Reverse-flow cylinder head

In engine technology, a reverse-flow or non-crossflow cylinder head is one that locates the intake and exhaust ports on the same side of the engine. The gases can be thought to enter the cylinder head and then change direction in order to exit the head. This is in contrast to the crossflow cylinder head design.

The reverse flow design is generally considered to be inferior to a crossflow design in terms of ultimate engineering potential for two reasons. Firstly, there is limited space when inlet and exhaust ports are arranged in a line on one side of the head meaning a reduction in port area compared to a crossflow head. This mainly affects power delivery at high rpm by limiting airflow. Secondly, since inlet and exhaust manifolds are both on the same side of the engine and in close proximity, the inlet manifold and carburetor (if applicable) are heated by the exhaust. This heating reduces the density of the inlet charge and hence the volumetric efficiency of the engine. 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.

Engineering has found a number of solutions to the first problem such as staggering the ports by placing the inlet ports on a higher level than the exhausts. This way larger ports can be used while still leaving enough room for flanges and fasteners. This brings with it the problem that the exhaust ports have a tighter turn radius. This problem is somewhat offset by the larger port. Another popular solution, as used in the BMC A-Series and Holden 6-cylinder engines is the siamesed port. In this configuration one large port feeds 2 adjacent cylinders. The gain in area comes from effectively removing the material between 2 adjacent ports. This solution encourages charge robbing, where one cylinder "robs" the charge from the port leaving the next one with less mixture. This happens because the 2 cylinders which share the port are not equally spaced in terms of firing order. For example, the Leyland Mini with its 1-3-4-2 firing order has the 1 and 2 inlets siamesed and the 3 and 4 inlets siamesed. First the number 3 sucks the mixture out of the port, then there is less left for number 4. While number 1 and 2 are sucking the port again fills with mixture and the process repeats leaving the number 1 and 4 cylinders always lacking mixture. Also the siamese port can make both acoustic and inertial ram tuning less effective due to the irregular pulse. One should keep in mind that larger ports are only necessary at higher rpm and that small ports are desirable at low rpm to improve air speed. Due to charge robbing and lower air speeds, large siamesed ports are more suited to high rpm race motors.

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Wikipedia