The performance piston business today is hot thanks to a strong racing market, the introduction of new OEM engine families such as Chevy's LS1/LS6 V8s, Ford's 4.6/5.4L V8s and Chrysler's Hemi, and the increasing availability and popularity of aftermarket engine blocks and cylinder heads
By Larry Carley
Piston manufacturers are introducing new performance pistons for all of these applications as well as refining existing piston designs to reduce weight, and improve strength, durability and ring sealing.
Piston design and manufacturing used to be a relatively low-tech process. Now, with the aid of finite element analysis and other computer modeling techniques, many aftermarket piston manufacturers are using the same tools as original equipment suppliers to design and customize pistons for specific applications.
Computer-aided design and manufacturing techniques combined with highly flexible CNC machining centers allows many piston manufacturers to create new piston designs in record time, to make shorter production runs profitably, and to offer engine builders "off the shelf" performance pistons that formerly had to be custom-made.
Virtually every piston manufacturer we contacted for this article said they were expanding their performance piston product lines and were offering more new pistons for more new applications than ever before. One manufacturer's spokesperson said his company has added more than 100 new performance pistons to its catalog in the past six months! That's good news for our readers because it means engine builders now have a greater variety of pistons from which to choose when building performance engines.
Though small block Chevys, big block Chevys and small block Fords have been the mainstays of performance engine building for many years, Chevy's LS1 is really coming on strong followed by Ford's 4.6L. These two engines have been around for a number of years now, and are being rebuilt in greater numbers. As more and more aftermarket performance parts become available for these engines, it makes it easier to upgrade performance and build motors that produce serious horsepower.
The ongoing popularity of stroker motors is another factor that's driving the piston business today. There's no substitute for cubic inches when it comes to adding power and torque. But lengthening the stroke of the crankshaft requires longer connecting rods and raising the location of the wrist pins in the pistons. That, in turn, requires redesigned pistons that can accommodate a longer stroke crankshaft. On engines like the LS1 that have a crankshaft position sensor reluctor wheel on the crank, piston clearance can be a problem when stroke is increased.
Short skirt "box-style" pistons are available from several piston suppliers and offer engine builders yet another alternative for stroker engines that need additional crankshaft clearance, or high revving engines that can benefit from reduced piston/pin weight. The short skirt pistons offer significantly reduced weight due to the short skirt length. Weight is also reduced from using a much shorter wrist pin. This also reduces pin flex and allows the piston to be much more rigid. The best way to reduce weight is to reduce compression height. Several manufacturers are doing this today. The thickness, angle and orientation of the structural reinforcing struts inside the piston allow the piston manufacturer to fine tune the stiffness and expansion characteristics of the piston so it can handle tighter piston-to-cylinder clearances for improved sealing and more power. Tighter piston clearances also reduce piston rock, which in turn reduces piston rattle and cylinder wear when the engine is cold.
One manufacturer said the new box style full round pistons are good for an average of 15 more usable horsepower at the crank, but also said these type of pistons are not for every application.
Pistons with longer skirts provide more contact area and spread the wear over a larger surface. A longer skirt also runs quieter (less piston rock) but adds weight and may not work with a stroker crank.
Pistons come in a variety of materials and designs, ranging from low cost castings to premium forgings. Many piston manufacturers offer both a line of "street" performance pistons, and a line of "professional grade" racing pistons. Street performance pistons may be ordinary castings, hypereutectic castings, or forgings.
Cast pistons are usually adequate for most passenger car applications, but when it comes to modified high output engines, marine engines, boosted engines (turbocharged or supercharged), or engines that are "bottle-fed" nitrous oxide, a piston upgrade is usually a must. Many experts say any engine that is capable of producing upwards of 400 horsepower, revs beyond 6,500 rpm, is boosted or uses nitrous must have pistons that can take the punishment. For these kind of applications, that usually means upgrading to some type of performance pistons.
Forged pistons have been around since the mid-1950s, and were used as original equipment pistons in many of the muscle car engines during the heyday of the muscle car era (mid 1960s to early 1970s). Today, forged pistons dominate virtually every form of professional racing from circle track to drag racing. They're also widely used in aircraft, marine and motorcycle engines as well as heavy-duty truck, diesel and agricultural engines because of their proven reliability.
Ordinary cast pistons are made by pouring molten aluminum into a mold. After the metal cools, the casting is machined to its final dimensions. Most cast pistons also receive a "T5" heat treatment which involves heating the pistons for a certain length of time (a process called "aging") to relieve stress in the metal.
Hypereutectic pistons (which are also cast) were introduced over a decade ago for OEM engines that required something stronger than an ordinary cast piston. Hypereutectic alloys contain a much higher level of silicon (16.5 to 18 percent versus 8.5 to 10.5 percent in a typical cast piston alloy such as SAE 332 or F-132). Silicon increases hardness for reduced ring groove, pin boss and skirt wear. Hypereutectic alloys are slightly lighter (about 2 percent) than standard cast alloys, and can be machined somewhat thinner to reduce overall piston weight about 10 percent.
Hypereutectic alloys also handle heat better than standard cast alloys and undergo about 15 percent less expansion when the alloy gets hot because its silicon formulation rejects heat. Since hypereutectic pistons don't conduct heat they can be installed with somewhat tighter cylinder bore clearances to reduce ring flex and piston rock for improved sealing. Hypereutectic pistons that are made for performance applications may also receive a heat treatment to increase their strength; but how much is necessary is a subject of some debate. According to some sources, a "T6" heat treatment can increase strength up to 30 percent, but other engineers say this strength is not permanent: T6 is only stronger than T5 for the first 100 hours. A T5 heat treatment, on the other hand, gives a linear increase in strength over the life of the engine, which may be a better choice for many performance applications.
Many late model engines today come factory-equipped with hypereutectic pistons. In many instances, the OEM hypereutectic pistons can handle engine modifications that boost power up to 30 percent or more over stock. Aftermarket hypereutectic performance pistons are available for upgrading a wide variety of engines including SB/BB Chevys and Fords, as well as many late model engines and sport compact engines. Some racers are using hypereutectic pistons successfully as a lower-cost alternative to forged pistons on circle tracks and drag strips.
One manufacturer said hypereutectic pistons can usually handle up to 1.5 to 2 horsepower per cubic inch of engine displacement. Beyond 2 horsepower per cubic inch, they would recommend upgrading to forged pistons. The manufacturer also said its hypereutectic performance pistons may be able to handle up to 1,000 horsepower provided the fuel mixture and timing are correct so the engine doesn't go into detonation or preignition.
Forged pistons are typically made from one of two alloys: SAE 4032 or SAE 2618. The 4032 alloy is most often used for pistons in street engines, drag engines, naturally aspirated engines and many sportsman class circle track engines. The 4032 alloy contains more silicon (11 to 13.5 percent) than 2618 (less than 0.25 percent), which reduces thermal expansion, improves lubricity and scuff resistance. The 2618 alloy, by comparison, is a low silicon alloy so it has a higher coefficient of thermal expansion and much more tendency to scuff. But it is about a 10-15 percent stronger material and is typically the alloy of choice for serious racing, marine engines, and boosted and bottle-fed engines that produce a lot of heat in the combustion chamber. However, these pistons are not to be used for street or mild racing applications where they won't be replaced routinely.
Forged pistons undergo a more involved manufacturing process than cast pistons (which makes them more expensive). The molten metal is first formed into bars by a continuous casting or extrusion process. The bars are then cut into 3" to 4" long slugs, which are then heated and fed into a forging press that shapes the slugs into raw pistons. The forging process increases the density of the metal, which significantly improves its strength, ductility and thermal characteristics. Forgings tend to conduct heat quickly and cool better than most cast pistons, but cooling also depends on the design of the piston and ring contact with the cylinder wall.
Several piston manufacturers said they now have forged pistons available for the more popular sport compact engines such as the Honda H22 and H23, turbocharged Mitsubishi and others. Feeding nitrous oxide into one of these engines is an easy way to add horsepower, but it's also hard on the pistons. Stronger pistons are recommended as the dosage of nitrous goes up, or more boost pressure is added on a turbocharged engine.
Friction-reducing coatings and thermal barrier coatings have been around for many years, but only recently have coatings come into the mainstream for many engine builders. A number of piston suppliers now offer pistons with friction-reducing moly, graphite, or Teflon scuff protection coatings on the skirts. Debate rages heavily here, as well. Some say coatings are a fad and aren't really needed if clearances are adequate. Others say the coating provides insurance against piston scuffing and help protect the pistons and bores if the engine overheats. Still others say coatings on 2618 pistons are absolutely unnecessary.
Coatings typically add about .001" to the piston diameter, so the question often comes up as to how this affects piston installation clearances. One leading piston manufacturer said it is not necessary to compensate for the coating when figuring piston-to-bore clearances. "Just pretend the coating isn't there," is their advice. Use the piston size on the box to calculate clearances, not the actual diameter of the coated piston.
For example, if a forged piston in a small block Chevy is normally installed with .004" of side clearance, a coated piston would actually fit the bore with .003" clearance if you ignore the added thickness of the coating.
Another type of coating that may be used in some applications is a thermal barrier coating on the top of the pistons. The idea here is to reflect heat in the combustion chamber to protect the pistons. Such coatings may help protect the pistons in supercharged or turbocharged drag racing engines, or ones that run on short bursts of nitrous oxide. Nitrous produces a tremendous amount of heat in a very short time, which can be very damaging to unprotected pistons. But a barrier coating also prevents heat from dissipating down into the pistons and rings, which may be counterproductive if the heat persists for a long period of time. That's why most piston manufacturers do not recommend a thermal barrier top coating on pistons for naturally aspirated engines (without nitrous) or ones that are built for endurance racing.
One manufacturer said they will soon be introducing a new performance piston with a thermal barrier coating on top for supercharged Ford 4.6L and 5.4L engines, and a new piston for Chrysler's new Hemi that has a thermal barrier coating in the top ring groove.
Another manufacturer said they apply a protective phosphate "dry film lubricant" coating to their performance pistons to reduce wear in the ring grooves and pin bores. The coating is applied to the entire piston using a dip process and is similar to anodizing which is sometimes used in the upper ring grooves to prevent micro-welding between the rings and piston.
Most piston manufacturers use CNC equipment and diamond tooling to machine their pistons. Most also fully machine the tops of their pistons, too. Consequently, the depth of the valve recesses, the shape of the dome or recess can be closely controlled with a high degree of precision. One manufacturer said the proliferation of new aftermarket cylinder heads is one reason why they've had to introduce so many new pistons. With different valve angles and locations, a piston that fits a standard head won't work. The top of the piston needs to be machined to be compatible with the aftermarket head.
Some people think that the same thermal characteristics that allow forged pistons to run cooler also cause them to swell more than cast pistons or hypereutectic pistons as they heat up. Consequently, there's a common misconception that forged pistons always require greater skirt-to-wall clearances. This isn't always true because clearances depend on the type of alloy used in the piston, the design of the piston itself and the profile of the piston.
The coefficient of thermal expansion for some forged alloys is actually not much different than that of an ordinary cast piston. Consequently, thermal expansion can be controlled by machining the piston with a certain amount of "cam drop" so its shape conforms more closely to the cylinder bore as the piston gets hot. That's why all pistons (cast as well as forged) are slightly elliptic rather than round. The diameter of most pistons measures anywhere from .018" to .035" shorter across the wrist pin axis than along a line perpendicular to the pin. This compensates for the greater mass in the wrist pin area which causes the piston to swell sideways as it heats up.
Piston growth is also influenced by the temperature differential between the top and bottom of the piston. The top of the piston typically runs up to 300° F or more than the bottom. To compensate, the side profile of the piston may be machined with a slight taper inward towards the top because the top is the hottest area that experiences the most swelling when the engine is running.
The location, dimensions and design of the ring grooves (slotted versus closed) also influences heat transfer and piston growth. Slotted oil grooves can act like a barrier to slow heat transfer from the top to the bottom of the piston. This may require less taper in the skirt (and/or less skirt clearance) to compensate for thermal expansion at the top of the piston. Oil grooves with drilled holes or small windows provide greater heat transfer down the piston.
In an endurance engine, heat transfer is a desirable thing to keep the piston from getting too hot and/or contributing to detonation. In an engine built for drag racing, on the other hand, it is less a concern because the engine only runs at full throttle for a short burst.
One of the most confusing areas of piston selection is how much compression to use. More compression equals more power - up to a point. But once compression exceeds the octane rating of the fuel, detonation takes over and that means trouble. Detonation causes a sharp increase in cylinder pressure that is counterproductive and slams the pistons like a sledgehammer. Detonation can break piston ring lands, flatten rod bearings and blow head gaskets.
For most naturally aspirated street engines, 9 or 10:1 is about the recommended static compression limit for 93 octane pump gasoline. An engine with aluminum heads and/or forged pistons tends to run cooler and may be able to handle a little more compression, but not too much.
Actual dynamic (running) compression will depend on cam timing and the breathing efficiency of the engine.
For a turbocharged or supercharged engine, most piston manufacturers say to reduce the compression ratio to about 8:1 or 8.5:1. Again, you may be able to run a bit higher compression with aluminum heads and/or forged pistons, but don't overdo it.
With nitrous oxide, alcohol fuel or high octane rating fuel, a static compression ratio of 13.5:1 or 14:1 is a realistic limit. Some engine builders use much higher ratios, but they have the necessary control to do it.
Piston-to-cylinder bore clearances are another concern that will vary with the engine application and type of pistons used. One manufacturer said they recommend .004" of clearance for their forged pistons in small block Chevys, .005" clearance for big block Chevys, and .003" clearance in smaller sport compact engines. Another manufacturer said they recommend only .002" to .003" of clearance for their 4032 street forged pistons in a small block Chevy, but as much as .004" to .006"? (or more) clearance for their 2618 forged racing pistons. A third manufacturer said some coated hypereutectic pistons may be installed with as little as .0008" to .001" of clearance. On a Chevy LS1, they recommend .0025" to .003" of clearance. So it all depends on the application, the type of piston, the alloy used and the piston profile.
Piston-to-valve clearance is also a very important consideration especially with long duration cams and oversized valves. The standard rule of thumb is to allow at least .080" of clearance for the intake valves and .100" for the exhaust valves. The valve pockets that are machined into the piston must also be the right one for the type of heads that are used. Consequently, aftermarket heads that have a different valve angle than the OEM heads will typically require different pistons.
We'll wrap up this article on performance pistons with a few words about ring sealing. Many performance pistons are now available with a "low drag" ring package (1.2 mm top ring, 1.5 mm second ring, and 3.0 mm oil ring). Smaller, thinner rings reduce drag for more usable horsepower, yet some racers remain skeptical about how well the low drag rings will hold up over the long run or what effect they have on piston cooling. The manufacturers who supply the smaller ring sets say they've seen no cooling problems or durability problems, provided the bores are finished properly and bore distortion is kept to a minimum.
Pistons with only two rings (a top compression ring and an oil ring) are also available, but are only used in drag motors. Eliminating the second ring reduces friction, but also heat transfer and the ability to control oil. Therefore, two-ring pistons are not recommended for any kind of street engine or endurance racing.
Several piston manufacturers also mentioned the importance of machining the ring grooves as precisely as possible. The flatter the ring groove, the better the ring seal and transfer of heat.
Some performance pistons feature gas ports to help seal the top ring. Combustion pressure blows through the port to help seal the ring from behind and underneath. Vertical gas ports have holes drilled from the top of the piston to the top ring groove just behind the ring. Lateral gas ports are drilled through the bottom side of the top land and extend to the back wall of the ring groove. Gas ports work best at high rpm (above 7,000 rpm) and are not recommended for street engines.
Some performance pistons also have "accumulator grooves" machined into the piston land between the first and second ring grooves. The added space traps blowby gasses and helps prevent the top ring from unseating and fluttering.
The latest thinking on ring end gaps calls for opening up the second ring groove a bit so blowby gasses can escape before they unseat the top ring. This trick works best on engines that are running a dry oil sump and pull a vacuum in the crankcase. One manufacturer said they recommend increasing the end gap on the second ring .002 inches more than the top ring.
For naturally aspirated engines, the same manufacturer recommends .0045" of end gap per inch of bore diameter for the top ring. For a turbocharged or supercharged engine, the top ring gap should be .006" per inch of bore diameter. For a nitrous oxide engine, the top end gap should be opened up even more to .007" per inch of bore diameter. Some manufacturers have decided to take the risk out of selecting piston rings by offering custom matched piston and ring sets.
Another alternative is to use a "gapless" top ring or second ring to reduce blowby even more. The people who supply these type of rings claim they typically allow less than 1 cubic foot per minute (cfm) of blowby, which is good for up to 10 to 15 horsepower or more on a typical street engine. On an alcohol-fueled circle track engine, using a gapless ring in the second groove helps keep alcohol out of the crankcase.