The main advantage of the reverse-flow cylinder head is that both the entering inlet charge and the exiting exhaust gas cause a tendency to swirl in the same direction in the combustion chamber.
The constant swirl during overlap which results in a reverse-flow cylinder head promotes better mixing, hence better scavenging of the end gas.
In a spark-ignition petrol engine the heat also increases the likelihood of pre-ignition or detonation which limits the allowable compression ratio reducing both power and efficiency.
Due to charge robbing and lower air speeds, large siamesed ports are more suited to high rpm race motors.
In the early days of turbo charging a reverse-flow head allowed the compressor outlet of a turbocharger to blow directly into the inlet manifold with either a blow-through or draw-through carburettor and no intercooler.
Modern turbocharged configurations utilizing intercoolers and fuel injection are more difficult to plumb up to a reverse-flow head and are ideally suited to a cross flow head where the turbo is on the exhaust side of the engine, the charge crosses through an intercooler in front of the engine and into the inlet manifold on the other side.
The reverse flow head was ideally suited to a production carbureted engine due to its low rpm performance and ease of manufacture.
However, the advent of fuel injection and electronic ignition has made most of the reverse-flow head's advantages redundant in a modern engine and as a result the design has lost its popularity.
The reverse-flow head still enjoys some popularity among enthusiasts including Leyland Mini, Chrysler Slant-6, Holden and Ford Inline 6 fans.