Understanding Engine porting and timing


The reciprocating motion (up and down) of an engine's piston allows it to act as an air pump. Initially, the air/fuel mixture is moved into the crankcase below the piston, and then it's forced into the cylinder (above the piston) where it's compressed and united. As the gases burn, temperature-and pressure-soars. This pressure forces the piston toward the bottom of its stroke to where exhaust gases are finally purged. It sounds simple, but very accurate port design-shape, size and location-and timing are vital if you want sensational engine performance.

INDUCTION PORT. RC nitro engines use the crankshaft rotary-valve induction system. Here's how it works: a port machined through the crankshaft journal's surface is aligned with the engine block's air-induction hole (under the carburetor) once per revolution. The air/fuel mixture flows through the open port in the crankshaft journal and then through a hole bored in the center of the crankshaft and, finally, into the crankcase.

TRANSFER PORTS. These are holes machined through the cylinder wall and that are alternately covered and uncovered by the piston. The air/fuel mixture in the crankcase (below the piston) is moved through bypass channels between the engine block and the outside of the cylinder to the transfer ports.

Nitro 2-stroke engines use a variety of transfer-port combinations. There can be as few as two and as many as 10 or 11 transfer ports in all shapes and sizes-- plus an exhaust port or ports (yes, there can even be several exhaust ports).


Dozens of transfer- and exhaust-port configurations have been used in 2-stroke engines, but RC engines use a basic configuration that's known as Schnuerle porting, so we'll discuss only this type.

In a Schnuerle system, two transfer ports are angled upward and away from a single exhaust port that's between them. The fresh fuel mixture is intended to be directed at a point that's farthest from the exhaust port. At this point, the fresh mixture loops toward the cylinder head and forces spent gases out through the exhaust port.


The boost port is an important improvement on the basic Schnuerle port arrangement. It's opposite the exhaust port and is easily distinguished by its sharp upward angle relative to the other cylinder ports. The boost port not only creates another passage through which air/fuel mixture can be transferred into the cylinder, but it also does so at an angle that forces the mixture toward the glow plug, which is at the top of the cylinder. This promotes better "packing" of the cylinder and improves exhaust scavenging.


More significant than the number of ports are port timing (when the ports open and close), duration (how long they stay open) and area (the ports' sizes), so don't be impressed by how many ports are advertised for a given engine. A properly configured 3-port engine can be more powerful than a less well-- designed 7-port engine. Don't base your purchasing decision on port count alone.


In engine terminology, "scavenging" means to clear a volume-in other words, to clear exhaust gases out of the cylinder and to move fresh air/fuel mixture from the crankcase and into the cylinder. To an engine designer, clearing exhaust gases from the cylinder is only half of the problem; simultaneously replacing these gases with fresh, cool, air/fuel mixture is the other. When a nitro engine operates, some of the fresh gases transferred to the cylinder mix with scavenging exhaust gases and reduce the engine's efficiency and power. Over the years, many porting systems have been tried to minimize this mixing and contamination; designs have improved, but the condition continues to impact 2-stroke performance. The size, location and direction of these ports determine how successful scavenging is and how well your engine performs.

Transfer and exhaust ports permit pressurized gas to escape from above and below the piston during the engine's cycle. Having adequate time (port duration) to do this is only half the story; having a large enough port (port area) is the other half. Stated differently: the time required to move a quantity of gas through a port depends on the port's area.

An analogy will be helpful: 50 people have 30 seconds to exit a room after the fire warning has sounded. If the exit door opens completely, they all file out smoothly in just under the time limit. If the exit door malfunctions and opens only partway, people can still get out, but there's a jam that ultimately allows only 35 to exit within the time limit. Arithmetic shows that the partly opened door allowed only 70 percent of the people to exit within the allotted time. A similar situation exists for gases attempting to pass through transfer and exhaust ports. If flow is too restricted, the port can be widened to increase its area, or it can be made higher to increase both its area and its duration. Each remedy has a different effect; deciding which is better is the subject of years of study and experience.

Len Brown
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