Jay Dickens tells us all about...
Jay Dickens tells us all about the various technologies involved with racing pistons.
The modern racing engine has several parts that are tunable as to design and configuration. The piston is one of these parts. We are required to hold to certain dimensioning for bore and stroke, crankshaft size, rod length, head and intake design, carburetors, and valvetrain geometry. Any tweaking we do has to be done to very few parts. Here, we will explain the parts and functions of the modern racing piston.
Of the few engine parts we can freely design, the piston, the cam, and our valvetrain geometry are the most popular. Since the cam design is best left to the manufacturer's professionals, and valvetrain geometry is often quite complicated, we have the ability to choose our piston design without possessing a college degree.
Not to oversimplify the process of choosing the right piston for your needs, the final say should be left to an engine professional. But with a little knowledge, we can influence the choices that are made by our engine builder. When we build our own motors, it is our own choice. Either way, it doesn't hurt to gain a little insight into the process of choosing the right piston for your application.
The choices involve the following:
Racing pistons come in all...
Racing pistons come in all shapes and sizes, and the design you need for your application is specific to the heat cycles and rpm you will experience.
The choice of the final weight and the alloy used in the production of the piston are determined in part by the overall strength needed for your application. Life expectancy is a major consideration. The two most common materials used in the aftermarket today are 2618 low-silicon and 4032 high-silicon.
The range of horsepower output of the engine, the intended rpm range, and the duration of the running time per cycle of the piston are all important considerations in determining the correct piston for your engine based on its use. Running one race cycle of 500 miles, an entire 12- or 24-hour endurance event, or a certain number of laps per weekend over an entire season are examples of different engine run time cycles.
The cut for relief of the...
The cut for relief of the valves should be to a minimum. This piston is designed for the Nextel Cup motor Jay built. It is quite radical from what short-track racers are used to.
For overall piston design, we will need to know maximum power and how high an rpm we will reach to determine, among other things, skirt length, the heat range or determine head thickness and the compression ratio for strength design.
A 350hp two-barrel Late Model motor topping out at 5,800 rpm and running 35-lap features would require a much different piston design than say a 650hp Super Late Model motor turning 7,200 rpm and running a 200-lap race.
The forces acting on the 350hp motor would be much less than that of the higher horsepower motors, and so the piston could be much lighter in design, which could help reduce wear and tear on the bearings, oil, and rings.
The height of the wristpin,...
The height of the wristpin, the length of the skirts, the dome shape, and the height of the ring grooves are all variables in the design of the piston. Each area is built specific to the application.
Piston weight, for example, can be reduced in several ways. One way is by the use of pin squirters that are machined into the block. These spray oil onto the bottom of the piston to cool it, thereby allowing us to run a thinner crown, thus saving weight.
Reduced skirt length for lower rpm applications saves weight over the use of pistons designed for higher rpm. The use of lighter pistons will result in less rotational weight and quicker acceleration of the drivetrain. Be careful in your choice of piston. Just because a piston is right for a high-horsepower motor does not mean it is the best choice for your 350hp two-barrel engine.
The pin diameter and length are also relative to the horsepower and rpm range your engine will experience. These determine the size of the pin boss, which affects the weight. Barrel and skirt design must not interfere with the crankshaft or connecting rods.
The valve reliefs should be cut to the minimum clearances for the lift and diameter of the valves to be used.
Different chamfers and radii here can be beneficial to fuel mixture and exhaust flow. And the dome or dish shape should match the combustion chamber exactly.
The ring package design is...
The ring package design is specific to the task the engine will be expected to perform. Ring tension, ring thickness, and ring material are all considerations when putting together your motor.
The overall purpose of the ring package is to stabilize the piston in the cylinder, lubricate the piston and pin, keep the oil in the crankcase, and maintain compression in the combustion chamber. We want a ring as small and thin as needed to control the issues of the package.
The closer to the top of the piston you locate the rings, the more cylinder pressure you can make. This is a good thing, but there are some drawbacks, too. This also puts the rings closer to the heat, making it necessary to run larger ring gaps or even thicker rings. It can also cause problems with valve relief to ring land thickness issues. A lot of thought should go into ring location.
On this piston, note the small...
On this piston, note the small holes drilled at the top ring groove. These provide added compression force to help seat the ring. This way, a lighter ring can be used to reduce friction during the other portions of the cycle besides the compression cycle.
Gas ports are often used so that a looser tension ring pack can be used. The ports help seal the rings under compression so you can run less ring tension, while the lighter ring package helps reduce drag at the other parts of the cycle.
The life expectancy of the rings is determined by the length of the racing cycles and how many cycles the engine will run. Rings tend to lose tension after a heat cycle, so knowing the heat cycle history will help you choose the right rings.
We measure ring tension with a scale in pounds of pull. This is done by installing the piston rings and rod in the piston, placing the piston in the bore upside down, and then pulling on the piston rod with a scale to see how many pounds of resistance it takes to move the piston. There are different goals for tightness, depending on the intended life cycle of the engine.
To measure ring tension, the...
To measure ring tension, the piston is installed in the bore upside down. A scale is then attached to the rod, and the amount of force in pounds needed to move the piston is noted. This is done on assembly and after the engine has completed a normal run cycle.
Notes on ring tension are taken when the engine is built and when the engine comes back. After a certain life cycle, it is checked again. If too much tension is lost due to normal operation (the engine was not run hot), then the rings must start out with more tension.
The choice of oil recovery systems influences the ring tension needed. A wet sump oil system requires a heavier oil ring tension, whereas a dry sump oil system that pulls crankcase vacuum requires very little oil ring tension.
Knowing the maximum rpm is important. Running at a higher rpm level is a factor in ring selection where special rings are designed to help reduce ring flutter.
We have the choice of using many different coatings on the pistons and pins. Here are a few determining factors.
The rings are carefully fitted...
The rings are carefully fitted to the bore. Minimum gapping should be the goal, while still maintaining enough clearance to prevent the ends from coming together under high heat conditions.
1. Break-in and scuffing Cold and dry starts contribute to skirt wear, or scuffing. Engines with external oil systems and piston oil sprayers that can be primed before starting every time cut down on this. Most dirt Late Model engines with bellhousing-mounted oil pumps do not offer this luxury, so a piston coating is necessary.
Thermal expansion and distortion of the cylinder bores occurs from a cold engine being started in the pits at ambient temperature and then raced at 200-plus degrees F. Changes in piston-to-wall clearances, at different heat ranges, occur constantly. Heating just the oil does not guarantee that there will be no wear due to improper clearances from a cold block. We are now urging our customers to use external water heaters so cold starts are eliminated. A unit we discovered is The Hot Setup(tm). This is a water heater for the engine block that heats the entire engine. The unit can heat the entire cooling system to 180 degrees F.
2. Frictional Losses (lubricity) Friction costs us valuable horsepower, causes excessive wear (galling), and creates unwanted heat. Areas where friction can occur include the piston skirts, the pin bores, and the piston pins. Ring lands are anodized to combat microwelding of the rings under extreme conditions.
Piston coating can reduce...
Piston coating can reduce the amount of heat that escapes from the cylinder as well as the friction from wristpin motion and piston travel.
3. Extended life Coatings that prevent wear keep the piston dimensionally accurate for a longer period of time. Coatings that control heat keep the piston's Rockwell hardness in range much longer. Depending on how extreme the operating conditions are, coating may be necessary to achieve the intended life cycle of the piston, not necessarily to extend its life cycle.
4. Heat control We want to keep heat in the combustion chamber and not transfer it into the piston if possible. A hot or glowing piston is a source for pre-ignition and detonation. Also, heat changes the hardness of the pistons (creating a lower life expectancy) and causes thermal distortion. By coating the top of the piston with a heat barrier, we address this issue by reducing the transfer of heat to the piston.
The top ring groove is Hard...
The top ring groove is Hard Anodized to prevent the ring from being microwelded to the piston. The small holes provide added ring pressure on the compression stroke, but also introduce a high-pressure flame directly to the ring and groove. Anodizing provides protection.
5. Oil Shedding Now that we have engineered a lightweight piston, we do not want oil attaching itself to the bottom of the piston, making it heavy. This is especially necessary when you have oil spraying directly on the bottom of the pistons. This is a very touchy subject, as there are many theories about how long the oil needs to be in place to do its job of cooling.
Piston coatings can reduce the time the oil spends on the bottom of the piston, thereby reducing piston weight and the possibility of oil cooking and building up on the piston. The oil does its job of cooling the piston, but doesn't hang around long.
Custom pistons are not for the novice engine builder. For low-buck engines and builders just starting out, perhaps a shelf stock piston would save you from having to figure this out. Custom pistons allow the professional and experienced engine builders the ability to control every aspect of piston design in order to maximize the package for a particular engine.
If in doubt, talk to your engine builder or your choice of piston manufacturer. They have experts who deal with making these choices on a daily basis and they really do want to help. Give a call and you just might find a better piston package for your application.
This piston is coated on the...
This piston is coated on the skirt and crown. The skirt treatment reduces friction while the crown treatment repels heat.
Jay Dickens is the owner of Jay Dickens Racing Engines. He was interested in cars from an early age, and actually built his first engine at the age of 12. During his high school years, Jay worked for the late Bobby Brown, helping to build racing engines for the likes of Bobby Allison and Grand Adcox.
A few years later, Jay started working on engines in his garage at home on a part-time basis. In 1991, his part-time business was growing and he built his first building dedicated to engine building. Finally, in March 1994, Jay decided to quit his day job and officially went into business as Jay Dickens Racing Engines (JDRE).
Over the past 12 years, JDRE has grown from a small company with a few customers into a large-scale business with customers nationwide. Along the way, JDRE customers have picked up track championships, series championships, and countless wins. The JDRE customer base ranges from dirt Late Model competitors to drag racers to competitors in NASCAR Nextel Cup racing.
Coating the bottom of the...
Coating the bottom of the piston helps shed oil to aid in cooling and to prevent the build-up of burnt oil on the bottom of the piston, which adds to the weight of the piston and causes unnecessary retention of heat.
Recently, Jay built engines for a Nextel Cup team as a side project. His motors stood up horsepower and stamina wise, with the best engine builders in Cup racing. As a short-track engine builder, he has again proven that money and exposure does not necessarily guarantee success. He has every bit as much expertise as any of the engine builders in stock car racing's top division.
Brad Loden started attending circle track races with his father at the age of 5. By the age of 13, he was racing BMX Bicycles and was the Mississippi BMX Bicycles State Champion. Brad's interest in motors started to take off at about the same time, and he built his first engine at the age of 13.
After a short stint in racing go-karts, Brad moved into drag racing in 1991, racing a bracket car that he built. From bracket racing, he progressed into racing in the IHRA Top Dragster division. When he was not racing his own car, he spent time serving as crew chief for Outlaw Promod/Top Sportsman cars. Finally, in the mid-'90s, Brad gave up his driving role and went to work for JDRE.