How a tuned pipe works
In the 1950's an engineer by the name of Walter Kaadan was consulted by motorcycle racers asking him to help them squeeze more power and speed out of their motorcycles. After some experimentation, he found that the 2-stroke engines in motorcycles were affected by its exahaust characteristics. He found that by varying the length of straight exhaust pipes, the performance also changed accordingly. After further experimentation, he found that a divergent cone instead of a straight pipe worked better, and this heralded the arrival of the 2-stroke tuned pipe.
Tuned Pipe Principles
The basic principle of the tune pipe is making use of the moving air masses
in the exhaust to assist in the scavenging (the process whereby the exhaust
gases are removed and replaced with unburnt mixture) of a 2-stroke engine.
Each time the exhaust port of a 2-stroke cylinder is uncovered, exhaust gases
rush out of the opening and through the exhaust pipe. This causes a high pressure
wave to radiate out of the pipe towards the exhaust opening. However, the
momentum of this moving mass of air also creates a low pressure wave that
follows behind it. If this is carefully timed, this low pressure wave can
be used to suck in the fresh fuel/air mixture from the transfer ports. This
process repeats itself at the same frequency at which the engine is running
and thus, if the exhaust pipe can be made to resonate at the operating RPM
of the engine, this will improve the engine's efficiency. It must be noted
that at a low (lower than the resonant frequency) RPM range, this low pressure
pulse would return too soon, bouncing back out exhaust port. The converse
is true for an RPM range higher than the pipe's resonant frequency, whereby
the low pressure pulse returns too late such that the exhaust port is closed.
It should be noted that the length of the pipe is what determines the operating characteristics of the pipe and this is independent of the engine's capacity.
The Divergent Cone Pipe
The next stage in the development of the tuned pipe consisted of adding a
divergent cone instead of a fixed diameter pipe. This had the effect of lengthening
the duration of the low pressure pulse, however it was not as strong as that
of a fixed diameter pipe. As the low pressure pulse was stretched to a longer
time frame, this gave it a higher chance of finding the exhaust port open.
The pressure was still sufficient to suck the exhaust out and a new charge
into the cylinder. Hence, the power band was somewhat broadened as the engine
could work over a wider range of RPM's.
The Twin Cone Tuned Pipe
There is a problem in the design of the divergent cone tuned pipe. At low
RPM's the stretched out low pressure pulse created by the divergent cone arrived
at the exhaust port early and it not only manages to scavenge all the exhaust
gases but also sucks some fresh charge out of the cylinder via the exhaust
port.
At this point, a second convergent cone was added after the divergent cone.
When designed properly, this convergent cone reflects the initial positive
pressure pulse back to the exhaust port thereby pushing the fresh charge back
into the cylinder just before the exhaust port closes. This in effect acts
like a supercharger with no moving parts and it improves the engine's performance
significantly.
Stinger
The stinger section of the tuned exhaust acts as a bleed valve, allowing exhaust
gases to escape. This is usually acheived by a pipe connected to the end of
the convergent cone. In some pipe designs, the convergent cone opens to a
second chamber and the stinger pipe is connected to this chamber.
The stinger also affects the performance characteristics of the engine by
controlling the amount of back pressure within the tuned pipe. The increased
pressure helps as the pressure waves travel at the speed of sound and the
speed of sound increases in a dense medium. This helps as when the engine
RPM increases the pressure within the exhaust created by the stinger also
increase due to the larger anount of exhaust gases generated. This allows
the pressure waves to travel faster and the overall exhaust timing is improved
over a larger range of RPM's, hence the power band of the engine is broadened.
