@KB02, the TL/2 is based on the time it takes sound to travel as well as the timing difference between cylinders. Remember Cylinder 1 is offset by two strokes from cylinder 2.
For example
Power - Exhaust - Intake - Compression
When Cylinder 1 starts the Power stroke, the bang sends shock waves down the pipe. At the end of its stroke the sound has travelled distance TL. If the bend is half way down the pipe, then a reverse sound wave will travel back up the pipe towards the piston, arriving just as Cylinder 1 is starting its Exhaust stroke. The arrival will try to "stuff" exhaust back into the chamber, thereby increasing compression until the power stroke is complete. A sound node will follow the sound front, hopefully arriving just as the exhaust valve is fully open, that reduces sound pressure and effectively "extracts" gases faster than would otherwise take place. In this way, the pipe prolongs the power stroke and speeds up the exhaust gases. The duration and amount of "stuffing" and timing of sound node arrival, depend on how sharply the pipe bends and at what distance it is away from the port.
Continuing, the bang from cylinder 1 also travels up the pipe of Cylinder 2 and will arrive just as Cylinder 2 enters its Power stroke. If the exhaust port of Cylinder 2 is shut by then, there is no stuffing or scavenging effect, however, if there is port timing overlap, then this wave front will (hopefully) push gases back into the chamber that would otherwise have been sent unburned into the exhaust pipe. Essentially, overstuffing the chamber just prior to the power stroke. If timed just right, the effect of course is a bigger bang. And therein is the art of tuning.
Also consider the bends have produced "echoes" that reverberate within the pipes. The echo effects also produce sound fronts and nodes of their own and interfere either constructively or destructively with other sound energy. These secondary (and even tertiary) echos may still have enough energy to add to or subtract from the primary effects. Again, only final tuning can really optimize these complex interactions.
Finally, because we have 4 pipes to consider, (not just the 2 in my example), that allows the pipe design to vary the lengths of complementary pipes in order to maximize the arrival of sound waves across more than one tuned length. This gives the system a broader optimal range. Pipes tuned in this manner don't have just one sweet spot but indeed can target peak horsepower and peak torque or can target for example, 6000 rpm
and 9000 rpm (the actual targets have to be carefully examined so that the TL of one pipe doesn't take away from the resonance of another in the target power range)
Pipes, especially stainless pipes, make me hot!
But, now you're getting into the advanced course.
