To maximize purification efficiency, the catalyzer (three-way catalyst) has been located directly beneath the exhaust ports and is activated early when the engine starts. This design lets the combustion gas pass through the catalyst while it is still hot. Although locating a catalyzer directly beneath the exhaust ports in this way was a first for large motorcycles, research into such catalyzers for small-displacement cub engines had been ongoing for some time. However, because of differences in absolute values such as heat load for large-displacement engines, adapting this technology was far from straightforward.
One of the reasons for this is that whereas in the case of just a 2-cylinder engine, a catalyzer can simply be placed directly beneath the two exhaust ports, it is difficult to locate the catalyzer efficiently within the limited space available on a motorcycle, and also disadvantageous in terms of cost. So in this engine, the two exhaust ports have been brought together inside the cylinder head and consolidated into a single port, directly beneath which it has been possible to efficiently place a single catalyzer of sufficient capacity.
The design of a catalyzer unit capable of withstanding the heat capacity involved, and the optimal port shape and layout, were arrived at using automotive heat analysis technology (CAE) to determine the optimal specifications for a motorcycle. Consolidating not only the air intake ports, as mentioned earlier, but also the exhaust ports, enabled us to arrive at our engine's streamlined and compact intake/exhaust layout.