If your Porsche engine produces more than 500-600 HP, a coated high-performance type rod bearing is required. Main bearings are not typically available in high-performance variants and Connecting rods are subject to extremely high loads. The highest loads are the inertia loadings when the rod is at TDC on the exhaust stroke. This is when the weight of the piston, pin, rings, clips and the small end of the rod all combine and try to pull collective masses out through the top of the engine. It puts extreme loads on the rod cap, bolts and tries to stretch the rods themselves. This stretching elongates the vertical dimension of the rod’s big end and narrows the width of the bearing across the rod parting line.
As this dimension shrinks, so does the clearance and the oil film. For this reason, high-performance bearings are thinner at the parting line to counteract this narrowing effect called “bearing eccentricity”.
Bavarian Rennsport recommends always upgrading to performance or aftermarket style bearing to help decrease the chances of suffering a catastrophic engine malfunction from a spun rod. Bi-metal rod bearings are recommended for those enthusiasts trying to get the most out of their car while also making sure they keep their engine happy. Tri-metal bearings are the next step up from Bi-metal as they are designed for higher-performance levels and typically used in race applications.
They have 3 layers and usually have a hardened steel back with tin or copper-lead underlay, and an overlay of babbit. These tri-layered rod bearings are designed to slightly deform under pressure allowing the bearing to “touch” the crankshaft journal under extreme pressure without friction or heat welding itself to the journal.