A conventional ring pack on...
A conventional ring pack on a piston uses three different sets of rings. The top two are designed to maintain compression in the combustion chamber. The bottom, thicker ring set is for oil control.
Sometimes the best solutions are deceptively simple. Take, for example, sealing the combustion chamber in an internal combustion engine. Extreme temperatures mean pretty significant expansion rates when it comes to moving components within an engine and the exacting tolerances required to make that engine work properly. It gets worse when you mix metals that have different expansion rates, which is exactly what happens when you have an aluminum piston traveling inside a cast-iron cylinder bore. Leave enough tolerance between the piston and cylinder when it's hot, and it will never seal well enough to even start when it's cold. Tighten up the tolerances enough to achieve a cold start, and the engine tears itself apart from the inside out before you can even get the car on the track.
Obviously, the answer is to use piston rings to provide proper sealing between the piston and cylinder wall. No news there; piston rings have been a part of the piston-engine formula almost since day one. But that doesn't mean you can throw a ring package haphazardly into any engine package and expect to be competitive on Saturday night. Properly choosing and installing piston rings is critical to making maximum engine power. Don't discount this fact even if you're throwing a rebuild kit into an old Chevy 350 for racing the Strictly Stock classes-it's probably even more critical then because it's so hard to make power in the lower classes.
A ring gap may look significant...
A ring gap may look significant when it's laying on the work bench, but compressed in the cylinder, it should be in the neighborhood of 0.018 inch.
The conventional piston configuration includes three sets of rings. The top ring is the most important. It provides the seal that is used to create compression, hold the burning-and rapidly expanding-air/fuel mixture in the combustion chamber on the power stroke, and push the burnt remains of combustion out of the chamber to complete the process. The second ring is a bit of a backup; whatever compression slips past the first ring, the second ring helps to contain. The third ring is commonly referred to as the oil ring. It is actually two scraper rings assembled with a corrugated spacer ring between them. The oil ring's job is to scrape excess oil off the cylinder walls so that it doesn't mix with the air/fuel charge.
Collectively, the rings ensure that fuel does not find its way into the crankcase (especially important if you're running alcohol, which has the unfortunate tendency to break motor oil down into useless gunk) and that the full effect of combustion is used to push the piston down and turn the crank with the full available force.
Here's a properly gapped ring...
Here's a properly gapped ring in the cylinder. As you can see, the gap appears minimal, but it is critical that each gap is the correct size-and gapped specifically for each cylinder. At operating temperatures, the metal in the ring expands, and the gap allows this without damaging the engine or the ring.
We are not talking about your stress level, although we do hope it's down and you're thinking peaceful thoughts. Ring tension is actually an important consideration when it comes to racing engines. Although a good hone job on the cylinder bores will hold a thin film of oil that does a good job of lubricating the rings as they slide along the bore, there is still a significant amount of friction involved. This is especially true because any friction created by a ring set should be multiplied by eight.
Piston rings for over-the-road cars are designed to last 100,000 miles rather than help squeeze every last bit of power out of an engine. But as a racer, you're willing to sacrifice a little durability for a chance at faster lap times. That's why low-tension rings are a virtual no-brainer. Low-tension piston rings are generally thinner than standard rings and, when properly gapped, do not exert as much pressure against the cylinder wall. You can actually feel the difference between low-tension racing rings and standard rings when you turn the crank in an assembled short-block with a wrench.