Difference between low-tension and ordinary piston rings
Low tension rings are thinner and exert less pressure against cylinder walls than conventional rings. This reduces friction, improving fuel economy and cylinder sealing. Low tension rings are used in most engines today.
Ring tension is described two ways. One is tangential tension, which is the amount of force needed to squeeze the ends of the ring together. The other is unit pressure, or the amount of pressure exerted by the face of the ring against the cylinder wall.
In the 70s, conventional piston rings had tangential tensions of up to 30 pounds. Compression ring tension specs for a Ford 302 V-8 used to be 22 to 26 pounds. It is 14 to 16 pounds on later model versions of the same engine. On some applications today, compression rings are rated at 5 to 7 pounds.
The amount of force the ring exerts against the cylinder wall (unit pressure) depends on tangential tension as well as ring thickness and cylinder bore diameter. Conventional oil rings exert pressures in the range of 180 to 240 psi. Low tension rings fall in the 90 to 160 psi range.
Most aftermarket low tension rings have a somewhat higher tension than the OE rings they replace. If an OE ring specification calls for 6 to 12 pounds, an aftermarket ring may have as much as 12 to 16 pounds. Higher tension is needed because rings are often installed in oversized cylinders. Cylinder bores may also have more distortion than a new engine, so extra loading
improves sealing. >
Low-tension rings require rounder cylinder bores, which may require the use of torque plates when honing certain engines. When heads are torqued, cylinder bores can distort up to 0.0015" or more near the bolt holes, throwing cylinders out of round.
This obviously makes it more difficult for rings to seal properly. Simulating bore distortion by bolting a torque plate to the block allows cylinders to be honed so they will maintain their shape when the engine is assembled.
It is essential that correct replacement rings be used. Conventional rings designed for standard grooves must not be used in shallow groove pistons designed for low- tension rings. Narrow, low-tension rings must not be used in deep groove pistons.