Although performance bearings aren't the first thing you think of when defining the specifications for a high-performance engine, and they probably won't do much to increase engine output, they are one of the most important items necessary for ensuring the reliability you'll expect and need when other engine modifications are made.

The right bearings are an all-important factor in a performance engine-in fact, they're vital. Bearings, of course, keep the crankshaft-which is turning somewhere around 7-8,000 rpm-spinning smoothly as the rods transmit combustion force to drive it.

According to Bill McKnight, Training and Motorsports Management for Mahle Clevite, the world's largest supplier of engine bearings, performance crankshafts and rods often feature lighter weight, and operate under much higher engine speeds and loads than those in everyday engines.

Bearing Characteristics
Because of bore-shape variables, the most desirable bearing shape is provided with a slightly oval inside diameter. As a result, bearings are manufactured with an eccentric wall. In most cases, the bearing wall thickness is greatest at the top and bottom (90 degrees from the parting line). The bearing ID tapers slightly at the parting line area.

Connecting rods are subjected to especially high inertia loads at the top of the exhaust stroke when the weight of the piston, rings, and wrist pin pulls at the rod big end. This dynamic force also tries to pull the rod's big end out-of-round, which causes the bearing housing bore to become tighter at the parting line area. In order to address this, the slight eccentricity is designed into the bearings. This prevents the bearing from contacting the crank journal when bore distortion occurs. Eccentricity is also important for oil film formation when the engine is first started.

In addition to a range of bearing eccentricity (low, medium, and high), performance bearings are offered with a host of additional design features to properly function in the high-load and high-speed environment of a high-performance/competition engine.

Designed for Interference Fit
Main and rod bearings are designed to provide interference fit with their housings. In half-shell bearings, this is called "crush." Each bearing shell (upper and lower) is made with a length that results in slightly more than a true half-circle, so that the ends of the bearing shell, when installed into a housing, protrude slightly beyond the parting line of the housing,

McKnight says, "When the cap is installed and the cap bolts are tightened, these protruding ends of the bearing shells are forced against each other and push the bearings against the housing, compressing the bearing shells. The outward force of the bearings, as they squeeze into place, causes a slight change in the size and shape of the housing. As you might expect, different housing materials (such as steel versus aluminum) provide different levels of bore distortion. Bore distortion is a natural by-product of the assembly. Compensating for this bore distortion can be tricky-a multitude of variables can directly affect main bore and the connecting rod's big end bore shape."

Engine blocks and connecting rods feature irregular shapes that surround the bearing's housing-bore. For example, connecting rods usually feature a cast-in beam at the top of their bore, notches for bolt heads and/or nuts at each side of the housing bore, and/or thin or thick ribs at the bottom of the cap and so on. In addition, dynamic loads (when the engine runs at various speeds and loads) change in both magnitude and direction. All of these factors combine to cause the bearing housings to develop an out-of-round shape under many operating conditions. Depending on the surrounding metal shape and mass, some housings will go out-of-round in either the horizontal plane or the vertical plane. This varies with each engine, and quality performance engine bearings are designed to compensate for these geometric changes.