If internal combustion is a war, then pistons are quite literally on the frontlines. Converting reciprocating energy into rotating force means that the four-stroke process tries to both eject the pistons out of the block deck and blow them out through the oil pan in brutal succession. At 6,000 rpm, this melee goes down 100 times each second. Furthermore, advances in cylinder head and valvetrain technology allow modern engines to turn more rpm and pack more cylinder pressure than ever. To top it all off, forced induction and nitrous often intensify the beat-down, and pump gas is getting worse by the day. Given these formidable circumstances, it's truly amazing that piston failure is so rare these days. It's companies like JE Pistons that are directly responsible for this impressive feat.
While the nickname "slugs" suggests that pistons are nothing more than archaic hunks of forged aluminum, the technology involved in their development is astonishing. To find out what it takes to design and manufacture premium-grade, race-bred aftermarket pistons, we tapped into the collective knowledge of Sean Crawford, Stephen Golya, and Alan Stevenson of JE Pistons. As we found out, there's far more to piston design than merely pounding an aluminum ingot into a cylindrical shape and calling it a day. Some of the design elements of a piston that hot rodders typically obsess over are insignificant, while factors that most people aren't even aware of can be the difference between being a hero and blowing up. To ensure you pick the right pistons for your next engine build, keep reading.
Race-Bred Technology
From Top Fuel and Sprint Cup to World of Outlaws and Pro Stock, JE has always been involved in high-end racing. Naturally, this magnitude of involvement has elevated the company's breadth of knowledge, which directly benefits the average hot rodder. "When it comes to professional racing, particularly at the top tiers, you can't get away with poor designs, performance, or quality. The expectations are so high that most piston manufacturers could not supply our professional customers with an acceptable product," Crawford says. Every one of JE's manufacturing steps, from design to final inspection, is strictly controlled to ensure accuracy, consistency, and quality.
Company Evolution
JE Pistons was incorporated in 1947, and although the company has grown over the years, its primary focus remains the manufacturing of high-quality custom pistons for the racing market. "In order to keep our products at the top, we've had to make constant improvements to our engineering, manufacturing, and inspection processes over the last 62 years," Crawford explains. "We started out by manufacturing both cast and forged pistons, but have since switched to forged and billet units exclusively due to the demands of modern racing engines. To ensure we're offering the best-quality pistons on the market, our manufacturing has also evolved. Decades ago, JE used all manual machines to cut pistons, but today we utilize over 55 modern CNC machines."
In addition to helping the company fine-tune its quality control, Crawford says that many new piston innovations developed for professional racing have trickled down to JE's off-the-shelf parts. "The most recent example of this is our SRP Professional piston line. We integrated features such as a new lightweight forging design, an advanced low-friction ring package, shorter wrist pins, a unique skirt profile, and ultra-flat ring grooves. On our Chevy 383 test engine, the new design resulted in a weight reduction of 63 grams per cylinder and a 13hp gain!"
Piston Alloys
Two of the most common alloys used in pistons are 2618 and 4032 aluminum. JE manufactures pistons from both alloys, and Crawford says that the pros and cons of each determine the applications in which they should be used. "The main differences between the two are their material composition and thermal and fatigue characteristics. JE's 4032 performance piston alloy has a silicon content of approximately 12 percent, while 2618 has less than 0.2 percent," says Golya. This means that the 2618 alloy expands approximately 15 percent more than the 4032 alloy when exposed to elevated temperatures. Some people prefer 4032 alloy for their street-driven vehicles since they require less cold clearance and reduce startup noise. Mechanically, both are very similar, with 2618 having higher strength at all temperatures. "When selecting the proper material for a piston, we don't just consider the room temperature strength but also strength at operating temperatures. This is where 2618 outperforms 4032, as it's significantly stronger at temperatures of 500 degrees F and more. Since many racing engines operate above that temperature range, 2618 has the clear strength advantage for these applications. Consequently, 2618 is used extensively in Formula 1, NASCAR, and ALMS, while 4032 isn't. Currently we're testing new materials that could potentially yield the benefits of both alloys."
Skirts
Piston skirts provide stability within the bore, but also create friction. When designing a piston, the trick is achieving a balance between maximizing stability and minimizing friction. In essence, the piston skirts allow the piston to perform its primary and secondary movements. The primary movement of a piston is when it traverses from TDC to BDC and back up to TDC again. Its secondary movement is the effect of the piston rocking in the bore. The rocking effect is caused by frictional and viscous drag, piston center of gravity location, constantly changing side loading, and changes in temperature. The key in controlling skirt wear, reducing parasitic losses, and improving ring seal is predicting the secondary motion. Frictional losses associated with the piston skirt are substantially influenced by its width and length, hence the surface area of contact between the skirt and cylinder bore. As the contact area is decreased, the viscosity will tend to fall and so will frictional force. However, as the bearing area and viscosity decrease, so does the oil film thickness. If the film thickness approaches the sum of the asperity heights on the two surfaces, this will result in boundary lubrication and an increase in friction. The contact area and the access of oil to that area therefore need to be optimized. There are two ways to reduce the contact area: reducing skirt length and changing the surface shape of the skirt. The first will reduce friction but increase the secondary motion effects influencing ring seal. The second is more successful, as the contact patch and secondary motion can be reduced. At JE, we are constantly developing new skirt shapes that focus on these factors.--Alan Stevenson
Ring Gap
Engine builders have been debating second ring gap for quite some time. For most applications, JE says it's better to run a larger gap on the second ring than the top. "Many tests have proven that a larger second ring gap increases the top ring's ability to seal. The larger second ring gap helps release cylinder pressure that passes by the top ring," Stevenson explains. "Without this relief, the pressure can lift the top ring and impair ring seal. In some situations, the top ring seals well enough to eliminate the need for a larger second ring gap. This is typical in naturally aspirated engines that do not experience high cylinder pressures. Ring bore conformity, thickness, and tension play into the equation as well, but for most performance engines a larger second ring gap is preferred."
Oil Control
The benefit of maximizing connecting rod length is debatable, but the practice persists nonetheless. While long-rod motors push the wristpin into the rink package, JE says that it isn't as detrimental to oil control as some people claim. "With the use of an oil rail support, there should be no problems maintaining oil control when the pin bore intersects the oil ring groove," says Crawford. "In fact, the extra openings can actually help the oil drain properly. Some piston designs allow the pin bore to intersect the third ring land, which can cause oil control problems since oil is fed above the oil control ring. For this reason, all of our pistons are designed with the pin bore below the third ring land to prevent this issue."
Wristpins
Some people tend to overlook the role of the wristpins in the overall piston equation. However, since they endure such heavy loads, they're a critical link in the rotating assembly. The wristpin sees the loading that each piston puts upon its respective crankshaft big end journal, a loading that includes inertial forces as well as the effect of combustion pressure.
The pin is loaded both by the rod and by the piston in a complex combination of forces varying both in magnitude and direction, the result of the applied and reactive loads. The loading on the pin promotes bending along its axis and also ovalization, and the combination of both can lead to frictional binding and twist.
Pin stiffness is a vital consideration. It not only impacts its ability to function as a journal; it also influences the stiffness of the entire piston and pin assembly. Increased pin stiffness can actually translate into a more stable ring platform, resulting in improved oil control and reduced blow-by.
A fully floating pin is able to rotate about its main axis and slide along that axis. Stock pins are often an interference fit in the small end. An interference fit means there must be heating of the small end each time the pin needs to be removed, which is not very practical for a race engine that is frequently rebuilt. Where there is an interference fit, the pin material upon which bearing and bending loads are imposed is subject to greater fatigue effects because it is always the same fibers being cyclically loaded and unloaded. Conversely, with a floating pin the fatigue cycles are more evenly spread around the outer surface fibers of the pin. In addition, if the pin is allowed to rotate, its velocity relative to the individual bearing surfaces will be lower. Rotation also has the effect of moving the oil around within the pin bores, reducing the possibility of dry spot formation.--Alan Stevenson
Gas Porting
Gas ports are small holes that feed cylinder pressure into the top ring groove. Their purpose is to allow pressure from behind the top ring to increase the sealing effect. Without gas ports, the top ring seals itself primarily from the pressure acting on its top face. Gas ports are usually needed in engines with high cylinder pressure or in conjunction with very narrow top rings. JE offer two types of gas ports, vertical and lateral. Vertical gas ports are most popular in drag race application where maximum pressure behind the top ring is desired. Lateral gas ports provide slightly less pressure on the ring and are more desirable in endurance applications. Both styles of gas ports significantly reduce ring life and are not recommended for street use. In addition to gas ports, JE also offers "gas distribution grooves." This is a small groove that intersects the entire upper half of the top ring groove and helps evenly distribute pressure around the circumference of the top ring.--Stephen Goyla
Compression Height
When juggling engine design parameters such as crankshaft stroke, rod length, and block deck height, the compression height of the pistons must be taken into account. Defined as the distance from the centerline of the wristpin to the top of the piston crown, compression height that's too short can conceivably compromise durability in the wake of intense cylinder pressure. According to JE, however, advances in modern alloys and forging techniques mean that "thick" pistons aren't always necessary.
"With shorter compression heights used in many of today's performance engines, the piston design should not suffer as long as there is enough room above the wristpin to provide the proper deck thickness and rod clearance. We have off-the-shelf pistons with compression heights of 1.000 inch that have proven to be very reliable," explains Crawford. "Occasionally, we run into a situation were there isn't enough room to meet our requirements. This is most common on forced-induction engines that require a very large dish and a short compression height. In these situations it is necessary to find a way to increase the compression height."
Power-Adder Pistons
Some people assume that as long as a piston is forged, it can handle anything you throw at it. Although forged slugs are much stronger than their cast counterparts, they still have their limits. Therefore, JE recommends using dedicated nitrous or blower pistons if you plan on using big-time power-adders on your engine combo. "The major difference between a naturally aspirated and a nitrous or blower motor is cylinder pressures and operating temperatures, so it's important to design a piston for this type of application," says Golya. "Higher pressures call for pistons with thicker crowns and more structural stiffening. In addition, the wristpins should be thicker in diameter and the rings will require more tension, thus making them thicker and more durable. We choose to manufacture most of our racing pistons from 2618 alloy, but also offer 4032 alloy pistons that are compatible with moderate forced-induction and nitrous use."
Design Factors
The two most popular piston designs for internal combustion engines are full round or forged side relief (FSR). Each design can offer unique benefits depending on the desired cost, application, power, and engine speed. A full round has a singular central void with a continuous circular band joining the skirts. An FSR has multiple external voids in addition to the central void. Typically a full round piston is the easiest to manufacture, is more affordable, and offers a greater degree of robustness. The FSR piston is designed mostly for specific applications that require a high level of performance and feature more complicated voids. It can be made lighter and stiffer and offers less skirt area than the full round. This is advantageous for inertial response and reduced friction.
Material quality is equally important to piston design. All of JE's material stock is physically and chemically certified and is traceable back to the producer. We verify physical properties and dimensional integrity in our onsite inspection laboratory prior to manufacturing.--Alan Stevenson
Chamber Volume
The trend in many high-end race motors is the use of small combustion chambers and a dish piston to achieve a relatively high static compression ratio. Compared to a more traditional setup that employs larger chambers and either a flat-top or domed piston, this arrangement yields dividends in efficiency, as the piston crown functions as part of the combustion chamber and the spark plug is positioned more centrally in the chamber. "The combination of a smaller chamber and bigger dish is more thermally efficient and reduces the amount of unburned fuel," Crawford explains. "Shorter flame paths generate higher combustion temperatures and increase fuel efficiency, which all goes to increase the energy output and reduce emissions."
Coatings
Pistons coatings are applied when the surface of the aluminum can't cope with the environment. JE's thermal barrier crown coating helps to maintain surface hardness and resist surface erosion and pitting due to detonation. This coating may allow a piston to last longer under high temperatures. Although the crown coating is beneficial to the piston, the engine builder should consider the effects the coating may have on the total system. Since less heat is being dissipated through the piston and rings, it is being reflected elsewhere in the combustion chamber. This extra heat may have an effect on other components in the engine. Many builders choose to coat their cylinder head chambers, valve faces, and exhaust ports for this reason. Skirt coatings help reduce cold start scuffing, surface friction, and wear. In some cases a skirt coating can also be used to decrease piston to cylinder wall clearance safely. JE's most popular skirt coating is our proprietary Tuff Skirt. This coating is up to 0.0005 inch thick per surface and is designed to have improved wear resistance when compared to other coatings on the market.--Stephen Goyla
Dishes
When it comes to dished pistons, the terminology can be confusing. While both dish and dome pistons reduce the static compression ratio, there are important difference between the two. "The main difference between the two is their shape, with a dished piston having a simple circular dish and inverted domes having a special shape that matches the cylinder head chamber. Many people refer to inverted domes as D-shaped dishes," explains Golya. "Contrary to some of the information out there, the shape of the dish has very little impact on valve deshrouding and airflow. Heat distribution has more to do with the thickness of the crown underneath the dish than the shape of the dish itself. The design and contour of the underside of the piston play a big role in how the crown distributes heat. Inverted domes are commonly known to provide greater combustion efficiency than full round dishes because they direct the air/fuel mixture into the chamber better, although this effect is greatly minimized with forced induction. Conical and spherical dishes have shown some power gains, but not across the board with every combustion chamber design. Unfortunately, their thin center sections and thick outer sections trap additional weight in the piston crown."