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Smaller main bearing diameter is more desirable at high RPMs. If you recall your geometry, the surface velocity of an arc increases as the radius of the arc increases at a given RPM. In other words, if you have a crank turning at 7000 RPMs, the surface of the crank journal against the bearing is traveling at a measurable rate of speed against the bearing surface. As the diameter of the journal increases, so does the actual velocity of the surface of the journal against the bearing translating into more heat and friction produced. Therefore, racing engines like to keep the diameter of the main journals as small as possible without creating a weakness in the crank.
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