many times a “high flow” converter isn’t quite what it seems. According to Brad Brand of Random Technology, "Replacement converters aren’t subject to the same requirements as original equipment models, so most standard replacement converters offer increased air flow potential. The 'high flow' label is a result of this increased flow capacity. However, a replacement converter designed for use on a four-cylinder engine will likely not have as high a capacity as an original equipment converter (with the same size inlet and outlet pipes) designed for a V8. Although converter manufacturers certify each converter type for a maximum engine displacement and vehicle weight, some dealers have no qualms about ignoring certification criteria. If a “high flow” converter has an extremely low price, chances are it’s not really a high flow model."

Obviously, the bricks within a converter create the major resistance to exhaust flow. Over the years, various brick densities have been used, with the most common now being 400 cells per square inch. Converters with bricks having 200 cells per square inch were once common, and might appear to offer high exhaust flow potential. However, the walls in 400 cell bricks are thinner, so flow capacity isn’t much different, given the same face area. And face area is a major player in determining the flow efficiency of a catalytic converter.

But another factor, and one that’s often overlooked, is brick length - longer bricks offer higher flow resistance. On the other hand, if a brick is too short, it won’t offer sufficient area to effectively control exhaust pollutants. Converter manufacturers use different precious metal loadings of washcoats and vary them according to brick length and density. Brand notes, "Most converters currently being produced utilize bricks that are between three and four inches in length. This configuration allows lighter wash coat loadings and trades air flow efficiency for cost.