In the simplest of terms, a piston is just a slug of aluminum that moves up and down in the cylinder and takes the brunt of the cylinder pressure. But in reality, there’s more than a little engineering that goes into that alloy slug. We’ll spend a few pages on these camshaped pieces that attract more than their share of attention from engine builders and enthusiasts alike.
MATERIALS AND PROCESSES
For your basic street engine, cast pistons work just fine. But we’re interested in more than just pedestrian street power plants. For any performance application, forgings are the only pistons worth discussing. Cast pistons offer minimal strength; and hyper eutectic pistons, while stronger, are very brittle and react to detonation by shattering, which is not what you want in any power plant, let alone a high-output performance engine. Given these limitations, forging is the only realistic choice. But there’s much more to ordering pistons than just picking a compression ratio.
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Cast and Hypereutectic
We will spend a limited amount of time with these pistons since they represent such a small portion of the big-inch small-block market. First of all, there’s no reason to even consider a cast piston for any kind of performance engine buildup. Cast pistons have their place in low-performance, stock production engines because they are inexpensive to build and are durable within the realm of low-RPM engines. The problem with cast pistons is that they are both heavy and brittle. The weak link with cast pistons is the ring lands. Combine a heavy dose of cylinder pressure with even mild detonation and the ring land will break every time. When cast pistons fail, they virtually explode due to their brittle construction. As a result, those pieces help destroy the rest of the engine. At least with forged pistons, if there is a problem, the piston tends to deform rather than break, which limits the damage to that cylinder, as opposed to trashing the rest of the engine.
Hypereutectic pistons came on the market several years ago as a bridge Keith Black hypereutectic pistons were the first, followed shortly by Federal- Mogul. Hypereutectic pistons are essentially cast aluminum pistons made with a high silicon content alloy that is significantly stronger than a typical casting. So much so, that machining hypereutectic pistons must be accomplished using diamond tooling. However, these are still cast pistons, and as a result are susceptible to breakage under detonation. The concept was that hypereutectic pistons were less expensive than forgings and were offered in several different rod length applications for stroker combos like the popular 383.
Where the hypereutectic piston fills a need is with a budget-oriented 383 small block where the engine is built with a cast crank, stock rods, and a mild cam and cylinder heads. Since this engine will not see a lot of RPM or high cylinder pressures, the hypereutectic piston is a good choice. Cost should always be a consideration as well. Hypereutectic pistons cost around $250 to $300 depending upon the application, which is less than a good forged SRP piston, for example, but about the same price as a budget forging. If you can find a forging for a budget application like this for the same price as a hypereutectic piston, then the forging will always be the better deal. This holds true even if the forging is slightly heavier than the hypereutectic. A stronger piston is always a better choice.
The first forged pistons were merely stronger clones of OEM castings. Forged pistons are created using a stronger-alloy aluminum squeezed in a press with thousands of pounds of force. Pressing the material eliminates voids in the material and creates a stronger base that is able to handle the stresses imposed by high cylinder pressures and engine speeds. Ironically, the old original TRW pistons weren’t actually forgings, but rather an extrusion, that wasn’t quite as strong as a true forging. Many race engines employed these pistons mainly because they were the only alternative to expensive custom-made pistons. Later, high-production pistons were created as forgings in the basic shape that required only minimal machine work to complete the rings, skirt shape, and pin boss size. While strong, these pistons tend to be heavy and are only offered in a limited selection when it comes to rod length, ring thickness, and other options.
Federal-Mogul carries on the tradition of excellent forgings at an affordable price with a tremendous general performance selection of pistons in its Power Forged lineup. Most of these pistons are constructed of what they call a VMS-75 alloy. This is an aluminum alloy that includes 11 percent silicon to add toughness and give the piston excellent skirt-area scuff resistance as well as reduced ring-groove wear. One additional advantage to these pistons is a high production rate that allows them to be attractively priced. Unfortunately, Federal-Mogul offers a limited selection when it comes to big-inch small block applications other than 377, 383, and 406 displacements with either 5.7- or 6.0- inch rod lengths.
Custom pistons aren’t part of the Federal-Mogul lineup, but recently the company has added a few CNC machined 2618 alloy race pistons. For example, the LW2604 383 piston, is designed for a 5.850-inch rod-length. It offers close to 10:1 compression ratio with a 64cc chamber, minimal deck height, and weighs a mere 441 grams. With the 2618 material, Federal-Mogul considers this a race-style piston and doesn’t recommend it for the street.
This brings us to the 2618 aluminum- alloy material that begins life as a forged chunk and is fully CNC machined into its final shape. While this has been the playground of the custom piston companies like Diamond, JE, Ross, Venolia, Wiseco, and others, even Federal-Mogul is now playing in this arena. There is evidence to support the fact that 2618 is not as scuff resistant as the 11-percent silicon-style pistons, and therefore perhaps not as durable for daily street operation. For a 100,000-mile engine, this is probably true. However, few of the engines that will be covered in this book are intended as high mileage power plants. Most hot street and track engines will probably see far less than 20,000 miles in their lifetime before a teardown and inspection. The advantage offered by the 2618 alloy is its excellent ductility, which allows it to withstand the pounding of high cylinder pressures without cracking.
Another, perhaps less well known piston alloy is 4032, which is similar in most respects to Federal-Mogul’s VMS- 75. The 4032 alloy is enhanced with 12- percent silicon that adds scuff resistance and excellent durability. While not well publicized, many of the custom piston manufacturers like JE, SRP, and Wiseco offer both the 2618 and 4032 alloy pistons. Using 4032 instead of 2618 really comes down to application. If the piston will see a majority of street use with some track time, the high silicon content 4032 piston is probably the better choice. If the engine will see limited street use and more track time, then the selection of a 2618 alloy piston may be the better way to go. Not all piston companies offer the 4032 alloy, but if you intend to build a big-inch street engine, you may want to ask your piston manufacturer if your piston choice is offered in this material.
DISHED OR DOMED?
The first thing everyone wants to know about any particular piston is the compression that it generates. While the piston helps generate the ultimate compression ratio, piston configuration is only one part of a greater equation determined by bore, stroke, deck height, combustion chamber, and head gasket volumes. We’ll deal with how to determine compression ratio in the accompanying sidebar, but a rule of thumb for any engine is that even a small change in bore or stroke with a big-inch engine tends to really bump the compression.
As an example, let’s say we have a 418ci small block with a 4.155 bore, a 3.850- inch stroke, 20cc-dished pistons, a deck height of 0.005-inch, a 64cc chamber, and a 0.041-inch thick head gasket. This creates a compression ratio of 10.08:1. Increase the bore to 4.185-inch and the compression jumps to 10.20 for an increase of 0.12 of a ratio. Tickle the stroke from 3.85 to 4.00 and the compression spikes to 10.43:1. In both cases we have increased the volume that the piston squeezes.
Since big-inch small blocks larger than 383 or 406 inches are only now becoming popular, the demand has not been sufficient to warrant off-the-shelf pistons. This means that a big-inch small block may need a piston with a 20cc or larger dish in order to place the compression around 10.5:1 with the increasingly popular 64cc chamber. An inspection of the major piston manufacturer catalogs reveals very few dished pistons with bore sizes larger than 4.155-inch as on-the-shelf items. This means these pistons would have to be custom ordered. With the widespread use of CNC machines, this does not mean the pistons will be extremely expensive, but you can expect to pay at least $600 for a set, depending upon your specific requirements.
For the sake of discussion, domed pistons aren’t really even a consideration with large displacement engines unless you are looking for a compression ratio above 14:1. As an example, just adding flat top pistons to our original 418ci example engine creates 12.5:1 compression. Obviously, as the displacement increases, there is less need for domed pistons, unless you are aiming at pure, drag-race compression ratios. This is another advantage of the large-displacement small block over a rat motor where a 100cc chamber is considered small. With 119cc chambers, rat motors require a domed piston just to bump the compression to around 10:1. Piston domes tend to get in the way of the flame front, which is why using flattop or dished pistons with small chambers is a much cleaner way to make compression.
It’s worth it at this point to discuss what JE calls inverted dome or D-cup style dished pistons. In the early days of piston design, the piston dish extended across the entire top of the piston. But since the small-block Chevy employs a wedge-style combustion chamber with a quench or flat portion facing the piston, designers quickly adopted the D shaped dish that leaves a flat portion mirroring the quench portion of the head. The dish ends up shaped like a D, which is where the “D-cup” moniker originated. The reason for the piston’s flat portion is to create turbulence within the combustion space as the piston approaches TDC. These matching flat portions squish or push the air-fuel mixture into the chamber, creating dramatic mixture movement that helps create a more homogeneous air-fuel mixture in the combustion space above the piston. An ideal combustion situation would be where the fuel droplets are all very small in diameter and they are evenly distributed throughout the entire combustion space. This homogeneous mixture would be easier to light and would burn much faster than a mixture comprised of large and small fuel droplets that are unevenly distributed in the combustion space.
Since smaller fuel droplets burn more quickly than large droplets, there is a significant power advantage to enhancing a more homogeneous mixture. A better mix of fuel and air in the combustion area creates an engine that is smoother and less susceptible to the onset of detonation.
This efficiency of this squish or quench movement is determined mainly by the piston-to-head clearance. Here’s a case where tighter is better. Obviously, we are limited by a safe piston-to-head clearance to prevent the piston from smacking the cylinder head. But after polling several professional race engine builders, it appears that the safest piston- to-head clearance for optimal quench movement is in the neighborhood of 0.037 to 0.039-inch. Anything tighter than this runs the risk of damage to the piston, while greater clearance reduces the quench effect. You can run as tight at 0.037-inch, but this assumes a rather tight piston-to-wall clearance to reduce piston rock at TDC. The benefit of a tight quench area and enhanced mixture motion in the chamber is a dramatic decrease in the engine’s sensitivity to detonation. This means you could run a slightly higher static compression ratio with pump gas and still run the maximum amount of ignition timing the engine demands. An interesting benefit of running tight quench clearances is a reduction in total ignition timing required to attain maximum power. For example, a tight quench engine could demand only 32 to 34 degrees of total timing as opposed to 36 to 38 degrees of lead to make the same power in an engine with a greater quench area. These tighter quench clearances also reduce the engine’s sensitivity to detonation. Combustion chamber shape also plays a big part in this equation, with the kidney or heart-shaped chambers the most popular right now. Keep in mind that less ignition advance can itself improve power by reducing pumping losses.
One of the many limitations placed on a big-inch small block engine builder is the distance between the top of the piston and the centerline of the wrist pin. This is called the piston’s compression height. This is important stuff because we are trying to stuff 10 pounds of displacement into a stock deck height block’s five-pound bag. Let’s say, for example, that you want to build a 434ci small block with a 4.155-inch bore and a 4.00-inch stroke in a standard 9.025-inch deck height small block. The formula for determining compression height is easy. Add the rod length and half the stroke together and then subtract this sum from the deck height:
[Deck Height – (Rod length + 1/2 Stroke)] = Piston Compression Height
Based on this equation, if we wanted to run a 5.850-inch long connecting rod in this 4.00-inch stroke engine with a stock deck height block, the compression height would be:
[9.025 – (5.850 + 2.00)] = 1.175-inches
Compression height must include all three piston rings while also leaving sufficient room between the top ring and the top of the piston. The absolute minimum compression height is 1.00 inch, but most engine builders prefer heights of between 1.125 to 1.250 inch. This creates more room between the rings as well as moving the top ring down from the top of the piston. This is important since the closer the top ring is to the top of the piston, the more combustion heat is transferred to the ring itself. Elevated ring temperatures are a main cause of ring distortion and loss of seal. This is also hell on the wrist pin and bushing.
All of this is important since several factors affect piston design. As we learned in the chapter on connecting rods, rod length is important to reduce an acute angle of the rod to the piston after TDC. Given this, as stroke increases (as in our 4.00-inch stroke example) we’d love to have a 6.00-inch long rod in this package to create a decent 1.5:1 rod ratio. If we plug this into our compression height equation, we discover that this requires a 1.025- inch compression height. Not only does this squeeze the ring package together on the piston, but is also moves the wrist pin into the oil ring groove. This requires an oil rail support to ensure the oil ring remains stable across the gap created by the wrist pin hole entering the oil ring groove. For street engines, this will work, but it’s not ideal.
The solution for this particular problem is to use a tall-deck block that measures 9.325 inches. This adds 0.300- inch to the height of the block, which now means we can have our 6.000-inch rod with a 4.00-inch stroke and generate a more stable compression height of 1.325 inch. But keep in mind that adding this deck height also means a raised cam bore and other changes to the standard small block that you must take into consideration before making that decision. This taller block also creates other problems. For example, this will also raise the cylinder heads, which means that even if you wanted to use production exhaust headers, they probably won’t fit the chassis because the taller deck moves the exhaust ports farther out and up relative to the chassis Things can get very complicated very quickly when you get into non-stock applications. None of this is impossible to overcome, it just means more work and added cost.
While we could get into a discussion of determining the exact spacing of the ring package between the wrist pin and the top of the piston, this is best left to the piston manufacturer. The most important consideration, and the only one that we will address here, is the distance between the top ring land and the top of the piston. Naturally aspirated drag race engine builders prefer to place the top ring land very close to the top of the piston, often less than 0.100-inch. For street engines, this distance should be increased to at least 0.200-inch merely to protect the top ring from the elevated temperature of combustion. Supercharged, turbocharged, or nitrous injected engines should place the top ring land farther down from the top of the piston. Consult your piston manufacturer for their recommendation for these applications.
The question of weight is especially important when it comes to pistons. The compromise comes on several different levels. While reducing weight reduces the g-force load imparted to the connecting rod, excessively lightened pistons also sacrifice strength and durability. Ultra-lightweight pistons may survive in a drag racing environment with minimal run time, but they would not last long in an endurance, road race, or street application. The other area to consider is crankshaft bob weight. The reciprocating weight of the piston, rings, pin, and the top half of the rod must come close to the crank bob weight or you will have to spend money on balancing. Piston weights vary dramatically based on variables such as piston configuration (dished vs. flat top), rod length, and several other details.
As an example, we compared a flattop, Federal-Mogul, 406 style-piston,to a JE flat top with a slightly shorter compression height and discovered a 54- gram difference between the two pistons (526 vs. 472, respectively). Another interesting point is that dished pistons tend to be slightly heavier than pure flattop pistons because the dished pistons require a thicker top. A flat-top JE piston with a 1.125-inch compression height comes in at a mere 431 grams, but a dished JE piston with the same compression height is 526 grams — a 95- gram difference. As you can see, it pays to have all the information before you decide which piston is best for your application.
While it may appear that rings have a simple task to perform, the physics of what piston rings have to withstand makes their job difficult at best. Conventional pistons utilize a three-ring configuration that seems to perform best. Some drag race engines can get by with a two-ring set, but these are the exception. For the more typical three-ring set, there is plenty of engineering that goes into the material, design, shape, and performance of these rings. There are volumes of Society of Automotive Engineers (SAE) papers on the subject, but we’ll spare you the tedium and cut right to the chase.
The top ring is called the compression ring and its job is simply to seal as much cylinder pressure above the ring as possible so that the pressure can be converted into torque and horsepower. The ring shape, material, weight, and ring gap all contribute to how well the ring can do this job. The second ring is easily the most misunderstood ring. While most consider it a secondary compression ring, the second ring’s primary mandate is to act as an oil scraper to prevent oil from reaching the top ring and polluting the combustion chamber. This ring’s secondary job is to also function as a secondary compression ring and to prevent blow-by, which is combustion pressure and carbon that enter the crankcase, neither of which is positive.
The third ring is the oil ring, whose job is to actually lubricate the piston, rings, and wrist pin assembly and to not allow oil to enter the combustion area. Oil rings also help in controlling piston temperature by using oil to pull heat away from the piston top. Many performance pistons actually employ wrist pin oilers that drill a lubrication path between the backside of the oil ring groove and the wrist pin hole. Piston movement then forces the oil through the path, which keeps the wrist pins lubricated.
The first area to cover in more depth should probably be ring material. Production engines use cast iron rings that do an admirable job at a low cost. For performance applications, the more malleable ductile iron is the next step up and is often used in mild performance applications. The most popular street ring system uses a ductile-iron ring with a moly face. The moly is usually sprayed into a small slot milled into the ring. The moly creates a better ring seal that seats to the cylinder wall very quickly. Beyond coated ductile iron rings, we get into the specialized world of race engine rings. For example, Total Seal offers a steel top ring designed for nitrous and other heavily loaded engines with extremely high specific outputs. These rings are designed for specific applications, but it is worth noting that these very hard rings also contribute to increased bore wear, which is a good reason not to use them in a street application, especially when there are plenty of ductile-iron, moly-filled rings from which to choose.
Ring width is a popular subject and also a source of some confusion. Until recently, production small-block Chevy engines have always used a 5/64-inch ring width. These large rings did an admirable job, but were heavy and required a significant amount of radial tension to seal properly. This increased tension is the primary source of internal engine friction. We’ve seen estimates of up to 60 percent of total engine friction attributed to the interface between the piston, rings, and the cylinder wall. Because of this, performance engine builders, and now even the OEM, are moving to increasingly thinner rings. As an example, many GM production engines now employ 1.5-millimeter (0.0590-inch) thick top and second rings, which equate to a ring thickness that is slightly thinner than a 1/16-inch (0.0625-inch).
The most popular performance ring thickness is the 1/16-inch thick top and second ring combined with a 3/16-inch oil ring. The difference in ring drag between a 5/64-inch ring package and a 1/16-inch ring package is nothing less than amazing. You can actually measure this with nothing more complicated than a simple fish scale hooked to the bottom of a piston with the rings installed and fitted in a cylinder. The reason that the thinner rings create less drag has to do with radial ring tension. With a given radial tension, a thicker ring presents more area to the cylinder bore, creating a given unit loading pressure on the bore. A thinner ring width offers less contact area to the bore, which means the ring designer can reduce the radial tension until the unit loading pressure is similar to the previous load. The gain is less ring drag due to the reduced ring thickness.
Another way to reduce ring drag is with low-tension oil rings. Another popular trick for drag race engines where the engine builder has more latitude, is to reduce the amount of oil ring tension. The down side to this is a thicker film of oil left on the cylinder wall that tends to create oil control problems for the second ring. Ultimately, a small amount of oil could be left on the cylinder wall that could contaminate the combustion process. The problem is that residual oil in the chamber increases the chances of detonation, which is never a good thing. You may want to investigate reduced-tension oil rings since many manufacturers offer several different grades. For street and endurance engines, standard-tension oil rings may be a better choice to ensure adequate oil control.
There are probably as many different ring designs as there are rabid University of Georgia Bulldog football fans. In order to save our sanity, we’ll limit our discussion to a few of the more popular designs that have a place in performance street engines. The accompanying illustrations will help you to better understand how each of these rings is shaped. The most popular performance top or compression ring is the tapered face ring. This ring employs a slight chamfer on the upper inside edge of the ring that creates a twist when installed in the bore. This twist then pushes the lower edge of the face of the ring into the cylinder wall. This, along with cylinder pressure applied to the top and inside of the top ring is what seals the ring to the cylinder wall as the piston travels down the cylinder during the power stroke. Cylinder pressure does the main job of sealing the ring during the power stroke. Twist is still important however because excellent ring seal is also critical during the intake stroke when the piston is again traveling down the cylinder, but this time cylinder pressure is not present to help ring seal.
Another variation of this design is the barrel face top ring that eliminates both sharp edges that come in contact with the cylinder wall. This ring also employs the twist by virtue of the upper inside chamber. A favorite of the drag race crowd is the Dykes top ring that uses a wide face thickness that is reduced where the ring fits into the piston ring land to further reduce ring weight.
For the second ring, there is a taperfaced ring that creates a scraper-like face to increase the amount of oil that can be removed from the cylinder wall. A variation on this idea is the combination of a taper face with a torsion twist. The placement of the torsion twist chamfer on the second ring is the opposite from top ring, however. Note that the chamfer is on the bottom inside diameter of the second ring. This is because we want the second ring to apply the sharp edge of the taper face toward the cylinder wall during the piston down stroke. This is why the chamfer is always placed up on the top ring and down on the second ring. Most ring manufacturers will indicate the top side of either a top or second ring with a pip mark. In all cases when you see a pip mark on a ring, that means it should be installed with the pip mark facing up. If there is no pip (which is often the case on the second ring) then it can be installed in either direction. This usually means it is not a taper face or a torsion twist ring.
There has been quite a bit of noise about the advantages of gapless rings within the engine building community. The net goal with any gapless ring is to increase cylinder pressure by minimizing pressure loss past the rings during the power stroke. Gapless rings are supposed to minimize this pressure loss. It is worth mentioning that the ring end gap established on typical gapped rings when the ring is slid into the bore is much wider than the gap that occurs during combustion. As heat builds in the cylinder during combustion, ring temperature increases which makes the ring expand. The ring end gap is created to ensure that the ring ends do not meet, or butt. If this happens, radial load increases radically which can create excessive ring wear, ring-land erosion, and cylinder wall scuffing, not to mention a drastic loss in cylinder sealing. Clearly, ring butting is to be avoided. However, it is possible to have as little as a few thousandths of an inch of dynamic ring end gap during engine operation, which will allow very little cylinder pressure to escape past the gap.
There is also some information to suggest that while a tight top ring end gap is beneficial to good ring seal, increasing the gap on the second ring may actually increase top ring seal, and therefore performance. The idea is that no top ring is 100 percent efficient in sealing cylinder pressure. As a result, a certain amount of cylinder pressure exists between the top and second ring. Any ring manufacturer will tell you that the greater the pressure differential between the top and second ring is what helps seal the top ring. If sufficient pressure exits in this captured area, top ring seal can be compromised. The theory suggests that increasing the gap in the second compression ring increases this pressure differential (reducing the pressure between the first and second rings) improving top ring seal. This is a rather simple thing to accomplish and the best part is that it’s free. All you have to do is open the gap up on the rings as you are file-fitting them to the cylinder. This is also the reason you will see a machined indentation in the ring land between the top and second ring of many performance pistons. If nothing else, it makes for a great bench racing topic that could last for hours.
There is also evidence to suggest that the Gapless top and/or second rings sold by Total Seal and the Zero Gap Second rings sold by Childs & Albert do in fact perform at the level that their proponents suggest. The author has witnessed tests where the C&A rings have both reduced crankcase pressure and actually generated a slight torque and horsepower increase. In theory, a big-inch street engine could benefit from a gapless ring package if for no other reason than to reduce crankcase pressure and crankshaft windage. The jury is still very much undecided when it comes to these ring packages from a power standpoint. However, what is clear is these rings are generally about twice the price of a standard moly ring package.
There’s plenty more to the subject of pistons and rings. If you do your homework and apply the information presented here, you should be able to avoid most of the common problems associated with big-inch small blocks. If you do this, you’ll end up with an outstandingly durable and powerful street engine that will make you the talk (or is that the torque) of the town.
Written by Graham Hansen and Posted with Permission of CarTechBooks