Modified Mustangs & Fords
Exploring Piston And Ring Technologies
A Better Understanding Of Piston And Ring Technologies Can Lead To A More Potent Engine Build
But there is more to a modern ring set than just thickness. There are new materials, new designs, and to be certain, different schools of thought. In the old days, the top two rings were typically cast-iron, or perhaps a better ductile iron for the top ring with a chrome or flame-sprayed molybdenum coating. Today, ductile iron is the standard bearer for performance top rings, and in all likelihood, with a plasma-sprayed moly coating-thus the term plasma-moly. A big step up for top ring material is stainless steel, particularly as the thickness narrows. Depending on the manufacturer, stainless will be chrome nitrided or plasma-moly, and in some high-end racing applications, can even be tungsten or titanium nitrided.
Second rings have long been cast-iron since the heat and abuse in this location is much less than the top ring. That's pretty well the case today, and yet along with thickness, perhaps the biggest change in the second position is the growing use of Napier rings-which dovetails with current thinking that the second ring is really more about oil control than compression. Napier rings are seen in OEM applications, and every aftermarket manufacturer we spoke to advocated their use whenever possible. Being a better oil controller than the typical reverse torsional taper face second, a Napier is designed to put less pressure on the cylinder wall, and also allows for a lower tension oil ring-a double whammy friction reduction!
Oil rings are still a three-piece affair, with an expander located between two rail rings. You might be surprised to learn that the majority of friction in a ring package is right here, sometimes on the order of 60-70 percent depending on oil ring tension. Is there a way to free up power and efficiency here? Yes, but going to a "low tension" oil ring isn't necessarily the answer, because the label doesn't apply equally across the board. For example, a 3/16-inch low-tension ring will certainly have less tension than a 3/16-inch standard-tension ring, but that 3/16-inch low-tension will actually have more tension than a 3mm standard-tension ring. Basically you can only compare terminology within the same sized product. Akerly and Childs' Ray Akerly explained that if when comparing a 3/16-inch and a 3mm oil ring, each exerted the same amount of pressure on the cylinder wall, the thinner 3mm ring would seal better since it is better able to conform to inevitable bore distortion. Total Seal's Studaker added that for a hot street engine with a good finish hone and all else in order, 15-16 pounds of pressure on the wall is adequate, which equates to a low-tension 3/16-inch, or, believe it or not, a high-tension 3mm. As a frame of reference, a standard-tension 3/16-inch oil ring is in the range of 20-24 pounds of pressure.
Gapless rings are considered by some as the ultimate top ring, and are sometimes used in the second position as well. They've won plenty of fans, while many others remain solidly in the traditional camp. The idea is that without any gap at all, you eliminate virtually all possibility of combustion getting past the top ring, resulting in more power. Likewise as the cylinder wears over time, there is no end gap to increase in size. It's a concept that would appear to have merit, as the goal in a conventional ring is to be as close to zero gap as possible at operating temps, while avoiding the ends butting together. Perhaps a test conducted by our sibling magazine Engine Masters Challenge, best illustrates the potential of a gapless top ring. In the test of a 450-horse Chevy (sorry guys), Total Seal's gapless top rings made 10 more peak horsepower when compared to a set of file fit rings-the latter being no slouch in their own right!
Back in the world of conventional rings, much more consistent opinions were offered when the subject of the second ring gap was discussed. In years past, it was common to run a tighter gap on the second ring, compared to the top ring, again trying to achieve a near zero gap. This could be done since the second ring is subject to less heat, and thus doesn't expand as much as the top ring. That concept has largely been replaced with the idea that the second ring gap should be bigger-perhaps on the order of 1.25 times the top gap, to allow any gasses which slip past the top ring a way out. Trapped gasses between the first two rings will tend to unload the top ring and diminish its seal, which is also why many newer pistons include an accumulator groove between the first and second ring grooves-it acts as a reservoir for blow-by.