Advantages
In carbureted engines, poorly atomized fuel reduces efficiency and power at lower rpm (at higher rpm the large air speed keeps the mixture in suspension). The inlet manifold of a reverse-flow cylinder head may be connected to the exhaust by a heat riser to transfer further heat, improving low rpm response and emissions as a result.
Costs can be reduced in production engines by casting the inlet and exhaust manifolds as one unit. This also transfers further heat to the inlet eliminating the need for manifold heating and other related devices. Such an engine is overall simpler and also has improved cold starting.
The main advantage of the reverse-flow cylinder head is that both the entering inlet charge end the exiting exhaust gas cause a tendency to swirl in the same direction in the combustion chamber. In a crossflow head the inlet and exhaust gases promote swirl in opposite directions so that during overlap the swirl changes directions. The constant swirl during overlap which results in a reverse-flow cylinder head promotes better mixing and hence better scavenging of the end gas. The fact that the inlet charge must change direction before going out the exhaust makes it less likely that fresh mixture will make it out the exhaust before mixing during overlap. Overall this improves volumetric efficiency and reduces emissions.
Read more about this topic: Reverse-flow Cylinder Head
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