Side or central exhaust port configurations.

In model engines, especially in glow versions, it is quite normal for the exhaust port is to be on the same axis as the drive shaft.
Compared to the motorcycle field, on the other hand, there is almost a total absence of cylinders with exhaust ports located on the same axis of the crankshaft.

From a functional point of view, the co-axial layout of driving axle with the exhaust port is often a big advantage, just think of aircraft's engines or in those for Hydro hulls installations.
In these situations, side exhaust discharge would create an increase in frontal section with significant loss of the aerodynamic profile, in such cases the center discharge is an extremely advantageous solution for the final performance of the model.
Many motor's manufacturers, offer both models: side exhausts or rear (center port) discharge. We will explain why identical engines, differentiated only by the position of the exhaust port in relation to the crankshaft, will often offer important performance difference.
We will focus on medium to large engine displacement (beyond the 23cc); in such class of displacement we find substantially only 3 types of intake:
a) Sub Piston Port
b) Flap valves (Reed)
c) Rotary disk (Zimmermann).
Basically the engines with piston-controlled suction (with supply port opposite side of the exhaust port), do not benefit by a coaxial exhaust position to the crankshaft but often this configuration tends to slightly reduce performance. The reason is in the fact that with the side exhaust configuration, the rod drives the piston onto the cooler side of the cylinder. The dynamics of fresh incoming gas promotes an excellent lubrication of the head of rod and especially, for engines with piston-controlled suction, the thrust of the latter against the intake port does contribute to its perfect sealing.

In the case of engines with rotary disk intake, the simplest mechanical solution with the disk coaxial with the crankshaft, we encounter considerable problems of fluid dynamics. The incoming gas column that is to reach the transfer ports (opposite to the disk intake) is hampered in its flow by both sides of the crankshaft and the movement of the connecting rod.
The mechanical impediment to the free flowing of incoming gas creates considerable symmetry problems in the flow supply to the cylinder. The ports closest to the intake /disk port will receive a cooler supply of gases than the one opposite to the disk.
While rotating the cylinder (side exhaust port) creates considerable obstacles from a mechanical point of view, of lubrication and of possible seizures, on the other hand the advantages of a symmetric flow does create the base for very important increase in performance.

In the motorcycle field, but only in competition level, the ideal solution has been found by placing the rotating disk on the side hence without rotating the cylinder. In this way, despite the addition of a more elaborated mechanical transmission they managed to kept all the advantages of an ideal piston and drive shaft layout and yet allowing a perfect symmetry of flow to all ports.

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