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.
|
