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Sizing Presently Used in Several V-8 Applications
Loosely associated with our last "tech" presentation relating to bearings is the sizing presently being used in several different V-8 applications. Recently three has been a great deal of rhetoric relative to the bearing sizing of the connecting rod journal and the main journals in racing engines. Although we are not engine builders or designers, many questions are presented to us as to the effect on the connecting rods. Therefore, it seems logical to offer some very practical and empirical information relative to these questions.

The most common questions relate to the reduction of the bearing shaft sizes and how it effects the connecting rod, bearing and crankshaft assemblies. Specifically associated with the connecting rod is the integrity of the big end housing bore. In general terms, the major element to consider is the "tunnel strength" of the big end bore. If we assume that all cross-sectional areas of the big end remain the same and you reduce the bore, the loads required to displace the bore increase. A simple example is if you had a steel tube that was four feet in diameter, one foot long and 1/4 inch wall, a person could flatten or "displace" the bore with simple hand pressure. If the same tube was one inch in diameter you could roll a milling machine across it! What this illustrates is the fact that we, as a connecting rod manufacturer, have little concern about reducing the bore of the big end relative to strength. The obvious concern would be the reduction of "strength" or torsional resistance of the crankshaft - Carrillo does not manufacture crankshafts!

There are other significant changes relative to the size alterations of the bearing surface. Most effected is the speed of the bearing against the bearing surface (crankshaft). The distance of travel of a point on the bearing shell around 360 degrees of a 2.100 crank pin diameter versus a 2.000 is .315 inch. At 7000 Rpm's that is equal to 183.75 feet in one minute per bearing surface. Since the area between the shaft and the bearing shell is to be cushioned and lubricated by oil the oil film creates friction and shear. This friction and shear generate heat and parasitic losses. The conclusion is that smaller is better, not always.

What are the drawbacks? Already mentioned is the most obvious, the reduction is torsional strength of the crankshaft. Other obvious considerations are relative to the increased bearing loads. Without using specific numbers, much of the bearing load can be altered or changed by variances in bearing clearances within the specific tolerances offered by the manufacturers.

Bearing loads, shear and friction are often more dramatically effected by bearing width. These critical elements will be significantly effected by the alteration of this dimension. It is important to generate percentage calculations relative to any width changes of the bearings. Changes perceived as relatively insignificant often generate major changes.

When considering a reduction in shaft diameters or bearing width consider the bearing availability, its geometry as well as its material design.

It is enlightening to engage in calculations of parasitic losses rather than those calculations of BMEP, fuel flow, airflow and other related "power" producing exercises.

Many readers may find the above information a bit sophomoric, but we have found that many of the simplest concepts are overlooked and the resultant errors are often catastrophic.

Thanks again for your interest and attention, come visit us again!

Regards,
Fred Carrillo and Jack Sparks
CARRILLO INDUSTRIES