Piston Ring Selection - Engine Builder Magazine

Piston Ring Selection

Through the proper selection of style, size and material, rings prevent blowby by sealing against the groove in the piston as well as against the cylinder wall. They should be as flat as possible, fit the piston grooves as tightly as possible, have the least amount of end gap that the engine can safely tolerate, and be as conformable as possible to seal against the cylinder wall.

Piston rings have a difficult, four-part job of forming a seal between the pistons and the cylinder bore. During the intake stroke, the rings prevent air and oil from being drawn into the combustion chamber. During the compression stroke, the rings make sure the air/fuel stays in the combustion chamber and is fully compressed before it is ignited. During the power stroke, the rings prevent pressure from blowing past the pistons as the burning gases shove the piston down. And during the exhaust stroke, the rings make sure all of the spent gases are pushed out of the exhaust port.

Rings that do not seal well during all four phases of the four-cycle combustion process can reduce an engine’s power potential significantly.

Leaks that occur during the intake stroke will reduce air velocity and volumetric efficiency – less air and fuel in the cylinder means less power. Rings that leak during the compression stroke will allow some of the air/fuel mixture to escape into the crankcase. Besides reducing compression and power, the unburned fuel that gets into the crankcase will dilute the oil. This will reduce the oil’s lubrication qualities while increasing the risk of engine-damaging sludge if the oil isn’t changed often.

Rings that leak during the power stroke will allow a loss of pressure that would otherwise be used to shove the piston down. The resulting blowby will also allow soot and moisture to enter the crankcase to further degrade the oil.

Finally, rings that leak during the exhaust stroke will reduce scavenging efficiency, allowing residual exhaust that remains in the cylinder to displace air and fuel during the next intake stroke, further reducing power potential. Blowby during the exhaust stroke will also allow more soot and moisture to enter the crankcase. If the engine has a turbocharger that depends on exhaust velocity for intake boost, ring leakage during the exhaust stroke can reduce exhaust flow, which reduces boost and power.

Through the proper selection of style, size and material, rings prevent blowby by sealing against the groove in the piston as well as against the cylinder wall. They should be as flat as possible, fit the piston grooves as tightly as possible, have the least amount of end gap that the engine can safely tolerate, and be as conformable as possible to seal against the cylinder wall.

In late-model OE stock engines, the rings are often moved closer to the top of the piston to eliminate the crevice where unburned fuel can be trapped. This is done for emission purposes and to help fuel economy. In a performance engine, the same logic applies. The more efficient the combustion process, the more power the engine will produce.

Power also produces heat – A LOT of heat in a performance engine. This can be murder on both the top ring and piston groove if the materials are not able to handle the heat. For performance applications, you want a top ring made of ductile iron or steel. Wear-resistant side coatings such as PVD or nitriding can help the rings survive this harsh environment. The top groove in the piston may be anodized or coated to minimize micro-welding and wear.

Another factor to consider is the amount of friction created by the rings against the walls of the cylinders as the pistons reciprocate up and down. Ring friction eats up more horsepower than the cam and lifters, the cam drive, the rocker arms or the crankshaft. Of the three rings in a typical ring pack, the oil ring accounts for 60% to 70% of the total friction created by the rings. Reducing ring friction by using smaller, thinner, low-tension rings can allow you to “find” horsepower that was previously lost. Low-tension rings reduce friction and allow an engine to produce more usable horsepower.

However, dyno testing has shown that it is possible to reduce static tension on the top ring too much, causing a loss of pumping efficiency (vacuum) on the intake stroke. If you don’t get all the air/fuel mixture you can into the cylinder on the intake stroke, it’s not there to make power during the power stroke.

Final Thoughts On Ring Selection

Though books have been written on this subject, we can boil it down to the following points:

1) What is your total engine budget, and what percentage of that budget can you spend for rings? For example, if you buy a set of exotic, one-piece oil rings that cost almost $600 for a V8, the rings alone might cost more than the pistons! Such an expensive outlay for a set of rings wouldn’t make much sense for a typical street/strip engine, but it might be justified for a high-dollar Pro Stock engine.

2) Are you building a naturally aspirated engine or one that will be jacked to the max with nitrous oxide, a turbocharger or blower? The higher the power output, the stronger the ring material that will be needed to handle the heat and pressure.

3) How will the engine be used? Drag racing, truck pulling, circle track, marine, street driven, etc. If the application is a street performance engine, how much time realistically will it actually spend at full throttle? Choose a ring facing that provides the kind of durability that matches the application.

4) How often will the engine be refreshed? Will the rings have to last tens of thousands of miles (as they do on the street) or will they be replaced every season?

5) What type of rings are others using in similar applications? If a certain type of ring or facing material is working great for everybody else, trying to be a pioneer may prove to be expensive!

6) What kind of rings are available for the engine’s bore size and piston configuration? For some applications (like engines with unusually large bore diameters or extremely short lightweight pistons), this may limit your options quite a bit.

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