High efficiency catalytic converters are the key to high output and low exhaust emissions. They’ve been cursed, damned, gutted, removed and praised. And depending upon your priorities, they’re either the best or worst thing to happen to automobiles in the past 30 years. Since they first appeared on 1975 model year vehicles, catalytic converters have made a significant impact on both pollution and performance. Unfortunately, that impact hasn’t always been positive on both accounts. Owing largely to early designs, which were very restrictive, catalytic converters are widely viewed as horsepower killers. And while even the most free-flowing converter will increase exhaust back pressure, the effect on performance can be minimal.

That statement may seem to fly in the face of reality but catalytic converters have changed dramatically over the years. The first converters to find widespread usage were filled with pellets coated with precious metals. As hot exhaust gases pass over the pellets, (also called beads) their coating serves as a catalyst and instigates a chemical reaction intended to transform exhaust pollutants into harmless compounds. Specifically, when unburned hydrocarbons come in contact with platinum and/or palladium, the resulting oxidation process transforms them into carbon dioxide and water. Similarly, when carbon monoxide meets palladium and/or rhodium, the resulting oxidation process converts it into carbon dioxide.

Initially, catalytic converters addressed only hydrocarbons and carbon monoxide. But oxides of nitrogen constitute another compound that fouls the air we breathe and in the early 1980s, rhodium, another catalyzing agent, was incorporated with the resulting converters being known as “three-way” because they address three, rather than two pollutants. Rhodium functions as a reducing, rather than an oxidizing agent. In “chemistry speak” that means it separates oxygen from a compound instead of adding oxygen to it. Consequently, nitrogen oxides are broken down into nitrogen and oxygen. However, oxygen is as fickle as Lady Luck and tries to dance with any available partner. In the exhaust stream, that’s usually carbon monoxide, which the footloose oxygen atoms convert to carbon dioxide.

Loose oxygen also combines with unburned hydrocarbons in the exhaust stream, so they’re fully oxidized before exiting the exhaust pipe. In theory, with a properly functioning catalytic converter, and an optimized air/fuel ratio, all potentially harmful pollutants are converted to nitrogen, oxygen, or water vapor. However, a number of other compounds found in fuel and air don’t participate in the catalyzing process, and pass through the converter unchanged.

The most influential component of the catalytic reaction is the “loading” of the washcoat that’s applied to ceramic substrate. Heavier concentrations (loadings) of the precious metals that cause catalytic reactions increase the effectiveness of the process, with no increase in substrate surface area. Rhodium, platinum and palladium, which are used in various concentrations in the washcoat, aren’t bargain basement metals, so converter manufacturers must tradeoff cost and effectiveness to produce converters that meet operational requirements, yet are affordable.

Most converters produced in recent years contain two monolith "bricks" spaced several inches apart from each other. The washcoat on the forward brick typically contains rhodium which causes nitrogen oxides to break down into nitrogen and oxygen. After passing through the first brick, exhaust gasses pass through an air chamber before entering the second brick. In some converters, known as “oxidation” types, a small tube passes through the chamber and injects air pumped in by an engine-driven “smog pump”. (In some vehicles, the "smog pump" incorporates an electric motor, which reduces accessory drive complexity and also allows for remote mounting.) Injected air simply brings additional oxygen into the exhaust stream to assist in the oxidation process.

Although “three-way plus oxidation” type converters were prevalent during the 80s, that’s no longer the case. With improvements in washcoat technology, and improved control of air/fuel ratios, the need for additional oxygen has been eliminated. Some vehicle manufacturers have continued to use oxidation converters on some models, but typically that has been done to use up inventory. As an example, the Corvette and Camaro Z/28 were equipped with oxidation converters through 1991 and 1992 respectively. But when the LT1 engine replaced the L98 (1992 in Corvette, 1993 in Camaro) three-way converters with no air tubes were incorporated.

Physical damage is the easiest to diagnose. If a converter is bounced off a curb or speed bump, or is struck by freeway flotsam, the ceramic bricks can be fractured. Once that happens, it’s just a matter of time before the bricks start rattling around inside the case, beating themselves into oblivion.

Fuel, oil and antifreeze cause a different type of brick destruction. Under normal operating conditions, the catalytic process doesn’t begin until temperatures inside a converter reach 500 to 600 degrees (F). If air/fuel ratio is on target, and the exhaust is free of contaminants, internal converter temperature stays at about 1200 degrees. But when unburned fuel enters the picture, temperatures can reach 2200 degrees and either burn the precious metals out of the washcoat, or literally cause a melt down of the bricks. Extremely high temperatures can also result in destruction of the mat that's wedged between the bricks to the converter case.

Oil and antifreeze also cause elevated temperatures, but as the converter tries to burn (oxidize) these compounds, a residue, which plugs up the bricks is formed. At this point, the converter not only looses its effectiveness, it also becomes very restrictive to exhaust flow, which kills horsepower.

When a replacement converter is required, a high flow model is the typical choice if performance is a consideration. But many times a “high flow” converter isn’t quite what it seems. 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. 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.

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