Expansion Chamber (Tuned Exhaust Pipe)

This article will show you how an expansion chamber (also known as a tuned pipe) actually works through the use of a step-by-step explanation and a few graphics.

What it does

An expansion chamber which has been properly matched to an engine will effectively supercharge1 the mixture of gas and air inside the cylinder. This is done by pulling extra mixture into the cylinder and by pressing wasted mixture back in through the exhaust port.

How it works

As the descending piston opens the cylinder exhaust port, the action of the exhaust pressure wave rushing through the tuned pipe at the speed of sound initiates a sequence of events that enhance the engines torque and horsepower output.

The positive pressure pulse leaves the exhaust port

As the mixture is combusted inside the cylinder, the piston is forced downward and opens the exhaust port. The force of the exhaust pressure leaving the cylinder creates a positive pressure pulse as it moves out into the expansion chamber.

The time between exhaust port opening and and transfer port opening is called "exhaust lead". Exhaust lead allows the high-pressure exhaust gases in the cylinder to blow-down (reducing pressure) before the transfer port opens. If this doesn't happen, exhaust particles may back-flow into the crankcase and contaminate and heat the incoming fuel/air charge, thereby robbing the engine of power.

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Positive pressure pulse leaves the exhaust port forcing exhausted gas and air into he chamber

Negative pressure waves

The inertia of the out flowing exhaust particles out into the divergent code creates negative pressure waves and a strong partial vacuum (about minus 7psi) near the still opening exhaust port shortly after the transfer port has opened. In addition to the sucking out tail-end exhaust gases, these suctions cause fresh air/fuel mixture to be sucked through the transfer port into the combustion chamber.

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Negative pressure is created as the waves expand into the divergent cones

Positive pressure waves

The remaining energy in the negative pressure wave continues through the pipe and is reflected off the convergent cone at the rear of the unit and returns to the engines exhaust port. During this point, the negative pressure will actually end up pulling extra fresh gas and air into the header of the expansion chamber.2

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Positive pressure waves echo back from the convergent cone, towards the engine

Supercharging through the exhaust port

Finally just as the transfer ports inside the cylinder are closing, the returning positive pressure wave compresses the extra fuel and air mixture back into the cylinder through the exhaust port. The cylinder now contains the extra mixture for the next combustion—effectively supercharging the engine and increasing efficiency and performance.

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Positive pressure forces over scavenged mixture back into the cylinder

The moving example

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Tuned pipe design

Simply screwing a tuned pipe onto your engine wont guarantee supercharging. Pipe volume, manifold pipe length and diameter, the angles of the divergent and convergent cones, and stinger dimensions all play an important role in tuned pipe design. Fortunately, all of these critical design factors have been taken care of by the pipe manufacturers.

Rules of Thumb

  1. Gently tapered divergent and convergent cones will provide the widest torque and horsepower range but not the best engine performance overall.
  2. Steeply angled divergent and convergent cones produce intense but short lived negative and positive pressure pulses at the exhaust port. These pipes can potentially produce superior torque and horsepower but they reduce an engines useful operating rpm range. In addition to being more difficult to tune they also tend to fall out if tune especially if you run at different altitudes, or if the weather changes.
  3. The faster you run an engine, the shorter the pipe you'll need. The slower you run an engine the longer the pipe should be.
  4. The percentage of octane in the fuel affects tuning by altering the temperature inside the pipe and therefore the speed of the pressure pulses. Elevation, ambient air temperature and humidity all affect pipe operation.

List of Expansion Chambers

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