of piston science, we dream of the perfect piston--the piston that creates very little friction (drag), weighs very little, carries just the right amount of oil up the cylinder walls, and provides a perfect cylinder seal. In the real world, it is nearly impossible to achieve all of these elements at once.

Piston development and research is a science that has been going on since the dawning of the four-cycle engine more than 100 years ago. The objective has always been to squeeze the mixture and spin the crank without losing power in the process. Older, modern-day piston engines had big, heavy slugs in their bores. Take the FE-series big-block, for example. Our old 352s, 390s, and 428s had heavy pistons with long skirts from the factory. These pistons were stable, but heavy.

Piston technology began to change in the '70s, with close attention paid to fuel economy and cleaner emissions. Heavy pistons consume fuel. They also contribute to emissions because it takes more fuel to move them. As a result, Detroit began designing lighter pistons with less skirt. Less weight, less skirt to create friction. The up side to all of this is greater efficiency with less power lost, and less fuel burned.

Manufacturers have managed to shave a lot of weight out of pistons because aluminum technology has improved a lot since the '60s. however, not much has changed when it comes to cast pistons. If you're rebuilding an old FE Ford or Y-block using cast pistons, expect to see virtually the same kind of piston Ford installed to begin with. If you are stepping up to hypereutectic or forged, expect to see significant changes in piston design.

Piston design has always varied because engine building needs vary. Piston crown design effects what's going to happen on the upstroke. Anticipated compression ratio effects piston requirements. It is a common misconception that compression ratio is effected by combustion chamber size alone, piston crown, deck height, and compression height also contribute to compression. When you are shopping pistons, it is important to remember these issues.

Another advance in piston design and manufacturing is high-temperature coatings that protect pistons. This is where the space age meets the automobile. There are coatings available that protect the piston crown from temperature extremes if you are going to push the engine hard. These coatings also protect the piston skirt from wear issues, reducing friction considerably. See your piston manufacturer for more details.

Riding In Cars With Pistons
Anytime you are shopping pistons, there are all kinds of factors to consider: chamber size, valve sizing, how much stroke. This all plays into the relationship your pistons will have with the rest of the engine.

You have to check clearances to make sure everything is going to work well together. It's always a good idea to do a dry run before engine assembly is complete. Valve-to-piston clearances must be checked, especially if you're working with a custom application.

Checking valve-to-piston clearances calls for a mock-up assembly of the short-block and heads. You can do this during engine assembly, or with the pistons without the rings before assembly. The benefit to an initial mock-up is not risking ring damage if you have to disassemble the engine because the pistons weren't right for the application.

Another way we check clearances and timing is with a degree wheel. When we "degree" an assembled short-block, we are checking valve timing events, valve lift, piston timing, and piston deck/compression height. In fact, this is something you should check before checking valve-to-piston clearances. When we are doing this, we want to check true top-dead-center (TDC) with a TDC indicator. This, coupled with the degree wheel, teaches us the absolute truth about camshaft specifications and crankshaft indexing.

Five Golden Rings
We tend to glance over piston rings because they just aren't that terribly interesting. But in the interest of reliable operation and performance, piston rings need as much attention as the rest of the engine. Think of piston rings as seals between the combustion chamber and the crankcase. The two compression rings seal the combustion chamber. Each of the compression rings is made out of a different material, based on what each ring is exposed to.

Piston rings have a tricky job to perform. We have to achieve a delicate balance between oversealing and undersealing the chamber. If we overseal the chamber, we risk engine damage via detonation and friction. if we underseal, we lose the important heat energy and pressure necessary to make power.

To achieve proper cylinder sealing, we have to understand piston rings, the materials they are made out of, and what happens to rings in operation. The top compression ring has the toughest job: to seal in combustion gasses and thermal energy. It also carries the heat of combustion to the cylinder wall and water jacket. The second compression ring assists the top ring in containing hot gasses. It also grabs some of the oil from the cylinder wall for lubrication purposes, even though it isn't called an oil ring.

The oil rings, as their name implies, carry oil up and down the cylinder wall. As the rod journal comes around, it throws oil on the cylinder wall. This is called splash lubrication. The oil rings wipe the cylinder wall with the splash lubrication.

Piston ring materials have evolved for improved durability. As a rule, compression rings have always been made of cast iron, ductile iron, or steel. What has changed is how these rings are treated in manufacturing for durability. Chrome piston-ring sets have a top compression ring that is chrome-plated for reduced wear and scuffing at the top of the bore. We see this a lot in racing applications.

Moly piston-ring sets sport a top compression ring impregnated with molybdenum, which has a very high melting point of nearly 5,000 degrees. This reduces the potential for scuffing. Because molybdenum is a lubricant, it helps piston-ring life immeasurably.

In the old days, piston-ring widths were typically 5/64-, 5/64-, and 3/16-inch. Racing engines went with lighter stock: 1/16-, 1/16-, and 3/16-inch ring packages. These days, it gets even thinner in the interest of friction and weight reduction.

Piston-ring-groove design is also important to smooth ring operation. We want ring grooves that offer stability without causing ring bind. Rings have to maintain specific clearances in the grooves to keep a smooth marriage with the cylinder walls.

Keeping proper cylinder sealing has always been a challenge for engineers. This has been approached with different kinds of materials and manufacturing techniques. One means to maintaining good piston-ring contact with the cylinder wall has been gas jets drilled into the piston crown and top-ring groove. When fuel ignites above the piston, heat and pressure enter the gas jets and force the top compression ring outward against the cylinder wall. This is intended to maintain cylinder pressure during initial light-off. This works well in racing applications, but doesn't live long on the street.