Here are a couple of statistics that will blow your mind. Up to Eighty percent of the friction in a reciprocating piston engine stems from the piston rings, depending on which study your read. What’s more, The highest percentage of the drag in a given cylinder comes from the oil ring.
That’s incredible when you think about it. One guy who thinks about it a lot is Keith Jones, a technical specialist with Total Seal Piston Rings. He thinks fast and he talks faster, and we caught up with him to learn more about engine friction and the current state of piston ring technology.
For racers, friction is good when it comes to tires. Friction means grip. But it’s bad when we’re talking about the inside of our engines. Friction accelerates wear and produces heat, but most important, it decreases power output.
“Each one of those compression rings makes about 6.5 to 7 pound-feet at 5/64”. The oil ring makes about 20 pound-feet at 3/16”, and that’s what was in your typical small-block Chevy, for what, 40 years?” Jones said. “By comparison, let’s move up to today and look at the ZR1 Corvette, a 650 horsepower, supercharged factory GM engine with a warranty, that engine uses a 0.8, 0.8 2.0 mm combination. That was the same ring pack we ran in an NHRA Pro Stock engine and a NASCAR engine just 10 years ago.”
Much of the advances in piston ring technology and friction reduction stem from materials science. Whereas piston rings used to be cast iron, and they were used for every application from a street car to all kinds of racecars, now engine builders have a choice. They can choose application-specific materials based on how the engine they’re building will be used. Manufacturers now offer piston rings made from hardened ductile irons, hybrid irons, and different grades of steels, such as carbon steel, stainless steel and even tool steel.
“It’s modern technology. We’re dealing with much, much better materials,” Jones said. “Back in those days you were dealing with plain old cast iron. Today we’re dealing with steel rings. Just about everything has gone to steel ring technology, steel tops, steel seconds. There are some iron-based second rings out there, but most of it is steel.”
Force and Pressure
When talking about piston rings and friction, it’s impossible to not talk about force and pressure. It’s a little wonky, but stay with me.
Force is a measurement of the energy, or the force, it takes to compress the part to its given bore size. Force is the effort exerted against the cylinder wall.
“We have machines that will literally take that ring and there’s a big band that goes around it,” Jones said. “It’s kind of a pneumatic system and it will pull that part down to its given bore size and we’re measuring the pound force it takes to compress that part to its bore size, i.e. force.”
Pressure is the ratio of force to area over which it is applied. The force you exert on the floor when standing on both feet is equal to the force you exert on the floor when standing on one foot. However, when standing on one foot you put twice the pressure on the floor as if you were standing on two feet. Think of pressure as you would a tire’s contact patch. Pressure is the psi across the part itself, which also is known as unit pressure.
With modern materials, manufacturers now can make all the rings much thinner, and that lowers the force, but keeps the unit pressure up because the contact patch is smaller. The rings exert force across a smaller areas, so they can maintain unit pressure with less overall force.
“It’s a narrower contact patch. That’s the crux of it all right there,” Jones said. “We’re going to a narrower contact patch. Again, going back to force versus pressure, I can keep the pressure up with less force because I’m putting it across a smaller area, and that’s the key to reducing the friction with smaller rings. That’s the key to thin-ring technology.”
Thin-ring technology is everywhere now. OEM’s use it for reducing emissions and increasing mileage. Race engine builders use it for increasing power output and longevity.
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Jones said it’s easy enough to get pistons machined to accept thin-ring technology. He suggests making piston and ring choices the same way you would choose a cam or an intake, which is to say, with your specific application in mind. Make them one of your first considerations, not your last.
Total Seal offers thin-ring technology in gapped or gapless ring styles. It also offers thin-ring packages for older engines, called the Advantage Ring System, which is good for older engines, vintage racing applications or racing series in which the pistons are spec.
“We can apply that to these older engines and get significant increases in power. It’s amazing, the difference,” he said. “You’re talking 20, 30 horsepower gains depending on ring sizes and how many cylinders it is, from the old ring packs compared these new ring packs.”
For years, there wasn’t much choice. You just bought whatever the speed shop stocked or whatever the machinist put in it. Of course, the needs of a guy who wants a reliable engine for his street rod are different from those an endurance racer who runs a wide-open throttle for hours at a time. Now we can choose — and that choice is more critical than ever.
“In the big picture, it really doesn’t cost any more. You’re buying pistons and rings anyway,” Jones said. “Why not put the modern stuff in it? It’s reduced friction in the engine, reduced wear in the engine. It’s just a lot better.”