The internal-combustion process is downright brutal, and the pistons are quite literally on the front lines of the battlefield. The nature of converting reciprocating energy into rotating force means that the four-stroke process tries to 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 the quality of pump gas has degraded in recent years with higher ethanol content and lower octane ratings. Given these formidable circumstances, it’s truly amazing that piston failure is so rare these days.
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Although something nicknamed “slugs” suggests that pistons are nothing more than archaic hunks of forged aluminum, the technology involved in their development is astonishing. 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 other factors that most people aren’t even aware of can be the difference between being a hero and blowing up. With the easy horsepower and high-RPM potential of the Gen III/IV small-block, selecting the right pistons is an important step in successfully designing any engine combination.
Like the stock crank and rods used in most LS engines, the factory pistons are adequate up to roughly 500 hp. They’re cast from hypereutectic aluminum, meaning that they incorporate a highsilicon content of about 11 percent. Compared to a traditional cast-aluminum piston, hypereutectic pistons are stronger, more resistant to detonation, and have half the thermal expansion. This allows for tighter piston-to-wall clearance, as well as reduced blow-by and piston slap. The downside is that the higher silicon content makes them more brittle, and hypereutectic pistons are not nearly as strong as aftermarket forgings.
The LS9 is the only factory Gen III/IV engine that came equipped with forged pistons from the factory. Nevertheless, because larger-diameter pistons must be used anytime a block is overbored, the virtues and drawbacks of stock pistons are somewhat irrelevant in a stroker motor.
For stroker builds where pinching every last penny is essential, companies, such as Keith Black, offer quality hypereutectic pistons for as little as $300. They’re built from a stronger alloy than factory pistons and are subjected to a T6 heat-treating process that makes them 30 percent stronger. Available in many popular bore sizes, these pistons work just fine in most naturally aspirated stroker buildups. That said, it makes little sense to invest in a forged crank and rods, only to top them off with a set of cast pistons. A more effective means of budgeting is to match a cast crank with forged pistons because the pistons are naturally exposed to more extreme abuse.
For the ultimate in durability, however, forged pistons are substantially stronger than their hypereutectic counterparts. At $600 to $800, forged pistons are at least twice as expensive as hypereutectic pieces, but they’re well worth the premium in a high-performance stroker motor.
During the manufacturing process, they start out as hunks of aluminum that are compacted in a mold by a press. This eliminates porosity and forces the molecules of aluminum together, creating a denser and stronger material. Afterward, they’re precisely machined into shape. A key benefit of the forging process is that it yields an extremely robust piston that’s actually lighter than a casting. Because the overall structure of a forged piston is stronger, this allows removing material from the piston skirts to reduce mass.
Two of the most common alloys used in piston forgings are 2618 and 4032 aluminum, and many companies manufacture pistons from both materials. The main differences between the two are found in their material composition and thermal and fatigue characteristics. Pistons made from 4032 alloy have a silicon content of approximately 12 percent, and 2618 pistons have less than .2 percent. 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 street-driven vehicles because the pistons 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, manufacturers factor in its strength at both the room temperature and operating temperature. According to its composition, 2618 outperforms 4032 by a large margin, as it’s significantly stronger at temperatures of 500 degrees F and above. Because many racing engines operate above that temperature range, 2618 has the clear strength advantage for these applications. Consequently, 2618 is used extensively in Formula One and NASCAR, and 4032 is better suited for naturally aspirated street motors that don’t see much track time.
Piston skirt design is often a compromise between providing piston stability within the bore and reducing 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.
To control skirt wear, reduce parasitic power losses, and improve ring seal, piston manufacturers must accurately predict the secondary motion of the pistons. Frictional losses associated with the piston skirt are substantially influenced by its width, length, and by how much of its surface area contacts the cylinder bore. As the contact area is decreased, viscous drag from the oil tends to fall, along with frictional forces. However, as the loadbearing area and viscosity decrease, so does the oil film thickness. If the film thickness approaches a critical point, it results in boundary lubrication and an increase in friction. Consequently, the contact area and the access of oil to that area need to be optimized. The two methods of reducing the contact area include reducing skirt length and changing the surface shape of the skirt. Reducing skirt length decreases friction, but it increases the secondary motion effects that impact ring seal. Changing the shape of the skirt is more effective, as the contact patch and secondary motion can be reduced.
Aftermarket pistons continue to improve by the day, and gas porting is one of the latest innovations now offered by most manufacturers. However, gasported pistons are not always a good idea for street cars. 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 of the rings. Without gas ports, the top ring seals itself primarily from the pressure acting upon its top face.
Gas ports are usually needed in engines with high cylinder pressure or in conjunction with very narrow top rings. They can be vertical or lateral in design, each with its own benefits. Vertical gas ports are most popular in drag race applications 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, some piston manufacturers also offer gas distribution grooves. These small grooves intersect the entire upper half of the top ring groove and help evenly distribute pressure around the circumference of the top ring.
The job of keeping the connecting rods attached to the pistons falls on the wrist pins. They don’t look like much, which makes it easy to overlook the important role they play in the overall piston equation. Although pistons transfer energy to the connecting rods, the wrist pins serve as the only link through which this transfer takes place.
The wrist pins see the loading that each piston puts upon its respective crankshaft big-end journal, which amounts to a combination of inertial forces and combustion pressure. The pin is loaded by both the rod and the piston in a complex combination of forces varying both in magnitude and direction. The loading on the pin promotes bending along its axis and also ovalization, and the combination of them can lead to frictional binding and twist. Consequently, pin stiffness is extremely important. Pin stiffness not only impacts the pin’s 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.
Premium aftermarket pistons feature full-floating pins that can rotate and slide inside the wrist pin bore. On the other hand, the pins used in many stock LS pistons are interference fit in the small end. This requires heating of the small end each time the pin needs to be removed, which isn’t very practical for a race engine that is frequently rebuilt. Additionally, press-fit pins are subjected to both load-bearing and bending forces. Conversely, with a floating pin, the fatigue cycles are more evenly spread around the outer surface fibers of the pin. 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.
Defined as the distance from the centerline of the wrist pin to the top of the piston crown, the compression height of a piston must be changed whenever the connecting rod length, crankshaft stroke, and the deck height of the block are changed. The reason for this is simple. As crankshaft stroke or rod length is increased, the wrist pin bore of the piston must be repositioned closer to the piston crown. Otherwise, the top of the piston will protrude through the top of the block at TDC. Likewise, whenever stroke or rod length is decreased, the wrist pin bore of the piston must be lowered to prevent the piston crown from sitting below the top of the block at TDC. To make things easier for engine builders, piston manufacturers usually publish the compression heights of their pistons, along with the stroke and rod lengths that they’re compatible with. Calculating the piston compression height is as easy as subtracting the length of half of the stroke and the length of the rods from the block deck height:
Compression Height = Deck Height – (Rod Length + 1/2 Stroke
Using this formula, a 402 stroker motor that uses a standard-deck 9.240- inch LS block, a 4.000-inch stroke, and 6.125-inch connecting rods would need pistons with a compression height of:
9.240 – (6.125 + 2) = 1.115 inches
Decreasing compression height also decreases weight, alleviating stress on the rods at high RPM. For example, a JE forged LS piston with a 1.340-inch compression height weighs 434 grams, while a piston from the same family with a 1.050-inch compression height weighs 390 grams. Multiply that 44 grams of weight savings by eight pistons, and the reduction in reciprocating mass of 352 grams is significant. This is one advantage of using a longer connecting rod that can’t be refuted. That said, reducing compression height too much compromises piston strength, so there is a practical limit to how much this critical dimension can be reduced.
Because the area of the piston above the wrist pin must accommodate all three piston rings, reducing compression height forces them closer together. As a result, the wrist pin bore actually intersects the oil ring groove, which necessitates reinforcing it with a separate support rail. Such an arrangement has proven to be durable in both street and race motors, but the bigger problem at hand is that having too short of a compression height pushes the top ring closer to the piston crown. The closer the top ring is to the top of the piston, the more combustion heat it absorbs. Not only is that heat hard on the wrist pins, but it can also distort the top ring and compromise its sealing ability.
Flip through the catalog of any piston manufacturer, and it’s extremely rare to find any off-the-shelf piston offered with less than a 1.000-inch compression height. That figure is generally accepted as the bare minimum. Thanks to advances in modern alloys and forging techniques, as long as a piston has at least a 1.000-inch compression height, it has the ability to provide acceptable oil control and cylinder seal. Even if you don’t subscribe to the long-rod theory, the popularity of engine combinations that utilize long connecting rods has forced the aftermarket to maximize piston strength with a minimum of compression height.
Dishes and Domes
For the most part, the shape of the piston crown is determined by an engine’s target compression ratio. Flattop pistons are most prevalent, as the desired compression ratio can usually be achieved by manipulating head gasket thickness or combustion chamber volume. This isn’t always possible, however, in which case the piston crowns can be dished or domed. When it comes to dished pistons, which are sometimes called inverted dome pistons, the terminology can be confusing. This generic phrase refers to a piston where a portion of the crown is recessed into a “D” shape and sits below the deck height at TDC. Dished pistons reduce the static compression ratio, so they’re most commonly used in forced-induction applications. Likewise, increasing the size of the bore and stroke bumps up the compression ratio, so larger motors often require a dish to maintain a pump-gas-friendly compression ratio.
Some pistons feature a simple circular dish, as opposed to a D-shaped recess, which is referred to as a concave dish. The advantage of an inverted dome piston is that its D-shaped recess closely follows the contours of the combustion chambers in the cylinder heads. The contour of the crown helps create turbulence in the air/fuel mixture during the compression stroke. This improves mixture homogenization, making a motor more resistant to detonation. With a concave dish piston, this effect is greatly reduced.
However, concave dish pistons are widely used in many high-end race motors, including in NASCAR Sprint Cup, where they have posted gains of 3 to 5 hp over inverted dome pistons. The reason for this is because concave dish pistons are used in conjunction with very small combustion chambers.
In essence, the dished portion of the piston’s concave design functions as part of the combustion chamber itself, which positions the spark plug centrally inside the chamber and allows for more even flame travel. The combination of a smaller chamber and bigger dish is more thermally efficient and also reduces the amount of unburned fuel. Additionally, shorter flame paths generate higher combustion temperatures and increase fuel efficiency, which increases energy output and reduces emissions. Even so, concave dish pistons are very uncommon in street motors because their effectiveness is highly dependent upon the shape of the combustion chamber. Also, the thin center sections of a concave dish piston trap additional weight in the piston crowns. The combination of a small chamber and a large dish can also be employed with reverse-dome pistons.
At the other end of the spectrum, domed pistons—in which the piston crown protrudes into the combustion chamber at TDC—are used in engines where maximum compression is desirable. With the current trend favoring engine combinations with lots of cubic inches and small combustion chambers, very high static compression ratios can be achieved with flat-top pistons. For instance, a 408 stroker LS motor that features a 4.030-inch bore, a 4.000-inch stroke, .039-inch head gaskets, 66-cc combustion chambers, and flat-top pistons with -4-cc valve reliefs will yield an 11.7:1 compression ratio. Depending on camshaft selection, that’s really pushing the limits of pump gas.
Consequently, the only reason to use a domed piston is in a race-gas application where the compression ratio approaches or exceeds 13.0:1, so they’re even less of a necessity in bigger-inch motors. A tall-deck stroker build that uses a 4.200-inch bore, a 4.500-inch stroke, .039-inch head gaskets, 66-cc combustion chambers, and flat-top pistons with -4-cc valve reliefs will have a 13.89:1 compression ratio. A major drawback of domed pistons is that their piston crowns tend to interfere with flame front propagation during the power stroke.
Power Adder Pistons
There are far more naturally aspirated engine builds than forced-induction and nitrous-oxide combinations. Consequently, most off-the-shelf pistons are designed for naturally aspirated engines. Even though forged pistons are remarkably stout, power adder applications call for extra-rugged pistons designed specifically for boost and nitrous. The major difference between a naturally aspirated engine and a nitrous or blower motor is cylinder pressure and operating temperatures, so it’s important to design a piston accordingly. Higher pressures require pistons with thicker crowns and more structural stiffening. Additionally, the wrist pins must be thicker in diameter, and the rings require more tension, making them thicker and more durable. For moderate doses of boost and nitrous, a 4032 alloy is sufficient, but a 2618 alloy piston is a necessity in heavy-duty power adder combinations.
With the endless combinations of bore diameters, stroker cranks, rod lengths, block deck heights, cylinder heads, and power adders that can be integrated into an engine design, there are certain instances when the ideal piston isn’t available as an off-the-shelf forging. Fortunately, piston manufacturers are more than happy to build custom pistons for your specific application for not much more money than off-the-shelf pistons. The process of making a piston is highly automated, with CNC machines whittling raw forgings into the final shape of a piston. As such, a set of custom pistons might only set you back an extra $100, and in some cases, might cost the same as a catalog piston. Furthermore, some piston companies can turn around a set of custom slugs in one or two days. By comparison, custom cranks and rods are astronomically expensive, so it’s far easier to design a piston around the rest of the rotating assembly and short-block than doing things the other way around.
In recent years, engine builders began testing the virtues of applying coatings to various engine components,including pistons. They’re typically used on the piston crown and skirts to protect the aluminum from the rigors of internal combustion. Coating the piston crown with a thermal barrier helps maintain surface hardness and resist surface erosion and pitting due to detonation. The coating also allows the pistons to last longer under high temperatures.
Although the crown coating is beneficial to the piston, engine builders must also consider the effect the coating has on the rest of the engine. Because less heat is being dissipated through the piston and rings with a crown coating, that means the heat is reflected elsewhere in the combustion chamber. This extra heat may adversely affect the combustion chambers, valve faces, and exhaust ports, which may require coating. In contrast, 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.
Reducing reciprocating weight takes stress off the crank and rods, but if saving a few grams of weight requires laying out a big wad of cash, it might not be worth it. Because weight is often directly related to strength, there’s a risk of compromising piston strength to reduce mass. According to JE, in an engine that turns 9,500 rpm and produces 1,764 psi of cylinder pressure, a 3,500-gram piston imparts 16 kilo-newtons (kn) of tensile force and 67 kn of compressive force on the small end of the connecting rod. On the big end of the rod, the tensile force is 25 kn, and the compressive force is 58 kn. With all else being equal, increasing piston weight to 400 grams changes loads to 18 kn of tensile force and 65 kn of compressive force on the small end of the rod and 27 kn of tensile force and 56 kn of compressive force on the big end of the rod. So while there is a difference in loading, it’s not always worth spending lots of money to reduce piston mass by a few grams.
Piston technology is evolving by the day. To stay on top of the latest trends, leading manufacturers are actively involved in the most elite forms of racing such as NHRA Pro Stock and NASCAR Sprint Cup. Fortunately for hot rodders, the technology and innovation gathered from on-track competition eventually trickles down into the grassroots market. Thanks to highly automated manufacturing processes, cutting-edge pistons can be had at a very affordable price. Here’s a look at some of the most popular LS pistons on the market today.
In addition to having a decorated history that dates back more than 40 years, Diamond offers what’s possibly the most comprehensive line of off-the-shelf pistons for Gen III/IV small-blocks. Even if you think your application requires a custom piston, chances are Diamond already stocks what you need. Not surprisingly, Diamond is a very popular choice among LS enthusiasts. All Diamond pistons are forged from rugged 2618 aluminum and feature offset wrist pins for quiet operation. Furthermore, Diamond pistons come in a dizzying array of compression heights and bore sizes with valve reliefs for both cathedraland rectangle-port heads. Available in flat-top, dished, and domed crown designs, Diamond’s diverse line of naturally aspirated, nitrous, and forcedinduction pistons have a proven track record of quality and durability.
Founded in 1947, JE is one of the most respected names in performance pistons. From NHRA Top Fuel to NASCAR Sprint Cup to the World of Outlaws, JE has always been actively involved in the highest levels of motorsports. Over the years, the company has evolved tremendously, leaving behind its manual cutting machines for a warehouse full of dozens of CNC machines. Like most high-end piston manufacturers, JE can build a set of custom pistons for any application, but it also stocks a full line of off-the-shelf LS-series pistons in flat-top and inverted dome configurations.
Forged exclusively from 2618 aluminum, most of JE’s Gen III/IV pistons are designed for standard-deck-height short-blocks utilizing 4.000-inch-stroke cranks and 6.125-inch rods. Available bore sizes range from 3.905 to 4.130 inches, and the majority of JE’s pistons have valve reliefs machined for standard 15-degree cathedral-port heads, although the company also offers off-the-shelf pistons compatible with 12-degree LS7 heads.
JE pistons can be had with standard skirts or the company’s proprietary Forged Side Relief (FSR) skirts. The standard full-round skirts feature a singular central void with a continuous circular band joining the skirts. FSR skirts have multiple external voids in addition to a central void. The full-round skirts are easier to manufacture, and less expensive. On the other hand, FSR skirts are pricier, but offer reduced mass and friction.
During the heyday of NHRA Top Fuel in the 1970s, just about every competitive team relied on blocks or complete motors from Keith Black Racing Engines. A true pioneer in the realm of high-end drag racing, Keith Black felt that there was a shortage of quality blocks and pistons on the market, so he made his own. The lessons learned on the track while powering legendary racers, such as Don Prudhomme, to victory are evident in the high-quality, yet affordable, pistons in the Keith Black catalog today.
For enthusiasts on a tight budget, Keith Black’s Silv-O-Lite cast hypereutectic pistons are an excellent choice in naturally aspirated applications. Technically, the Silv-O-Lites aren’t stroker pistons, as they’re designed for stock-style rebuilds. That means that they’re offered for mild .5- to 1.5-mm overbores in stock 1.330- inch compression heights for 5.3L, 5.7L, and 6.0L LS-series small-blocks. Nonetheless, for hot rodders looking to build a performance III/IV engine with a stock crankshaft, the Silv-O-Lite pistons cost half as much as a comparable forged piston set.
The company also offers forged 2618 pistons under its KB Performance label. They feature fully machined crowns and valve reliefs and drilled oil drain backs. Available in both flat-top and dished configurations, KB Performance LS-series pistons come in diameters ranging from 3.905 to 4.030 inches in the popular 1.115-inch compression height. Consequently, they represent an affordable option for 383-, 402-, and 408-ci stroker builds.
A long-time OE supplier, Mahle recently entered the LS market with a comprehensive line of quality pistons. The company offers pistons for just about every bore, stroke, and rod length combination conceivable for cathedraland rectangle-port heads. Forged from 4032 aluminum, Mahle pistons are offered in both flat-top and inverted dome designs and feature the company’s Grafal low-friction skirt coating, Furthermore, all Mahle pistons are handdeburred prior to shipment, and they require minimal prep work before installation. Further enhancing their value, Mahle includes matching rings with all of its piston sets.
For Sportsman racers and enthusiasts on a restrictive budget, JE’s line of SRP and SRP Professional pistons offers an affordable alternative to JE’s top-ofthe-line pistons. SRP pistons are forged from a high-silicon 4032 alloy for low thermal expansion, although they do not perform as well at high temperatures as 2618 alloy pistons. All SRP LS-series pistons have a 1.115-inch compression height and large dishes ranging from 25 to 29 cc. This makes them best suited for moderate boost applications. Unlike SRP Gen III/IV pistons that are only compatible with cathedral-port heads, the SRP Professional pistons work with L92-style heads. Other differences include FSR skirts with a lowfriction coating and integral accumulator grooves for improved ring seal. SRP Professional pistons are available in 1.115- and 1.315-inch compression heights with either flat-top or inverted dome crowns. Bore sizes range from 4.005 to 4.070 inches.
Buying ingots of forged aluminum from outside suppliers, then machining them into shape, is common practice for piston manufacturers these days. Wiseco, on the other hand, has its own forging facility in-house. This allows Wiseco to custom-design raw forgings around the intended use of the pistons, instead of the other way around. Thanks to this flexibility, Wiseco pistons are characterized by their combination of low mass and high strength. Furthermore, Wiseco’s comprehensive catalog of Gen III/IV pistons has made the company one of the most popular brands among LS-series engine builders.
Wiseco’s LS off-the-shelf piston catalog is too extensive to list the products one by one in this book. The company offers pistons for 3.905- to 4.200-inchbore diameters and compression heights accommodating strokes ranging from 3.622 to 4.125 inches. All Wiseco LS-series pistons feature a strutted skirt design that allows for pistonto-wall clearances as tight as .004 inch to minimize slap and wear. The skirts are also coated with ArmorGlide, a moly-based compound that reduces friction. Other highlights include radiused and de-burred valve pockets for optimized airflow and detonation resistance, anti-detonation grooves around the ring lands, and valve reliefs designed to fit both cathedral- and rectangle-port heads. Also, complete ring packs are included with every set of Wiseco pistons.
Written by Barry Kluczyk and Posted with Permission of CarTechBooks