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Why Fixed Tuned Pipes Have to Compromise
Because two-stroke motors rely upon the movement of the piston to control critical engine functions such as intake and exhaust timing, a "tuned" pipe is a critical factor in engine performance. The "tuned" nature of a pipe refers to the pipe designers' decisions regarding the desired performance characteristics for the motor and the subsequent pipe geometry required to obtain those goals.
The tuned pipe is critical to the performance of two-stroke motors because it greatly enhances the motors' ability to remove exhaust gases from the cylinder as well as improve the flow of fresh mixture into the cylinder for the next combustion cycle. This process, although easily summarized, is intensely complex from a design perspective. The interaction of the inherent characteristics of the motor and the chosen geometry of the tuned pipe is a fine balance between the competing goals of low, mid and high RPM power.
The design of the tuned pipe is essentially a compromise, because any of these competing goals can usually be accomplished quite easily on its own but at a substantial expense to overall performance. It is not difficult to design a "torque" pipe that will have great low-RPM performance, but this design will guarantee poor top speed. This is also true of high-speed pipes, with the resulting loss of low-RPM torque and mid-range power. This reality is based upon the laws of physics and the inherent characteristics of the two-stroke motor.
A tuned pipe operates by sending pressure waves (in the form of sound waves) back to the motor to accomplish a specific task at various times during the combustion cycle. The first wave is a negative pressure wave that is produced by the sudden increase in cross sectional area of a tuned pipe. This section of the pipe is known as the "divergent" cone. This negative pressure wave or vacuum wave actually sucks the lingering exhaust gasses from the cylinder after combustion and even helps to pull fresh mixture from the transfer ports into the cylinder for the next combustion cycle.
The second wave produced by the pipe is a positive pressure wave created by the sudden drop in cross sectional area. This section of the pipe is known as the "convergent" cone and many times is not visible because it is contained within the rear section of the pipe. This positive pressure wave acts to prevent the fresh mixture from escaping from the exhaust port before it is closed by the piston, thereby producing a little extra compression of the gasses just before the final compression stroke and combustion.
The effect of these pressure waves is essential to the performance of a two-stroke motor. In order for a tuned pipe to properly assist the motor in aspiration, the timing of the pressure waves arrival at the cylinder must be exactly right (See an animation of how tuned pipe length affects engine performance). If the timing of these waves is off even slightly then their effect will be greatly diminished and the motor will lose the resulting boost in performance. For the most part, the shape of the power curve is controlled, if not dictated, by the geometry of the accompanying tuned pipe. So where is the compromise?
The compromise results from these dual realities:
  1. Proper timing of the arrival of these "pressure waves" at the cylinder is critical to the performance of a tuned pipe and,
  2. The length of the tuned pipe dictates the timing of the wave arrival.
The waves travel through the pipe as sound waves. They are NOT exhaust gas flows! Because the pressure waves are sound waves, their speed, and the time it takes to travel through the tuned pipe and back to the piston, does not change with the RPM. These two realities mean that a pipe builder must choose an RPM range for maximum effect and design the corresponding pipe length that is most likely to guarantee arrival of the pressure waves at the correct moment. Generally, longer tuned lengths are suitable for low-RPM applications and shorter lengths are beneficial to higher RPM ranges.
So what's the difference between a pipe designed for low to mid speed and a pipe designed for mid to high RPM power?
Probably less than you think. The illustration below shows the cross-section of two well respected small block exhausts. The real difference in the tuned length of these pipes is only about 1/4 of an inch. Virtually all pipes designed for small block motors will have very similar proportions, with minor variations depending on the designers' intent. This is because for the most part small block engines as a class are internally designed to produce maximum power at very similar RPM ranges. The reality is that a small change in tuned length can yield surprising changes to the power output of a motor. Many experienced drivers know that a significant variation in the power curve can be noticed with a change in the tuned length on as little as 1/16 of an inch. Just imagine the incredible spectrum of power delivered by the unprecedented 1/2 inch of internal travel in the Buku Power Exhaust small block model!
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