Repeated cycles of searing combustion heat alternating with cool incoming air and fuel, extreme load reversal, thrust forces that slam the pistons into the cylinder walls—it’s quite amazing that aluminum pistons can survive in a performance engine at all. That they do is a tribute to today’s aftermarket material science and advanced manufacturing techniques. Technology has advanced to the point that what just a few years ago were considered custom “race-only” parts are now available at relatively affordable prices for even everyday performance use. Big-time companies like Federal Mogul/Speed-Pro are introducing new-generation lightweight mass-produced parts, while on the extreme high-end, new-tech niche manufacturers—such as HTC Products and CP Pistons—are working with advanced design processes and exotic materials to merge rocket science with race science.
Developments have focused on better materials, shaving weight, improved manufacturing precision, computer-aided design, and CNC-machining processes. Better materials and weight savings go hand in hand; stronger materials can be made lighter without sacrificing durability. And tighter manufacturing tolerances ultimately translate into better, closer-fitting parts that can be installed with reduced clearances for better sealing. Computer-aided design combined with finite element analysis allows “testing” the part before it actually leaves the drawing board, ensuring it is both as light and as strong as possible. With CNC milling, custom pistons are easier than ever to make, with lead time often in days instead of weeks and months. And what used to be custom orders are now in many cases in-stock items.
Pistons
The first step up from the common cast piston is the hypereutectic casting, which is strengthened with additional silicon content added to the aluminum brew. As with a conventional cast piston, a properly designed hypereutectic piston permits relatively tight piston-to-wall clearances, making for less noise under cold-start conditions.
All cast pistons—be they standard or hypereutectic—are made by pouring melted aluminum into a mold that shapes the slug into a piston. In contrast, forged pistons are formed using a giant press that takes a block of metal and pounds it into shape under thousands of tons of pressure. The tooling needed to do this is much more expensive than the tooling used to make a casting, and it wears out quicker. This makes forged pistons more costly than castings.
However, forgings have inherent advantages in terms of density, ultimate strength, and durability. Forging eliminates metal porosity, improves ductility, and generally allows the piston to run cooler than a casting. Within reason, forgings can be lightened without adversely affecting structural integrity. However, forged pistons expand and contract more under changing temperatures, so they traditionally require greater piston-to-wall clearance than cast pistons. In recent years, CNC-machining processes, diamond tooling, and careful attention to piston skirt profiling has given piston makers the ability to finely adjust skirt contact areas for more even loading. Barrel-type profiles now accommodate greater expansion at operating temperature. One result is that today’s short-skirted pistons have better contact areas than the old long-skirt designs, and wear is reduced even as piston-to-wall clearances are tightened up.
Forged pistons are generally made from one of several different aluminum alloys, with each offering different benefits depending on the application. The two most popular alloys are 4032 and 2618. Speed-Pro typically uses VMS-75, which is fairly close to 4032—both contain about 11 percent silicon, which helps with ring groove and skirt durability. These are the best choice for applications expected to have decent longevity, such as street vehicles and entry-level bracket racing and oval track combos. Although 2618 has better high-temperature characteristics, it contains virtually no silicon. This material expands and contracts more, so greater bore clearances are needed to prevent scuffing. Pistons using 2618 are best suited to nitrous, blowers, or higher end race applications where frequent inspection and replacement are not a problem.
A recent innovation is “Ultralloy,” a secret patented ceramic-aluminum alloy presently available from HTC Products, a premier manufacturer and distributor for most brands and types of cranks, rods, pistons, and rings. The silicon particles in Ultralloy have unprecedented uniformity in terms of their size, shape, and dispersion in the aluminum matrix. The new alloy’s strength is on par with titanium (and costs less) and parts can be made much lighter.
One of the most important advantages of forged pistons is what happens at the point of piston failure. Under extreme conditions—like detonation—forgings tend to “go plastic” and fail gradually. You generally have time to replace them before the entire engine is toast. Hypereutectics, although relatively strong in terms of ultimate tensile strength, have less ductility and are prone to fracture when their limits are exceeded. However, a lightweight hypereutectic piston specifically designed for high-performance use can withstand considerable cylinder pressures if the tune is right. When considering which style of piston is right for your application, you should consider how much abuse the piston will see, your budget, and the need to remain competitive in your form of racing. Sustained heat is the biggest piston-longevity limiter.
Rings
Piston rings perform a number of important functions. They seal the gap between the piston and cylinder wall to prevent combustion gases from blowing by into the crankcase. They stabilize the piston as it travels up and down in the bore. They help cool the piston by transferring heat into the engine block. And they scrape oil off the cylinder walls. That’s a tall order, and in recent years the theory on how to make rings best carry out these tasks has undergone revision.
Old-school thinking followed a brute force approach: Make everything as rigid as possible to force the rings into contact with the walls. Today, the trend in current production and racing engines is towards a more flexible ring package that better conforms to the cylinder wall. Back in the musclecar days, most production engines used a 5/64-5/64-3/16-inch package. The 1/16-1/16-3/16-inch packages were for all-out racing. These days, Detroit automakers and many racers are gravitating toward even thinner “metric” rings. Standard-tension oil rings have been replaced by low-tension rings. Many of the new ring packages feature reduced radial wall thickness. Besides decreasing friction, this makes for a more stable package—assuming the piston rings, piston profile, and cylinder wall finish take advantage of these improvements. In the custom piston world, most build-to-order pistons can be ordered for reduced radial thickness rings; otherwise, spacer stock can be used to convert conventional pistons.
Ring groove design is far more important than it may appear at first glance. Properly designed ring grooves have a small degree of vertical uplift, which compensates for uneven temperature growth as the piston reaches operating temperature. Ring groove smoothness is likewise extremely important; any waviness or roughness causes poor ring seal and can lead to microwelding—a destructive situation where, under extreme pressure, the rings momentarily attach themselves to high spots on the ring groove. There also should be a small radius where the vertical and horizontal portion of the ring grooves meet. Pistons without this radius are more prone to groove distortion and ring land breakage.
Thinking on piston ring gaps has also changed. In the old days, second ring gap specs were tighter than those for top rings because they didn’t see as much heat. But this didn’t account for inter-ring gas-pressure buildup between the top and second rings. If the pressure between these rings equals or exceeds the pressure above the top ring, it can cause the top ring to lift off the bottom of the piston ring groove and lose contact with the sealing surfaces. It also inhibits the ring’s ability to transfer heat from the piston. To keep inter-ring pressure from becoming a problem, the current trend is to create an easy escape path for the built-up pressure by gapping the second ring larger than the top ring. Another benefit is that because gas pressure is now directed downward towards the sump, any oil that has collected in the ring pack areas will go with it.
Of course, normal ring wear causes the gaps to open up, allowing more combustion gases to escape. At least one ring manufacturer—Total Seal—offers gapless rings. Traditionally, these gapless rings went in the second groove along with a conventional top ring, but ring technology refinements plus the new thinking on ring sealing has led Total Seal to revise this installation scheme and introduce a new line of gapless top rings that achieve significantly less blow-by under real-world running conditions.
The ultimate in ring seal is drilling the pistons for gas ports. Compression rings normally need about 0.002-0.004-inch (vertical) ring-to-groove side clearance to allow cylinder pressure to get behind the ring and force it to seal against the groove and cylinder wall. Gas ports apply combustion pressure directly to the back of the ring, allowing the virtual elimination of side clearance. Since the ring is restrained by the groove itself, there’s less opportunity for high-rpm ring flutter. Very thin, narrow, and lightweight 0.043-inch–thick rings are needed to reap gas-porting’s full benefits. Gas ports work best with short piston-compression heights (under 1.200 inches) on engines running 7,000 rpm or higher. The major drawback is that all this positive pressure greatly shortens ring life, so it’s not recommended for street use.
No matter the specific thickness and configuration, most high-performance and racing engines now use moly-faced rings in the top groove. Plasma-sprayed moly over a ductile-iron base material is the preferred choice, but steel is becoming more popular because it’s at least as strong and easier to machine.
Chrome-plated rings still have a place in off-road or dirt-track applications. Just as high-end pistons are now machined to close tolerances, many racers now custom-prep (remachine, if you will) piston rings to higher tolerances to reap the full benefits of the new high-tech pistons. For example, precision ring grooves allow a reduced back clearance if ring thickness tolerances are likewise more tightly controlled. Custom-prepped precision-gappable rings are also offered by several aftermarket ring makers such as Total Seal.
Matching rings and piston design only reaffirms what we’ve been stressing for years: When it comes to engines, there’s no magic bullet or individual component. Everything has to be considered as part of a total package. Lightweight, close-tolerance pistons demand higher-quality rings. But to work they require a higher level of cylinder wall preparation. And reducing friction by running low-tension oil rings may mandate trick oil pans with windage trays, crank scrappers, adjustment of bottom-end bearing clearances, lightweight synthetic oils, and even positive crankcase evacuation pumps. Nevertheless, custom piston makers assert that the right high-end piston and ring combination can be worth up to 30 hp on a 1.5hp/ci engine if the rest of the combination is optimized to take full advantage of them.
