Tuned Pipes


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2 Stroke Engine Expansion Chamber Tuned Pipes

Tuned pipes have been used to increase performance on 2 stroke engine for many years.  Another term that is used to refer to a tuned pipe is: expansion chamber.  It was discovered that a specially shaped exhaust system that was tuned for a specific 2 stroke engine could significantly boost power output.  This technology is employed on stroke engines used in applications such as motorcycles, scooters, snowmobiles, personal watercrafts, and just about anywhere a 2 cycle engine is used and additional performance is desired.  On smaller applications like chainsaws, leaf blowers, or weed eaters, a tuned pipe is not normally used but rather a small muffler.  There is usually not room on these handheld equipment applications to have an expansion chamber tuned pipe sticking out.  Not to mention, a protruding exhaust would be more likely to cause burns on these handheld power equipment.    

Motocross Tuned Pipe

Example of Motocross Motorcycle Tuned Pipe

Since most hobby nitro powered vehicles also have 2 stroke engines, tuned pipes are also used in an effort to extract more power from these small 2 cycle engines.  Some low cost nitro powered hobby vehicles come with a simple muffler, but significant increases in power of 50-100% are possible by replacing a restrictive muffler with a good tuned pipe properly matched to the nitro engine!   

RC Tuned Pipe

Example of an RC Truck Tuned Pipes

Now let's look at how a tuned pipe actually works to increase performance in a 2 stroke engine.  As the name Expansion Chamber implies, the shape of a tuned pipe includes an "expansion" or increase in the size of the exhaust pipe.  The expanding part of an expansion chamber is technically known as the divergent cone.  In this section of the tuned pipe, the exhaust pipe diameter increases in a tapered cone shape.  As the exhaust flows out of the engine, it reaches the divergent cone section and expands which creates a vacuum (suction) at the exhaust port.  It's sort of like a venturi on a carburetor.  The divergent cone acts to help suck out more exhaust from the exhaust port of a 2 stroke engine.  This scavenging effect helps to remove burnt exhaust gases and makes room for a fresh charge on incoming air and fuel.  In fact, the scavenging effect can be so effective that it also sucks some of the fresh air/fuel mixture right out the exhaust port where it would normally be wasted if it were not for another portion of the tuned pipe - the convergent cone.  The convergent cone is also a tapered cone shape, but the taper is a reduction in diameter.  This funnel shaped reducing size takes some of the exhaust pulses and reflects them back towards the engine exhaust port.  If the tuned pipe is tuned just right, then these reflected sound waves will return back to the exhaust port while it is still open and it will act to push back in some of the fresh air-fuel mixture that was scavenged out of the engine earlier.

Simplied Tuned Pipe Design Diagram


So, in a way, the divergent cone section (sucks gases out of engine) of a tuned pipe works against the convergent cone section (crams gases back in engine), but in the end, if the shape of the tuned pipe is designed properly, then the net result is a supercharging effect of the 2 stroke engine and a significant increase in performance.  There are other sections to the tuned pipe that affect the tuning.  The belly section can be lengthened or shortened to tune the exhaust pulses so that the return pulses from the convergent cone are more effective.  Also, the stinger diameter and length also affect performance some.  The simplified diagram above shows the tuned pipe with an inlet and outlet (stinger) of the same diameter.  In reality, the stinger is usually smaller in diameter.  This reduction in size of the stinger is used as a part of the tuning and is beneficial.  Probably the most important parts of the tuned pipe are the divergent cone and convergent cone.  The angles (how quickly the cone expands or contracts) affect performance significantly.  Typically, abrupt and more extreme angles on the divergent and convergent cones will lead to a more intense increase in power, but this power spike usually occurs at a narrow RPM of the engine.  This can be described as a "peaky" powerband.  Generally speaking, more gradual & gentle angles (smaller degree angles) will lead to more moderate power increases but they will be effective over a wider engine RPM range.  In other words, a smaller angle will typically result in an engine with a wider powerband that is not so "ON/OFF" peaky in it's power delivery.  So, by varying the angles on the divergent and convergent cones, varying the length of the belly section, and varying the diameter and length of the stinger, the tuned pipe can be "tuned" to give the desired performance at the desired RPM range.  This simple concept of expansion chambers and tuned pipes can be a significant performance factor on 2 stroke engines of any kind.