The first upgrade to a cast-iron ring is applying a molybdenum or "moly" coating to the ring. A moly coating slows the cylinder-wall wear rate and reduces friction. Moly also has the highest melting point of any of the face coatings used, but can flake off if the engine experiences severe detonation. Moly-faced rings work extremely well in high-rpm or high-load conditions. The main disadvantage of moly-faced rings comes into play in dusty or highly abrasive conditions because its porosity can trap dirt in the face of the ring. This is a good reason to always run an air cleaner.
After moly-faced rings comes cast rings with a hard chrome facing. The chrome, like moly, is applied to the ring and although it displays better adhesion characteristics and fewer tendencies to flake than moly, it cannot withstand the same high temperatures and does not have the high resistance to scuffing like a moly-faced ring. Chrome is very hard making it the best choice for engines run in highly abrasive environments. Chrome-faced and moly-coated cast rings both require a specific bore finish to seat properly and may not actually last any longer than an uncoated ring.
Racing RingsDuctile-iron rings rank considerably higher on the performance ladder than cast-iron rings. Ductile-iron rings can also be coated for good break-in and wear characteristics, but they cost more. Ductile iron is probably the most widely used ring material in racing engines today because it can take the abuse of higher compression and detonation. That's because ductile iron does not shatter or break like cast iron. If you were to bend both a cast-iron and ductile-iron ring by hand, the cast ring would snap, while the ductile ring would merely flex.
Ultra-high-performance racing rings are usually manufactured from one of two different steels: stainless or H-11 tool steel. Both of these steels are very durable but can take a long time to seat and can wear the cylinder walls. Most of these rings are chrome-faced, although some are available nitrided. Stainless and tool steel rings work well in engines that will see high heat from turbos, superchargers, or large doses of nitrous oxide. Steel rings also have the highest tension.
A widely used racing ring material is HF479 iron composite. Most ring manufacturers are currently offering HF479 race rings specifically geared towards very high-compression and/or nitrous racing engines. The HF479 material is a special ductile iron with Ni-chrome plasma facing that delivers unmatched wear and flaking. Although HF479 rings may be the best race rings going, they're not available to fit standard rings grooves or in small bore sizes which makes them less than ideal for your street car.
Gapping For PowerOddly enough, even "gapless" rings still have end gaps. To be effective, the proper ring gap is more dependent on what usage the engine will see rather then on the rings themselves. Generally, a top ring gap of at least 0.004-inch per 1 inch of cylinder bore is sufficient on a normally aspirated street engine. If you do the math on a typical small-block that has been bored 0.030 inch over, (0.004 x 4.030) you'll find that a top ring gap of 0.016 inch is what to use. However, it's wiser to increase that factor by about 0.002 making 0.018 inch your minimum top-ring gap. This compensates for piston and bore tolerances that could be a little off. As engine demands increase, so should the ring gap.
Since the second ring is not responsible for the majority of combustion sealing, it will not see as much heat and consequential expansion. Therefore, the second ring can run a smaller gap than the top ring. The same small-block street engine described earlier would do well with a second ring end gap between 0.010 and 0.012 inch (0.0025 inch per inch of bore).
Ring sets for mild-performance engines can be purchased in pre-fit packages, which will not require grinding the end gaps to fit. But any serous engine will run custom, file-fit rings. Generally, a file-fit ring set will be 0.005-0.015 inch oversize, requiring the builder to check, custom file, and then recheck each individual ring set in each specific bore. Many aftermarket companies offer piston ring grinding tools that can make the task of file-fitting rings much easier. Total Seal offers its own battery-powered ring grinder that makes custom ring filing fast and even kinda fun. The old fashioned method of dragging the ring across a mill-file clamped in your bench vise is highly inaccurate. If you're planning on file-fitting your rings, a ring grinder is a good investment.