piston speeds over 4000fpm, tend to be a long term durability issue. over 4500fpm generally ARE getting into excessive stress levels
http://www.theoldone.com/articles/engin ... nfacts.htm
Custom Pistons 101: Everything You Need to Know!
JE Pistons stocks thousands of pistons in what is called the shelf stock program; each of which has been built for a high-volume application, such as those pistons built for a traditional small-block Chevrolet engines. Many of the applications where shelf-stock pistons are traditionally used can actually share the same forgings, as the bore dimensions across some of these applications are very similar, as are the build requirements of the piston when it is complete.
Many times, a simple $5 change can help get you what you need. Click here
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But when you’re building an engine that’s slightly off the beaten path, a custom piston is just what the doctor ordered. A custom piston order form
is required for custom pistons but if there’s a reference engine application, you don’t have to fill out every line.
Custom Piston Options Include:
- Ultra Crown Domes & Inverted Domes
- 3D Under Crown Milling
- Vertical & Lateral Gas Ports
- Ultra Groove
- Weight Reduction Options
- Contact Reduction Grooves
- Accumulator Grooves
- Double Pin Oilers
- Pin Fitting
- Oil Squirt Notch
- Bottom Oilers for High RPM/severe duty applications
JE offers full under-crown milling to remove as much weight from pistons as possible.
Forged? Or Billet? Which Is Best For Me?
Over the last 70-plus years, the JE Pistons design team has seen just about any engine combination which can be dreamed up and can likely support your engine build with a piston forging to satisfy the parameters required.
A forged piston begins with a mechanical process which requires many hundreds of tons of pressure, whereupon a piston blank – which is shaped somewhat like a hockey puck – is preheated, then smashed into a rough shape which, then cooled and set aside until it is needed for a customer (unless it is a shelf stock part number). There are many steps to the machining process, but a quick rundown has piston blank going through the forging process, then to heat-treating, then into the end-to-end machining following a trail where several steps all come together to produce a ready-to-run, fully CNC-machined piston at the end of the line.
Custom pistons are machined from our wide selection of forgings. However, if a suitable forging is not available, a forged billet can be utilized.
However, there are instances where there is not a piston forging available to work with a particular engine combination, and that’s where billet pistons come into the picture. For instance, when a race team is working with an engine program that has been specifically tailored to their needs, a billet piston may make the most sense for that particular usage scenario. Many of those teams are constantly refining their engine programs and testing various configurations to develop the most efficient burn cycle and flame front.
Want to learn more about piston technology? Head over to the JE Blog!
“The main advantages to choosing a billet over a forging are: first, you don’t need to have a large quantity of parts to justify the cost of making a new forging die and second, any changes to the billet design can be made quickly compared to making a new forging die. No one would consider spending the thousands of dollars required to make a forging die to only make a few pistons. The billet process also allows us to test out potential forging designs before spending the money for a new forging die. Using billets, we can manufacture the pistons and test them out in real world conditions before committing to the cost of making the new forging die,” says Eugene Henson, JE Pistons’ Manufacturing Engineering Manager.
Bead blasting of the underside of billet pistons can help to remove sharp edges which can become stress risers.
Shaping a Billet
In terms of construction, a billet piston starts out as a heat-treated puck, similar in shape to a hockey puck. But unlike a forging, all of the material in a billet piston that doesn’t end up as part of the finished billet must be removed from the puck. This requires a bit more engineering time to design the billet properly, and time from the CNC programming team to create accurate toolpaths to machine the puck into its final configuration.
JE’s advanced CNC machines and cylinder head scanning capabilities allow them to create precise, 3D piston domes.
“If you are developing a racing program you can make slight changes to the strut angles, valve pocket locations, and virtually any part of the structure as you see fit. When working with a forging you are constrained to what the forging you start with can handle. If your engine needs a strut with a more diverging angle than the forging supports, the billet can be just quickly designed to accommodate,” says JE’s Global Automotive Product Manager, Nickolaus DiBlasi.
Available Piston Coatings
- Top Groove Hard Anodize
- Skirt Coating
- Electroless Nickel
- Thermal Barrier Crown
- Tuff Skirt
- Oil Shed Coating
This piston has been treated to gas porting, 3D crown design, and has also had precision “ultra-groove” ring lands machined.
Ordering a set of custom pistons is as simple as providing the measurements you require for your application. For more information, please visit www.jepistons.com
What Makes a Racing Piston?
That clearly is an open-ended question because the answer will single out the uber-sophisticated pistons designed for Formula 1 and World Endurance Challenge (WEC), yet to be completely literal the response must acknowledge hand-modified cast factory pistons used in Hobby Stock racing at the local dirt track.
For the sake of argument, let’s define a racing piston as one that works best for any builder assembling an engine for motorsports competition. From there the discussion can move toward more specific talking points.
“Typically [a racing piston] is a highly developed design that has undergone many iterations to optimize its use under a very specific set of conditions,” said Alan Stevenson, senior technical account manager.
A simple comparison of pistons from the muscle car era of the ‘60s to today’s more advanced products will certainly validate that position. Manufacturers have improved materials and designs through advanced computer modeling to improve power and increase fuel efficiency.
To start this bench racing session, consider the basics of aluminum piston construction. Unless there’s a specific rules restriction, a forged piston will always be preferred in racing over a cast. “Forged is the standard, offering excellent design flexibility and the best cost-to-performance ratio,” said Stevenson. “Cast has no flexibility and can therefore not be developed or optimized. Cast pistons are also heavy and brittle, but they’re cheap to produce.”
Forged pistons can be made of either a forged blank (right), where the entire shape of the piston is machined out of a billet aluminum cylinder, or created from a net forging that is squished closer to its final shape via a forging die (left).
Low-power engines can survive with cast pistons since they won’t see the abuse of blistering heat or high RPM. There’s not much you can do to improve performance with a cast piston, other than grind out a little metal in the crown to clear the valves if a more aggressive camshaft is installed.
In the ‘80s and ‘90s, hypereutectic became a big buzzword in the piston industry. A hypereutectic piston is one that has greater than 12.5 percent silicon in its metal composition, usually around 16 to 18 percent. A standard cast aluminum piston has around 8-to-10 percent silicone content, which improves the hardness and helps reduce wear around the ring grooves, skirt and pin boss. The hypereutectic piston is still manufactured through casting but the alloy is a little lighter, and due to the improved strength the casting can be machined a little thinner to cut down even more weight.
Still, a forged piston will be much stronger and that’s why it dominates the performance market. There are two aluminum alloys popular with forgings: SAE 4032 and SAE 2618. Here’s where another choice is made that often distinguishes, or at least makes a good argument, for a racing piston—although one alloy isn’t always “better” than the other.
“The term ‘better’ is tricky. If only one alloy were better at achieving every goal, there wouldn’t be multiple offerings,” explains Stevenson. “4032 expands less so it requires less cold clearance which tends to run quieter. This could be important for EFI engines with knock sensors. 4032 is less dense so a given design would be lighter than 2618, however 4032 doesn’t have the high-heat annealing resistance that 2618 does.”
Again, silicon content is the differentiating factor. A 4032 piston has about 11 to 13 percent silicon while 2618 has less than .25 percent. “2618 has higher ultimate strength, better annealing resistance at elevated temperatures and has better ductility. In a racing environment, the goal is often to minimize weight without sacrificing durability in high-heat environments. For these reasons 2618 often gets the nod,” continues Stevenson.
The piston on the right uses a 5/64in ring pack. This is a very common size used during the muscle car ere. The piston on the left uses a much thinner 1.00mm top ring. This reduces friction, freeing up horsepower and allowing the engine to rev quicker.
While a majority of racing pistons are made from 2618 aluminum, there are some exotic materials used in high-end racing—or at least they were tried before being banned. Aluminum beryllium alloy, which is exceptionally light and strong with superior thermal properties, was developed by Mercedes/Ilmor for the McLaren Formula 1 team in the late ‘90s. However, the alloy was quickly banned because beryllium dust is extremely hazardous, and in a fire the element will turn into beryllium oxide, which is extremely toxic.
The latest advanced alloy to draw banishment in motorsports is aluminum metal matrix composite, or MMC. Another very stiff, lightweight alloy, MMC is also banned by Formula 1 but continues to draw interest in other areas of motorsports where the rules are more open.
For many years, top engine builders have preferred billet pistons over forged versions—and probably not for the reason you might think. “Billet is not just a simple option to a forging,” said Stevenson. “Billets are approached as complete engineered solutions that are put through several architecture design iterations using FEA modeling to optimize design to a very specific set of environmental conditions. Most popular configurations are offered as a catalog item, but if a combination comes up that doesn’t match, that’s where a forged custom piston can be designed and manufactured at in just a couple of weeks. This is half our business.”
In other words, billet pistons are primarily used in tight-timeline development projects where critical changes can be made quickly without worrying that a proper forging isn’t available. The debate over billet vs. forged in terms of strength will have protractors and detractors on both sides, but generally speaking, a properly executed forging will have inherent strengths in grain structure that a billet piston will not. “Probably 98 percent of racing formulae have a forged option available in the aftermarket,” confirms Stevenson.
Anti-detonation grooves (arrow) are designed to help reduce pressure spikes while contact reduction grooves (just below the crown) reduce how much of the piston’s crown surface make contact with the cylinder wall.
In addition to improved materials and advanced construction methods, specific features have been developed to boost horsepower through either thermal dynamics, reduced weight or reduced friction. Following are some examples of design elements often unique to racing pistons:
Thinner ring packages
Although some exotic engines run 2-ring packages, the majority of racing applications stick with the tried-and-true 3-ring setups. What has changed dramatically is the thickness of those rings, especially in Pro Stock and other non-endurance engines. “You reduce friction and mass, freeing up power and allowing the engine to accelerate faster. The secret to power in any N/A engine is to use the thinnest rings possible, lapped flat and matched with a super-flat ring groove, and ensure minimal axial and radial clearance,” adds Stevenson.
Anti-Detonation Grooves, Constant-Pressure Groove and Accumulator Groove
“Accumulator grooves work on every piston, but work best on gas-ported pistons,” explains Stevenson. “They work according to Boyle’s law; pressure and volume are inversely proportional. Through normal secondary motion (piston rock), the top ring tends to become momentarily unsettled as it rapidly changes direction through TDC. As this occurs near peak firing, combustion pressure tends to make its way past the top ring until it re-settles. Combustion pressure also gets through at the end gap of the ring. Accumulator grooves nearly double the volume below the top ring, reducing pressure according to Boyle’s law and preventing the top ring from becoming pressurized from the underside, which promotes ring flutter.”
Smaller, Lighter Wrist PinsLess weight is always desirable but using a smaller or thinner wrist pin may compromise strength and engine durability. “This often comes at a cost of using better, more expensive materials but also reduces mass to help the engine accelerate faster,” adds Stevenson.
Shortening the piston skirt is a good place to remove mass, but also can be mandatory in stroker applications. Racing pistons can also have different skirt profiles to help them stay stable in the bore at high rpm.
Again, more design changes to reduce weight and friction but these efforts are usually dictated and restricted by the engine architecture, such as length of the cylinder sleeves and the stroke. “The gauge point of the piston skirt must remain captured in the bore at BDC,” said Stevenson.
Some engine builders want to reflect the heat away from the piston, preferring that the valves and cylinder head dissipate the heat to the coolant instead of the pistons and rings through the cylinder wall. Thermal coatings are designed to repel heat from the piston crown, and in some cases the combustion chamber as best as possible.
“By far the most friction in an engine is from the rings. A distant second are the bearings. Skirts have minimal friction since they ride on an oil film,” says Stevenson. On racing pistons, top teams use coatings as an insurance policy against overheating. In OEM and daily driver conditions, they are used to protect against dry starts and other situations where oil on the cylinder wall is limited.
This piston features vertical gas porting which allows combustion pressure to get behind the piston ring, forcing it into the cylinder wall and increasing ring seal. It is most commonly used in drag racing applications.
Application Specific Modifications
“Some modifications are inappropriate for one type of racing while done routinely on pistons made for other types of racing,” cautions Stevenson. “Some examples are spinning the bosses, plunging the bosses, drilling holes in the skirts or struts, and 3D under-crown milling.”
3D profiling is a precise milling procedure where the piston crown maintains the same thickness regardless of the dome profile. This step ensures that the crown has the necessary strength and heat resistance for competition with the least amount of weight. “This is especially important when trying to maximize compression ratio, and it can also be used around valve reliefs to promote smoother flame travel,” adds Stevenson.
The actual crown design and valve relief dimensions will be dictated by the combustion chamber and valve geometry. Another racing piston cue is that the engine builder will send a mold of the combustion chamber to the piston manufacturer so that the dome design exactly follows the profile of the chamber.
Finally, some engine builders call for very slight adjustments in the overall piston shape and dimensions to suit their needs.
“Skirt cam/barrel shapes and ring-land diameters are part of the black art of optimizing a design during a development program,” says Stevenson.
As you can see, a racing piston may have one or many of the features and modifications mentioned. The key is designing a piston that meets the needs of the engine builder assembling an engine for a specific competition.
This article was sponsored by Wiseco. For more information, please visit our website at wiseco.com
the piston moves twice its stroke distance per revolution, 4000feet per minute = 48,000 inches, 4500fpm = 54000 inches
example a 350 sbc with its 3.5" stroke =7" per revolution, so 4000fpm =6850 rpm as a fairly reasonable limit with stock components, swap to all balanced and forged components and ARP fasteners and you can push the limits briefly to 4500fpm or 7700rpm but that is very unlikely to allow the engine to live long and remember the valve train probably won,t hold up near that rpm level and your unlikely to make effective power because just filling the cylinders becomes extremely difficult because you need to open the valve, fill the cylinder , burn the mix, exhaust and refill the cylinder 64 times a second at that rpm
RING END GAP CLEARANCE needs to be carefully checked
The piston ring's end gap can have a significant effect on an engine's horsepower output. Rings are available both in standard gap sets, and in special "file fit" sets. The file fit sets allows the engine builder to tailor the ring end gaps to each individual cylinder. Ring gaps should be set differently dependent upon the vehicles use, within the range of .003" (for the 2nd. ring) to .004" (for the top ring) per inch of cylinder diameter. The more severe the use, the greater the required end gap (assuming the use of similar fuels and induction systems). Engines having low operating temperatures, such as those in marine applications is too small. The chart below is a general guideline for cylinders with a 4.00" bore, adjust the figures to match your engine's cylinder diameter:
Top Rings (ductile iron, 4" bore)
Nitromethane .022 - .024"
Alcohol .018 - .020"
Gasoline .022 - .024"
Normally Aspirated - Gasoline
Street, Moderate Performance .016 - .018"
Drag Racing, Oval Track .018 - .020"
Nitrous Oxide - Street .024 - .026"
Nitrous Oxide - Drag .032 - .034"
2nd Rings (plain iron, 4" bore)
Nitromethane .014 - .016"
Alcohol .012 - .014"
Gasoline .012 - .014"
Normally Aspirated - Gasoline
Street, Moderate Performance .010 - .012"
Oval Track .012 - .014"
Pro Stock, Comp. .012 - .014"
Nitrous Oxide - Street .018 - .020"
Nitrous Oxide - Drag .024 - .026"
INSTALLATION NOTES -
CYLINDER WALL FINISH
When installing new rings, the single greatest concern is the cylinder wall condition and finish. If the cylinders are not properly prepared, the rings will not be able to perform as designed. The use of a torque plate, head gasket, and corresponding bolts are necessary to simulate the stress that the cylinder head will put on the block. Main bearing caps should also be torqued in place. The correct procedure has three steps. First the cylinder is bored to approximately .003" less than the desired final size. Next it is rough honed within .0005" of the final diameter. Then a finer finish hone is used to produced the desired "plateau" wall texture. Use a 280 - 400 grit stone to finish cylinder walls for Plasma Moly rings.
Note - the "grit" number we are referring to is a measurement of roughness, it is not the manufacturers stone part number (a Sunnen CK-10 automatic hone stone set #JHU-820 is 400 grit). The cylinder bores should be thoroughly scrubbed with soap and hot water and then oiled before piston and ring installation.
Piston ring grooves are also sealing surfaces, and must be clean, smooth and free of defects. Ring groove side clearance, measured between the ring and the top of the groove, should be between, .001" and .004".
Id call the piston manufacturer and get their tech guys feed back, you'll generally want about a .005 piston to bore clearance with forged pistons but every piston design and manufacturer will have the required specs on file for every piston they make ,that should be carefully followed
don,t forget to ask about ring end gap,ring thickness,ring back clearance and ring groove size, quench distance, compression height, valve notch clearances and cylinder bore hone grits to be used,what piston pin and retainer should be used and what the pistons should weight, always ask "what else should I know?" and "what would be YOUR suggestions here?"
ID suggest trying for a MINIMUM of .038-.044 on a 496 BBC for quench as the long stroke bbc, or any BBC run over about 6000rpm with a 4" or longer stroke) tends to allow the quench clearance distance to tighten up a bit at high rpms, as parts stretch under stress/inertial loads
damage like this is rather frequently the result of the piston rings gap being too tight, the rings ends touch as they expand,
due to excessive heat and with no clearance the rings temporarily lock against the bore wall, the result is a broken ring, and fractured piston ring groove