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.