In the realm of piston selection, there are a series of basic choices that need to be made first, followed by a lengthy list of detail choices that define the best part for the task. Or you can simply choose a goodquality forged piston and be done with it. But that would be too easy.
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Material and Design
The first part of the selection process is material choice, where you have to choose both the manufacturing process and the alloy. Pistons can be either cast or forged. Virtually all pistons today are made from aluminum, but there are several alloys to choose from based on the manufacturing process and the expected usage.
I should quickly define the difference between the piston manufacturing process, the metallurgy, and the design features. Each of these is interdependent with the others and have a significant impact on piston performance. But they are each different subjects, and you cannot make blanket statements about one process or alloy being better than another one without the proper context. Misleading advertising from various manufacturers has further clouded this issue.
Manufacturing Process— Cast or Forged
Cast pistons are made from molten aluminum that is poured into a mold and then cooled. Cast piston molds are fairly expensive but are extremely durable, and once they are finished the cost per piston is very low. Cast pistons are, thus, considerably less expensive but have limits to their ultimate durability under the stress inherent in a high-performance application. When married to sophisticated electronic engine management, which prevents detonation, a cast piston can handle quite a bit of power.
Forged pistons are extruded into their shape under extreme pressure. They can be formed either in a mechanical or a hydraulic press. The forging process delivers a part with greater ductility (the ability to “bend” or deform) and material density than cast parts. Forging tools are quite expensive, wear out in service, and the process is slow compared to casting. All these factors mean that forged pistons cost more money than a similar cast part.
A cast piston and a forged piston may well have the same compressive and tensile strength in testing. The advantage of forging lies in ductility and fracture resistance. It’s not so much the ultimate strength as it is the mode of failure once the limits have been exceeded. Forgings tend to go into plastic deformation, while castings tend to fracture when overloaded. This forgiving nature allows forged pistons to survive extreme power levels and the occasionally marginal tune ups. Engines beyond 500 hp are almost always going to have forged pistons.
Piston Alloys– Hypereutectic, 2618, 4032 and More
Cast and forged pistons can be manufactured from numerous aluminum alloys. Alloys are a separate subject from the manufacturing process. Some aluminum alloys may be either cast or forged, although in most cases the alloy is optimized for the intended purpose.
Many original-equipment cast pistons and most performance aftermarket ones are cast from hypereutectic alloy. Hypereutectic aluminum by definition has a high percentage of silicon dissolved within the alloy. While common casting alloys can only hold roughly 11 percent silicon in a dissolved state, the hypereutectic alloys contain up to 16 percent. The excess is dispersed throughout the piston in the form of “nodules,” which are microscopically small bits of silicon. Silicon in a piston increases wear resistance, improving both the durability and strength of skirts and ring grooves. The downside of the additional silicon is that each nodule becomes a stress point, and hypereutectic pistons tend to beless fracture resistant than those made from alternate materials.
Forged pistons also have alloy choices. The two most common ones are designated as 4032 or 2618 aluminum, although many others are possible. The 2618 is stronger at high temperatures, and has greater ductility. The 4032 contains about 11 percent silicon, and is the better choice in street- or high-use race applications in which long-term wear resistance is a factor. A piston formed from 2618 alloy, which has essentially no silicon, is the better choice for racing applications where high-temperature strength and ductility are more important. While the differences between the two alloys are significant, there are numerous other design factors that impact the performance of the piston— making either one a satisfactory choice in all but the most demanding race applications.
In the case of our target engine (a high-performance FE), a forged piston is preferable because of availability and durability of these pistons. For drag racing and limited street use, the 2618 alloy is a better option. Since this engine will not go 50,000 miles before service, durability is less important than the ability to handle high loads. Forged pistons, in most applications, can handle more than 2,000 hp.
Contrary to popular information, the type of alloy does not determine the piston’s strength. Forged and cast hypereutectic pistons can have identical tensile and compressive strength, while the alloy’s composition determines its ability to handle high temperatures. The difference is ductility and post-failure behavior. Also, the construction method, whether cast or forged, does not determine the expansion rate. In turn, forged pistons are not always installed in the block with looser tolerances because the skirt design primarily dictates the piston-to-cylinder-wall specification.
Piston Design– Bore Diameter and Compression Distance
After choosing a material for the pistons, you need to define the best design for a particular application. In order to do so, you need to identify several key features of the engine. First is the bore diameter, followed by stroke and rod length. The latter two help to select the compression distance.
Determining the bore diameter seems straightforward, and it is. If you are building a 390-based engine, the basic bore was originally 4.050 inches. On a 428, the bore started out as 4.130 inches. The 427 had an original bore of a somewhat unusual 4.233-inch dimension. When reconditioning the bores in an engine, the common terms are something akin to “.030 over,” meaning that the finished size is thirty thousandths of an inch larger in diameter than the original dimension. Most shops go larger in .010-inch increments on subsequent rebuilds, so a minimal amount of material comes out of the block with each overbore. It’s relatively common to take the 427 to a more popular 4.250-inch bore first, rather than sticking to the .030-over mentality. It is worth noting that the clearance needed for piston skirts is manufactured into the piston—the bore is always assumed to be the nominal specification.
Compression distance refers to the dimension between the piston pin centerline and the deck surface of the piston. This is measured to the piston’s flat top—not the top of the dome, if any. The desired compression distance can be calculated by subtracting the crankshaft stroke divided by two, the center-to-center connecting-rod length, and the desired deck clearance from the block’s deck height as measured from the main bearing centerline to the cylinder head mounting surface. It is usually best to allow for a small amount of deck clearance even if your desired target is zero, to accommodate any dimensional variances.
Compression Distance Calculation Example
Block Deck Height
Stroke divided by 2
(4.250/2 = 2.125)
Rod Length, center-to-center
Desired Deck Clearance
Desired Compression Distance
Skirt Design and Oil Return
Piston-to-bore clearance is primarily dependent on the skirt profile combined with the oilreturn configuration. The differences between various piston alloys in terms of growth are modest. The differences in growth between pistons made in a casting or forging process are even smaller. A 2618 alloy piston needs more clearance, and not so much because of growth—it is because the lack of silicon in the alloy makes it less tolerant of bore contact.
Oil-return design has a big effect on needed skirt clearance. A racestyle design uses a series of drilled holes into the back of the oil ring groove to permit return of scrapedoff oil to the pan. A passenger car piston uses a slotted oil-return passage for the same purpose. The drilled holes make for a stronger and less flexible piston skirt that needs more bore clearance. The slots make for a more flexible, but weaker, skirt. A flexible skirt can run tighter clearances and makes for a quieter engine when cold—good for your daily driver.
After the engine is up to temperature, there is no advantage in ring sealing from having a tight cold clearance. The same is true for noise. No matter what the alloy or manufacturing process, operating clearances, with the piston at normal temperatures, are going to be very similar.
Also most 2618 alloy pistons are designed for racing applications where noisy cold operation is less important than strength and friction reduction.
Dome Design, Compression Ratio Calculation and Selection
The next piece of the piston puzzle is to define the dome design. In order to do this, you need to choose the desired compression ratio and decide upon the cylinder-head chamber volume. All these things are interrelated and can have a dramatic impact on the engine’s performance. You need to do careful research when selecting engine parts because pistons and heads should be complementary, and at least compatible.
Compression ratio is defined as: The comparison between the amount of air volume above the piston when it is at the bottom of its stroke, to the amount of volume when it is at the top of its stroke. This ratio is a function of mechanical characteristics and does not change with RPM or fuel, or anything else.
To calculate compression ratio, add up all the volumes above the piston at each end of its travel. These calculations are commonly done in cubic centimeters (cc) so some conversion from inch measurements is required. One cubic inch equals 16.387064 cubic centimeters.
The volume areas to be concerned with are:
- Combustion-chamber volume of the cylinder head
- Effective dome volume of the piston, which can be “negative” if domed
- Crevice volume: the area above the piston rings but below the piston’s deck
- Volume of the head gasket’s cylinder opening
- Volume of the deck clearance above the piston at top center
The above items are referred to as “unswept volume” because the movement of the piston does not affect them.
The “swept volume” of the cylinder is defined as the cylinder’s volume as calculated by its bore diameter and the stroke.
This formulaic approach compares the cumulative volumes of all the above items to the cumulative volume of only the first five. Volume calculations for the cylindrical items can be done using the standard “radius squared x pi x depth (or stroke)” formula, but the other values need to be either actually measured or taken from manufacturer’s data (depending on your degree of faith in their published material).
Compression Ratio = (unswept volume + swept volume) ÷ unswept volume
The calculation can be done by hand, but these days it is commonly handled with a simple computer program. This allows you to quickly change the variables and find the combination that best meets your needs.
Below is an example using a 4.250-inch bore and 4.250-inch stroke, and common volumes for FE heads, gaskets, and a dished piston:
Combustion chamber volume
Effective piston dome volume
Head gasket volume
Deck clearance volume
Total unswept volume
Swept volume of cylinder
(4.25/2)2 x 3.14 x 16.387064 =
Remember to do the cc conversion!
Using the formula, the compression
ratio is 101.2 + 988.4 = 1,089.6
Compression ratio for this example is:
After calculating the compression ratio, what can you do with it? Raising compression is considered a guaranteed way to add horsepower. But that statement only holds true within certain constraints. A commonly referenced general rule is that you get about a 4-percent power increase per point increase in compression. This translates to perhaps 25 hp in a 600-hp engine by going from 10:1 to 11:1 in compression. This is by no means a definite value, and is not linear; the increase becomes progressively smaller as the compression ratio increases.
A number of factors determine the amount of compression for a given application. These factors include the quality of fuel, design of the combustion chamber and piston dome, weight and type of vehicle, and power adders if any— such as nitrous, blower, or turbocharger. It is far better to target a relevant range for compression, rather than fixating on one particular number.
Using pump premium gas for naturally aspirated applications is typically best when compression is set between 9.5:1 and 11:1. A heavy vehicle, such as a Galaxie with an automatic and highway gears, should be at the lower end of that range. A lightweight Cobra with a stick and steep gearing would be fine at the upper end of the spectrum. A modest sacrifice in power is well worth the ease, reduced expense, and convenience of running pump gas for a street car.
If you are going racing, the game changes. Power is the goal and race fuel is a necessary part of the equation. Drag racers normally find the best compression considerably higher than 12:1—often exceeding 14:1 on serious engines. Oval-track and road-race applications normally hold to somewhere between 12:1 and 13:1, in an effort to retain greater durability and handle the higher temperatures they encounter.
It is currently popular to use the calculated actual compression ratio along with the cam event data to generate a “dynamic compression ratio” (DCR) using any number of available calculators and a computer. These dynamic compression ratio numbers are sometimes useful for comparison of different build strategies. But this ratio is most valuable as a starting point, and not definitive by any means. The numbers do not take into account the various combustion chamber shapes, piston dome shapes, spark plug locations, intake configurations, or a host of other variables that enter into fuel tolerance. Use a DCR calculator to get close, but don’t become fixated on any particular number; you can be misled.
Piston Top Shape and Design
Current trends for piston top design are headed toward minimizing any dome, in favor of either a flat top or a dish. After decades of research, most OEM pistons have evolved to this style as they seek improved efficiency. I cannot find any reason not to follow this trend— “stealing” a good idea is a good idea in this case.
The FE is a good engine from the perspective of piston-dome design. With small cylinder-head chambers, you have the ability to get fairly high compression from a flat top or very modest dome on the piston. The earlier example showed a dish piston that achieved a good pump-gas compression ratio. If you chose to run a flat top in that particular application, you would be just under 12:1. This is the ragged edge for a street car and would perhaps need a bit of race fuel added to the tank.
Given the fact that a flat piston weighs less, and has been demonstrated to be more efficient in terms of combustion flame travel, it has become common for race builders to mill the heads, rather than add a dome when seeking more compression. For those not wanting to cut the heads, a modest dome of around .100 inch gets compression ratios approaching 13:1 with the more popular aftermarket head combinations.
A heavy piston does not offer any advantage once you reach an adequate thickness for strength. A lighter piston is easier on rods, bearings, and crank; and a lightweight package revs more freely. But the main advantage of a lighter-weight piston comes from its durability. Because load on the bottom end goes up with RPM, it is intuitive that a lighter assembly goes to a higher
RPM before those loads reach or exceed the limits of the components. In a race engine, RPM equals horsepower— more “bites” of air and fuel per minute.
Piston pins are important items that do not get enough attention in engine building. If the pin is not strong enough, it will flex, causing wear and possible failure of both rods and pistons. It is common for an inadequate pin to be the true source of issues that are blamed on the pistons.
The four ways to make a pin stronger are to increase its diameter, increase its wall thickness, reduce its length, or upgrade the materials. Lighter components allow the engine to rev more easily, but weight-fixated builders often go the wrong way on pins as a result, sacrificing the necessary strength and rigidity. Piston companies offer multiple pin designs, and are a good resource for proper selection. A larger-diameter, shorter, thin-wall premium-alloy pin is rarely a bad investment. Compared to the common 4340 alloy, a drag-race piston pin made from H13 tool steel can take advantage of the better material, using a thinner cross section to reduce weight without sacrificing strength.
Pin retainers are another subject worth touching on. The FE is blessed with floating pins from the factory. The original parts and some basic aftermarket pistons used a single Tru- Arc–style retaining clip. Higher-quality pistons went to a double Tru-Arc many years ago to prevent the clip from breaking and the pin from walking out of the piston. In the past 15 years, the Spiro-Loc clip has become the retainer of choice, often in double-clip designs for the insurance value.
Recently the round wire retainer has returned to the domestic performance market on both very-highend pistons as well as some basic cast parts. The round wire better distributes pin side loads across the entire pin boss, rather than focusing them on the edge of the snap-ring groove. But pin-lock failures with Spiro-Locs are scarce, and the important thing is to be certain that you are using the correct lock-clip design for the pistons you have, as well as matched piston pins. Piston pins used along with round wire locks require a large bevel/chamfer to work properly.
Pin oiling is normally of the “forced” variety on race pistons, with a feed passage from the oil-ring groove into the pin boss. While considered the norm in the race world, the forced design likely has little, if any, real advantage other than a limited-lube environment with a dry-sump or vacuum pump. Every OEM piston, along with many aftermarket parts, uses a broached groove for oiling the pin. And failures in those pistons from inadequate lube are very rare.
Piston-Ring Grooves and Piston Rings
The ring grooves on the piston are really a part of the piston-ring package. They just happen to be incorporated into the piston. The ring grooves are a sealing surface that is as important as the rings and the cylinder wall.
Ring grooves need to be extremely flat, non-wavy, and perpendicular to the bore in order to function correctly. A well-designed ring groove has a small radius at its root (where the back is) for strength, and has a small degree of vertical uptilt. Vertical uptilt is cutting the ring groove at a slightly upward angle to compensate for changes in the piston shape due to temperature. The piston always runs hotter on top and grows more there as a result. None of these features are readily visible to the naked eye, and all are extremely difficult to measure. You need to trust your piston supplier, and assume that there is a reason that cheaper parts are, well, cheaper. Thermal expansion rates are alloy dependent and are the same. But the better piston compensates for the dimensional change through design and machining, so that ring seal is not compromised as temperatures rise.
A piston ring is a sliding dynamic sealing device. It functions and seals through combustion pressure that forces it against the cylinder wall. The top ring does nearly all of the real combustion sealing work. The second ring is actually an oil control item and has a tapered face, which acts like a squeegee to develop and maintain a thin film of lubricating oil. The third ring is entirely devoted to oil control, scraping the majority of oil off the cylinder walls and sending it back down to the pan. Each ring in the “stack” can be optimized to do the best poss ible job.
Piston-ring dimension specifications can be confusing; ring “width” is considered to be the distance from the top to the bottom of each ring. For example, a common ring width is 1/16 inch. Radial-wall thickness is the dimension measured from the bore contact surface to the inside diameter. Radial-wall thickness dimensions are often specified to an SAE “D-wall” standard of bore/22. Thus a piston ring for a 4.250-bore engine would have a standard radialwall thickness of .193 inch. Current trends are for ring cross sections to be much smaller than in the past, with widths of .043 inch and smaller and non-standard radial-wall thicknesses of .160 inch or less.
Ring tension is measured as the average pressure exerted against the cylinder wall by each ring. Old theory held that higher-tension rings sealed and controlled oil better by forcing the rings against the cylinder bore. Current practice uses a reduced cross section (thinner in both width and wall thickness), which allows the rings to be flexible and conform to the bore— enhancing sealing while reducing friction. Friction reduction and sealing improvements result in more power.
The top ring in a performance build is made from either steel or ductile iron. Inexpensive cast rings fracture when subjected to severe operation loads and should be avoided in all but the most stock rebuilds. The top ring has a plasmamoly coating on a barrel-shaped cylinder contact face. The width of piston rings is measured “top to bottom.” The most popular dimension is 1/16 inch, and should suffice for the vast majority of builds. Reducing this dimension delivers improved sealing due to greater bore conformability and reduced ring weight. Going to a ring thickness of .043 inch or less is a good strategy in a race application, but has only modest benefits in a performance street car, considering availability and expense.
The second ring is almost always cast iron with no face coating. As referenced earlier, it is an oil-control device. The second ring also benefits from reduction in cross section— both in width and radial thickness as measured from the inside diameter to the outside diameter of the ring. A thinner, smaller second ring does a better job with reduced drag (more on that below).
Oil rings are comprised of three pieces: an expander and two rails. The expander is normally a stainlesssteel band that has been perforated, folded, and formed into a circle. It is designed to spring load the rails against the cylinder wall as well as against the oil ring groove of the piston. The rails are simple chromeplated steel rings that contact the cylinder walls and ring grooves. As an assembly, these parts clear all excess oil from the cylinder walls on each stroke. The tension of the spring expander determines how “hard” the contact is, and how much oil is cleared or left for the second ring to work with.
As with the other rings, the current trend on oil rings is for reduced radial thickness, smaller widths, and reduced tensions.
The ring summary goes as follows:
- On a race engine, I would go straight to a .043-, .043-inch, 3.0-mm ring package with a reduced radial wall thickness.
- On a street engine, I would stick to the traditional 1/16-, 1/16-, 3/16-inch package.
- On an “in between” deal, I might consider the 1/16-inch top, a reduced radial wall thickness 1/16-inch second (often referred to as a “back cut” ring), and a low-tension 3/16-inch oil ring.
This one might use a touch of oil at part throttle, but should still be tolerable on the street and run hard at the strip.
Piston rings make up a large percentage of engine friction, and are equally important for producing horsepower through adequate sealing during combustion. Reducing friction with low tension and smaller cross section parts yields measurable benefits, as long as you don’t go too far and sacrifice oil control. Oil in the chamber causes detonation and subsequent damage—you can go too far. In addition, oil smoke out the exhaust is never a good thing on a street engine. It’s the risk-versusreward scenario; it’s best to err on the side of caution if you are not willing to find or finance finding the “edge.”
Piston-Ring End Gaps
The quick strategy is to follow your chosen ring manufacturer’s suggestions for ring gap sizing. But there is a lot more to this than may first meet the eye. The top ring gap needs to be large enough to prevent ring butting at the highest temperatures the engine will see. In most naturally aspirated engines this translates to somewhere between .003 and .004 inch of gap per inch of bore diameter. At operating temperatures this gap area becomes smaller, approaching zero in an optimized race engine. On engines running boost or nitrous, the ring end gap needs to be increased due to the heat generated, usually by at least 20 percent.
Second rings used to be thought of in the same fashion. Because they run at lower temperatures the gap recommendations were smaller than those for the top. A change in theory, followed in practice by most OE manufacturers, has been to move to a second ring gap larger than the one for the top. This is done to relieve any accumulation of pressure between the top and second ring, and in turn allows the top ring to remain seated against the piston’s ring groove for better sealing. Recommendations are for second-ring gaps to be around .0050 to .0055 inch per inch of bore diameter.
Oil ring rails do not see much heat, and usually only need to be inspected for adequate gap. Roughly .010 to .012-inch total gap is sufficient.
Ring filing is best done with a rotary tool for the purpose. Filing only one side of the gap allows you to use the uncut side as a reference to ensure that you have the gap surface straight. Once you have the proper size, use a knife-sharpening stone to edge break any sharp corners left from the filing. You do not need or want a chamfer—just a clean, square edge with no burrs.
Alternative Ring Concepts—Gapless, Napier and Dykes
There are several specialty rings available in the performance market. Some of them make claims of greater performance, durability, oil control, or all three. Conventional pistonring designs have been under continuous development at the OE level for decades in pursuit of more power, efficiency, and reduced emissions, and they are fairly well optimized.
Others may differ in their opinions, but I am not a supporter of the gapless-ring theory. At the time of this writing there is not a single OE engine using a gapless-style ring, despite claims of tremendous benefits. It does not matter whether the engine is in a quarter-million-dollar exotic or a max-effort fuel-economy vehicle. Despite huge research budgets and intense competitive and government pressure, they all run a conventional-style piston-ring package. An engine like the one found in a Corvette Z06 or an NSX Honda can justify titanium connecting rods, so it is not a cost-driven decision. There is a lesson in that data for those caring to listen.
Generally speaking, if a piston ring claims more than an incremental gain in any performance metric it is likely to be too good to be true. If the claims read like something out of a nitrous catalog it’s virtually guaranteed to be marketing hype.
The Napier-style second ring is a different story. The Napier ring has an undercut around the outer diameter, which reduces the effective cross section of the piston ring while retaining the material thickness that lies within the piston’s ring groove. It is used in numerous OE applications and has comparable friction-reduction benefits in a performance engine.
A Dykes-style ring has an inverted “L”-shaped cross section with the cutout part going into the piston ring groove. This again reduces the ring’s cross section and allows combustion pressure to better reach the backside of the piston ring, thus enhancing sealing. This ring design is somewhat out of favor these days, but is still well accepted in race applications where gas-ported pistons are not permitted, along with use in supercharged race engines.
Leakdown testing is touted as a way to gauge ring sealing, but in practice has major limitations. Leakdown tests are performed with 100 psi of shop air, at room temperature, on a non-moving piston. There is no way that they serve to simulate the performance of rings that operate at temperatures of 700 degrees F, seal nearly 2,000 psi of combustion pressure, and are traveling over 4,500 feet every minute.
Your best bet functionally and financially is a conventional ring set with reduced radial wall thicknesses and ring tensions appropriate to the use of the vehicle.
Written by Barry Robotnik and Republished with Permission of CarTech Inc
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