Race Rings

Reducing friction and weight while maintaining needed sealing presents challenges to pro race engine builders.

by Mike Mavrigian

Evolutionary and revolutionary changes are constantly taking place with regard to pro-racing piston ring applications. To gain a bit of insight in terms of what’s happening at the moment, I spoke to two leading sources, including Mahle-Clevite’s Bill McKnight and Sealed Power’s Scott Gabrielson. Following are their comments.

Bill McKnight, motorsport marketing and technical advisor for Mahle-Clevite, notes “As far as circle track teams are concerned, most everybody in NASCAR is currently running 0.8mm top rings, and many are also running 0.8mm second rings. Weight reduction, increased engine speed and sealing are critical issues with these teams.

“Regarding materials, most applications use plasma moly ductile or plasma moly steel for just the top rings, while they continue to use ductile iron second rings.
“Most engine builders are using three-piece oil ring packages, using widths of either 1.5mm or 2mm.

“Addressing the need to reduce friction, low tension upper rings are in the 7-11 lb range, and ultra-low tension oil rings are in the 4-6 lb range. Our new U-Flex oil ring offers an extremely low tension while maintaining oil seal (it’s so flexible that you can almost tie knots in it).

“These builders are currently running very tight pistons (dimensionally), with short compression distances and hardly any skirt area. As a result, the rings are more tightly crowded together than ever before.
“The majority of builders are using barrel-faced compression rings, with the second ring style favoring a reverse twist taper faced style (80-90% of the second ring’s job is oil control).

“One of the really cool advances with regard to oil rings is our new
U-Flex ring design, which is a one-piece design that has a segmented appearance. It’s really flexible and offers very low tensions. Some of the Cup teams have started using this ring with great success.

“To avoid potential microwelding, everybody uses hard anodized rings lands, which should come as no surprise.

“The NASCAR Cup teams spend a lot of money having custom rings lapped to their specific sizes. It’s all about reducing friction. In the last three to four years, rules have placed severe restrictions on displacements, with engine builders faced with the need to produce more power within these rules. Most of these engines are now making well into the upper 700 HP range. Through extensive development, within the last seven to eight years, these Cup engine builders have found about an additional 70 HP. Certainly, part of those results involve extensive piston ring development.

“In the drag racing community, while top teams certainly spend a great deal of time and effort with regard to ring selection, they don’t spend as much (in terms of time or money) as the Cup teams. The drag racers’ concern focuses more strongly on sealing than in reducing friction. Most are still running dykes rings and gas ported pistons to achieve a good combustion seal.

“Many racers don’t fully understand the issue of ring end gap. While most understand the traditional rule of thumb of 0.003″ gap per inch of bore, most guys run 0.016″-0.018″ gap on top, while some builders run second ring gap at the same value, or more or less than the top gap. Mahle-Clevite recommends running 1.25 times up to 1.7 times more gap at the second ring than is used at the top ring. You should always run a bigger gap on the second ring as opposed to the gap used on the top ring. The reason: you don’t want a gas buildup to occur between the rings.

“Most racers are running moly top rings for extreme heat resistance. If you take a close look at the surface of a moly ring, you’ll see that it’s rather porous. This surface provides an area for oil to cling, resulting in a well-lubricated top ring.

“Also, to address the needs of the guys running nitrous injection, we offer the Perfect Circle PC479 nitrided ductile top rings designed to help reduce heat.
“Top Fuel builders continue to experiment with top rings in an effort to obtain optimum combustion pressure seal. Their research and development is kept top secret, since each team’s engine builder invests quite a bit in this process.

“When running good, top fuel engines produce in excess of 10,000 psi cylinder pressure. At end of strip, the driver is leaning on them hard. They commonly have problems regarding top ring seal, and in trying to make the land under the top ring survive, due to the enormous pressure that causes the top ring to push down on the land.”

According to Scott Gabrielson, product manager of cylinder head components for Sealed power/Speed Pro, “Pro racing is probably becoming class of their own…as opposed to amateur or semi pro racing…in the upper echelons of pro racing, such as Nextel Cup, most teams have fairly substantial budgets and can afford to go explore developmental territory where others can’t afford to go. Concerning the issue of piston rings, these engine developers are getting pretty exotic, demanding ring dimensions that can’t practically be produced on production equipment. This may involve custom-made rings that take advantage of special prototype material and unique dimensions. As an example, some of these compression rings are a mere 0.8mm in width (about 0.032″), and fairly shallow as compared with a ring produced from iron. Typically, we’re seeing steel or stainless steel with very exotic coatings to provide wear resistance on ring face, as well as some exotic materials designed to help reduce friction.

“Nitriding or titanium nitriding may be involved, where titanium is added to the nitriding process to create extremely hard and durable surfaces. In terms of cost, most of these rings range in the area of about $100 per cylinder. At the Nextel Cup level, this is what many Cup engine builders feel is required to be competitive.
“One of the reasons for thinner and lighter rings involves weight savings, but the smaller dimensions are also partially dictated by the available ring location on the pistons, where a minimal area between the pin bore and top is available. Basically, the rings must be “shrunk” to fit the available space.

“Per NASCAR rules, displacement is limited, so builders are forced to raise the RPM limits, which means reducing the rotating and reciprocating mass. Basically, everything is shrinking as a result.
“As far as second rings are concerned, builders seem to still choose an iron or intermediate higher strength iron to provide resistance to breaking. These builders seem to be favoring a napier style with an OD groove on the bottom corner.

“Oil rings are dwindling down to 2mm in width, again getting narrower to fit in the available ring land space.

“From a standpoint of trying to prevent microwelding, the only sure fix is to hard-anodize the ring groove. The crown and top groove area treated together. In OE applications where anodizing is featured, only the top ring groove is treated.
“As far as ring tension goes…it used to be that half the friction showed up in the piston and ring package. Now, that percentage has gone down quite a bit. Today, builders try to reduce friction with the use of coatings and special bore finishes, but obviously there is a limit. The top ring can be produced with a fairly low tension because it is sealed via compression. Nikasil has been tried on cylinder bore walls, but the jury is still out on whether or not this is helping to substantially reduce friction.

Our standard Speed Pro Pro Series (a package featuring 0.043″/0.043″/3mm) does a pretty good job on high-end pistons these days, ideal for those builders who don’t have an unlimited development budget.

“In Pro Stock drag racing, the same thing applies as with the cup guys…cubic inch and bore size is limited, so these builders need to keep upping engine RPM to get extra horsepower.

“In Top fuel, considering the 85% rule on nitro, cylinder pressures are increasing. Blower pressures are up, and combustion pressures are a real concern. For these applications, builders have moved up from 1/16″ to 5/64″ wide on the top ring. Cylinder pressure is so high that rings are twisting, requiring a larger cross section to resist that twisting motion.”

Tags: MAHLE CLEVITE, NASCAR CUP, PERFECT CIRCLE, PISTON RINGS, PRO STOCK, RING TECHNOLOGY, SPEED PRO

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