What’s In a Piston

Nov 24, 2009

If there is such a thing as glamour for an internal engine component, pistons might as well be cover models. No other performance-producing part seems to get nearly the attention or inspire as much conversation. Even those who know nothing else about a vehicle’s mechanics know that if you have a problem with a piston, that problem is serious and likely expensive.

What determines the price of a piston? Is it the material it’s made of, or the way that material is used to create the piston? Is it the engine the piston is designed for, or is it the way the piston is designed?

We asked a few of the industry’s top manufacturers these same questions, and their answers, as well as their insights on the future of pistons, may surprise you.

Types of Piston Lines

Before we dig into the materials pistons are made of and the method for making them, let’s take a look at the lines of pistons each company offers.

First, regardless of the motor being modified-from weed-eaters to 350 Chevys to ocean liners-there is almost surely a company making a piston for it, and their vast reach doesn’t stop there. In addition to an impressive range of current makes and models, builders are also able to find a selection going back years that stretch into decades. And, if for some reason a particular piston cannot be found, it can always be made custom.

Most aftermarket piston manufacturers follow a similar formula for their lines of pistons: a line of less-expensive pistons, a middle-of-the-road line and a line of high-end professional pistons.

Some companies also offer what may be considered a fourth line, consisting of custom pistons-usually commissioned by the highest levels of racing.

The difference in price between a top-of-the-line piston and its less-expensive cousins is determined by the material used to create the piston and whether the piston is cast, hypereutectic or forged.

Shawn Mendenhall of Probe Pistons in Torrance, Calif., explains the differences between forged and cast pistons. “Cast pistons have a mold that aluminum is poured into and allowed to harden in. A forged piston is made from extruded bar, and then a 55-ton press basically squishes it into shape. The difference is this: if you put a cast piston under a microscope, since it’s not cast with any pressure, it’ll have porosities and microscopic air pockets. By forging it, you’re using extreme pressures to get rid of these flaws and in turn making a much higher-quality and stronger piston. It will also make the grain structure flow in the direction you want it to, which will give it more strength where you need it.”

As for hypoeutectic, eutectic and hypereutectic, those are metallurgical terms that basically describe the amount of silicon present in the piston material and the way in which it is structured in the piston.

Hypoeutectic describes a molten mixture of alloy that contains a low quantity (up to around 10 percent) of silicon-to-aluminum ratio where the silicon can completely dissolve. Manufacturers don’t use hypoeutectic alloy much for cast piston construction.

The term eutectic means that the piston contains around 12.5 percent silicon, which is just about the point of total dissolved silicon saturation. This type of piston is common in production car engines. With older piston designs that have conventional ring lands, there is little need to have any more than this simple, reliable material.

A hypereutectic alloy is similar, but it has a much higher degree of silicon in its makeup, usually around the 16-18 percent mark. This allows for a high degree of free silicon in the end piston, lending itself to improved scuff resistance and, importantly, a relatively low expansion rate.

The downside to silicon in a piston is decreased durability.

Improvements Over Stock

Of course, it’s generally acknowledged that even the less-expensive aftermarket piston lines are a big improvement over stock pistons, which are often made of cast aluminum.

Explaining the aftermarket’s manufacturing superiority, Beeri Meza of Arias Pistons in Gardena, Calif., notes, “Our pistons are aftermarket, and as such, they give the customer a stronger piston to handle typical power-gaining setups such as turbo, supercharged or nitrous applications. Whenever engine builders want a piston not offered from OEMs, they come to us and change things such as cranks and rods, which in turn change pistons and their respective bores and/or compression ratios.”

It’s important to note that no piston alone will increase an engine’s efficiency and power as much as the synergic effect that comes from also using the other aftermarket internal engine components Meza mentioned.

Peter Calvert of CP Pistons in Irvine, Calif., says, “CP pistons are manufactured from forgings rather than castings. The forging process makes them structurally stronger and lighter. CP pistons can also be manufactured in any bore size and compression ratio.”

That flexibility is just as valuable an asset to aftermarket pistons as their superior alloys being forged.

Scott Sulprizo of KB Pistons in Carson City, Nev., provides an example to explain that point. “If you go to GM and buy an engine from them, they have a stock layout for that engine. But let’s say you go to my Premium Series Forged Line. On 350 Chevys, I’m offering different rod lengths and five different piston combinations for each rod length. So, what the aftermarket does is give the engine builder the flexibility to build the engine the way they want.”

That allows a skilled engine builder to make adjustments to an engine for a particular race format, track or other factors such as elevation. The higher the level of engine, particularly those made for competition, the more likely a builder is to take advantage of varied options for different combinations.

To further make his point, Sulprizo uses the 350 Chevy five-point seven-inch rod as an example. “For that one rod, I offer five pistons in our Premium Forged Series line. If we go to the Forged Head Relief (FHR) line, same motor and same application, I offer another four pistons. If we go to the Claimer Series, I offer another five pistons, and if I go to the hypereutectic series, I offer 16 more. For that one rod, I offer 30 different pistons.That’s tremendous flexibility.”

Forged, Hypereutectic and Cast Materials

However, flexibility is only one aspect contributing to the advantages of aftermarket pistons.

Trey McFarland of Mahle Motorsports in Fletcher, N.C., notes, “In most cases, it’s the fact that our pistons are forged.”

They will also usually have a more aggressive profiling and other attributes that have been built into their design for strength to handle the added power and the potential detonation that builders see when tuning a high-end performance application.

“It’s mostly strength, but also compression ratios. Stock pistons are conservative whereas forged pistons are application-specific,” says McFarland.

Those pistons aren’t forged from an average material. McFarland says that nearly all manufacturers use two basic aluminum alloys.

“There’s a 4032 alloy, which has a medium level of silicon. It’s not like one of the cast hypereutectic pistons; it’s a eutectic alloy. It’s got some silicon in it, but it’s only in the 13-percent range. Then, there is the 2618 alloy, which is what most custom pistons are made out of, and that has very low silicon content.”

McFarland goes on to explain that the two alloys are very similar, but they do have a couple of differing attributes-there isn’t one that’s better than the other, it just depends on the application.

He says the 2618 alloy is what the industry has known as the high-end performance forging material for years. A lot of people still share the view that if you don’t have a 2618 alloy piston, then you don’t have a true full-blown performance piston.

“But that’s not really true,” says McFarland. “Where that comes from is drag racers, who were the first to test the outer limits of performance in the engines because of the short race durations. In a four-second race, they can push things a lot further than someone who has to go 500 miles in a NASCAR stock car or drive for 24 continuous hours in an endurance race.”

The Piston Killer

The 2618 alloy is a softer, more malleable material, and it will endure detonation longer. Detonation, says McFarland, is the piston killer.

“Detonation is basically an extreme pressure spike in the combustion chamber. It’s almost like taking a piston and hitting it hard, really hard, with a hammer. A softer material will give a little, while a harder material, such as 4032, tends to fracture. The 2618 alloy is also stronger when cold, and in a drag application it’s never run very long, giving it another advantage, but that’s about it for its advantages over 4032,” says McFarland.

He adds that because 2618 is a little softer than its counterpart, under prolonged periods of high heat, its structural composition will move around a little more, making it difficult to keep it working at its optimum performance level over the long haul. Over repeated heat cycles, the 2618 starts to drop like a rock in strength.

“It’s truly a racer’s alloy. They’re not long-term pistons for use on the street,” says McFarland. “The 4032 alloy is harder. Its strength does not drop off as quickly over a number of heat cycles, so if you’re a street-strip guy, it’s a better alloy. It’s still a forged piston, and it’s still considerably stronger than a cast piston. It is still a performance alloy. We’ve made F1 pistons out of 4032, and the 24-hour endurance cars run 4032 alloy.”

Meza notes: “The 2618 is used for racing applications, also known as ‘non-silicon pistons,’ and the other, 4032, is used for creating what are known as high-silicon pistons. Non-silicon pistons, made from 2618 aluminum, are better-suited for high-performance (extreme abuse) and race applications (all types of racing). High-silicon pistons are better suited for street applications where power gains are significant but not extreme.”

Mendenhall adds that there are some high-end factory performance cars that come with forged pistons, but most factory pistons are cast hypereutectic.

“Probe’s Factory Replacement Series (FPS) line is made out of forged 4032 aluminum. Forged 4032 aluminum maintains a lower expansion coefficient compared to 2618 alloys allowing tighter piston-to-cylinder wall clearance, which helps eliminate any cold-start issues and maintains better ring seal. Probe’s Sportsman Race Series (SRS) are all made out of forged 2618 aluminum, which has a higher expansion coefficient and is a more malleable material, which will enable the piston to handle much higher horsepower and temperature loads including turbos, superchargers and nitrous. Cast and forged pistons made from silicon-based materials are more brittle than 2618 and are more susceptible to failures under extreme conditions, making them less resistant to bad tunes, boost and nitrous.”

Piston Industry Challenges

Aside from differences in design, construction and materials, piston manufacturers face a few different challenges in the current economic climate.

Calvert says the economy is the biggest challenge right now, not only affecting the piston industry but all forms of manufacturing and racing in general.

“Advise customers to spend their money wisely and buy a quality piston that will perform well and be durable,” he says.

Mendenhall says staying up-to-date with all of the new applications, engine configurations and customer needs are among the main challenges piston manufacturers face today.

“Back in the old days, manufacturers would not start developing aftermarket pistons for four or five years after a new vehicle was introduced. The thinking was that no one would want to dig that far into the engine until the car was paid for and the warranty was up. That’s just not true anymore. We’ve seen people drive the car off the showroom floor, drive it for a month, and then take it to their local performance shop to have major engine modifications done to make more power. One of the biggest challenges for shop owners today is simply staying up to date with the products available to service their customers.”

Sulprizo notes that as consumers become more conscious of fuel efficiency and small cars become more popular, there are going to be challenges developing parts for these vehicles. “That’s the area I think will really change,” he says.

McFarland also notices an environmental impact, noting that piston technology has come a long way in just the last two years.

“The OE, in a number of cases, is leading the front. The EPA has gotten so stringent and so tough on the OEs, they’re having to make the engines much more efficient, controlling oil consumption, lower drag and everything else, including pistons.”

Whatever challenges being faced by piston manufacturers, make sure your shop doesn’t face the challenge of trying to sell with an uninformed sales staff. All of the manufacturers in this article are proud of the customer service they offer, so if someone is uneducated on any aspect of piston technology, there’s an avenue of learning available to them.

People who know more, sell more.

Understanding Piston Rings

With all this talk of pistons, it’s only fair to discuss piston rings as well.

Keith Jones at Total Seal in Phoenix notes that an aftermarket piston ring can complement a piston upgrade just as beneficially as any other internal component, if not more so. And don’t worry about whether they’ll have one for your application.

According to Jones, “While our forte is the gapless piston ring line, we can supply a piston ring for just about anything out there. We’ve got a very diversified inventory. Literally, from about 2-inch bore to 7-inch bore.” He adds the company creates custom rings as well.

Commenting on the advantages of the gapless piston ring, Jones says, “What we’re trying to do with any piston ring is to seal the engine better. There are many aspects to sealing the engine. One is elimination of blow-by, which limits the contaminants in the oil.  Another advantage is that the gapless ring creates more cylinder pressure, which in turn creates more torque. The engine acts like it has a higher compression ratio.”

However, where Jones says significant power gains come from is the way the rings seal on the intake stroke. Unless the engine is supercharged or turbocharged, it’s the ring’s job to seal the intake stroke; to pull the fuel and air in.

“If the ring seal isn’t as good as it can be on the intake stroke, you’re missing out and not making the maximum amount of power.  We want the ring to seal the best it possibly can, because that’s going to draw more air and fuel, just making the engine more efficient,  and that’s the whole bottom line of the gapless piston ring: making that big air pump we call an engine more efficient.”

Materials & Precise Engineering

Jones says that piston rings can be crafted from many different materials. Basic piston rings are plain cast iron, which is an inexpensive material and easy to machine, but does break and wear relatively easily.

“For durable race engines, builders use rings made out of ductile iron instead of cast iron, or one of the main different grades of steel that we offer,” he notes. “As engines get higher and higher output, they create more and more heat in the combustion chamber, and then things like detonation and pre-ignition become problems.”

He adds that high-end piston rings are made to very exact tolerances. “We do have rings built to SAE specs, but we try to hold everything to the tight end of that. Then we go beyond those specifications with our top lines, such as our Diamond Finished line, which is used in NASCAR, NHRA and other high-ends of racing. To give you an idea, for an SAE spec, the height of the ring is plus or minus five ten-thousandths of an inch. The Diamond Finished line is held to a plus or minus fifty-millionths of an inch.”

Get What You Need

In order to make sure your customers get the ring that best fits their application Total Seal has catalogs and a comprehensive Web site.

“What we really offer is great customer service by guys who know these products and know their applications because they’re racers. But, the most important question is usually the same. ‘What is it you’re doing, and what do you expect out of it?’ We’ll do what we need to do to get him in the right package,” says Jones.