Ring tension is a critical element of combustion sealing, but myths surrounding it run rampant. We clear the air and set the record straight on ring tension.
Ring tension is among the more misunderstood dynamics of a performance engine—so much so that some engine builders aren’t sure which rings they’re actually talking about. “Oil rings are typically where tension is discussed,” says Alan Stevenson, Senior Technical Account Manager. “The top and second rings don't have tension ratings on them. The only ring of the three that has the tension rating in the industry is the oil ring. And so, in any given size you could have varying degrees of tension with the oil ring.
Depending on the bore size, you'll have some 3/16 rings that are high tension, standard tension, and low tension. But exactly what that translates into in terms of pounds of force is going to change relative to bore size.” In other words, comparing ring tension between engines can be misleading. For example, a standard tension 3/16 ring for a 4.000-inch bore could be 17 pounds. However, a standard tension 3/16 ring for a 4.500- or 4.600-inch bore could be in the 20- to 22-pound range.
These pound ratings for the oil rings are often mistaken with the effort needed to extract a piston from the cylinder. Engine builders have long installed a piston, flipped the block over and attached a fish scale or other weight-measuring device to the underside of the piston. They slowly pull out the piston and take note of scale as method of comparing the friction ring packs and selecting the one that provides the lowest friction.
Another method of testing ring-pack friction is to install the rotating assembly, then use a beam or dial torque wrench to turn over the engine, taking note of the torque required. “So, if you look at a tension rating on a ring, it's really a different type of tension. When some think of tension in pounds, they think that's pull tension,” explains Stevenson. “That's not what tension is. It's not pull tension. It's what's called tangential tension. It's the instantaneous tangential force that the ring exerts on the cylinder bore. And you need special equipment to be able to test that. Now there is a translation between tangential tension and pull tension. But it's not a direct correlation. As mentioned earlier, there are three rings on the piston but only one has a rating.” And which ring contributes the most to pull tension? “Remember, there’s a difference between tension and friction, between tension and drag. The ring that has the most drag, that is, the most effect on resisting engine rotation, believe it or not, is the second ring,” says Stevenson. “Because the second ring has either a tapered face or a Napier face, and it has a sharp edge. That ring is designed with a bevel on the inside. It's called a reverse twist ring. So. It's actually meant to kick that sharp edge into the cylinder.”
Another misnomer about the ring package is that the second ring acts as a compression ring. Basically, the second ring has very little if any effect on combustion sealing. That’s the sole purpose of the top ring. The second ring is designed to scrape oil off the cylinder wall. “The top ring has a barrel face to it. So, as that ring twists and moves around in the groove, that barrel face will always keep in even contact with the cylinder wall. It's not a sharp edge, it's a rounded edge,” explains Stevenson. “So, if you would have measured the tension of a top ring and a second ring, both of the same material and the same size, they would both have equivalent tension. But, they don't create the same amount of friction and drag because of the contour and the shape of the ring faces.” And here’s where much of the misunderstanding has spread about the difference between standard and low-tension rings.
It’s difficult to predict what the actual friction or drag, that is, the pull tension, of any given ring pack. “Yes, the oil ring definitely has tension to it. And that's what kind of gets to the crux of the matter. A lot of people don't understand when they ask for low-tension rings. A lot of people believe that that means that all three rings are low tension, and they're not,” reminds Stevenson. Low-tension ring packs don’t make horsepower but rather free up the acceleration of the reciprocating parts.
Stevenson says that’s a dynamic that can’t always be measured on a standard dyno. “However, it’ll show up on a racetrack,” says Stevenson. “Let's say you built two engines side by side. One has all stock components, the other with a rotating assembly that is 15 to 40 pounds lighter. Both engines will make the same peak power. The weight of the components doesn’t make horsepower. That's a function of the intake, heads, cam, compression ratio and so forth. But, if you were to take both engines to the track, the engine with the lighter components will get off the corner better and accelerate faster. It will out preform the engine with the heavier components.”
If the top ring is too thin, it simply won’t seal up the combustion gasses, especially in a high-horsepower engine. That’s where gas ports that help force the ring against the wall become a critical design factor in a racing piston. Another upside to gas ports is that the added pressure forcing the rings against the cylinder comes only on the power stroke. “So the downside to gas ports is that they increases friction and increases effective tension on the power stroke,” says Stevenson. “But, on the other three strokes you're getting the benefit of that lower friction and lower tension.”
Thinner, lower tension rings are the current trend, even in production engines where fuel economy issues are driving the ring and piston design. The lower drag allows the engine to get more work out of the same combustion. The automakers, however, have sophisticated computer modeling and test equipment that allows not only the reduced ring size but also ensures durability to meet the powertrain warranty.
For those building a street or race engine who want to take advantage of the low-tension rings, the decision begins with talking to a knowledgeable tech rep who has an understanding of the lubrication issues involved with the specific application. Also, some newer engines will have fewer choices. “Because there are certain types engines with deep-skirted blocks. The modern engines doe a better job of controlling windage, which has a direct correlation to how much friction and tension the second ring and the oil ring needs,” says Stevenson. “If you were to go back to a traditional small- or big-block Chevy and try to run an .043 second and a low-tension 3 mm oil ring without giving the rings any help in terms of windage, you're probably not going to be able to control oil very well.”
That’s why oil pan accessories such as a crank scraper and windage tray are valuable to performance engines, and vacuum pump is especially helpful on a race engine. “It just depends on what you’re going to do with the engine,” sums up Stevenson. “Is it a street car? Is it a drag race only car? Is it a wet sump? Is it a dry sump? So, it really needs to be a consultative process to match up how small the second ring you can go with, and how low the tension in the oil ring you can go with in order to still maintain oil control.”