Tech Professor: Two-Stroke Exhaust Hardware

In the 1960s, it wasn’t uncommon to see a piece of flex pipe hanging out the side of a snowmobile hood, obviously retrofitted by the sled’s owner. It was loud, and if the pipe happened to be about 15 inches long, it made better power than the muffler that was originally attached to the exhaust port. More often than not, however, the super exhaust modification only made more noise, but that usually seemed to satisfy the racer well enough. If people ran in horror when a racer started his sled, they assumed it was fast.

Flow Of A Sonic Wave

When a two-stroke engine’s exhaust port opens, a sonic wave begins traveling out the exhaust system. Behind this wave is a low-pressure area, about minus 7 psi, which is used to pull the spent exhaust gas and help draw the fresh fuel-and-air charge up from the crankcase. This is known as the extractor effect.

A strong extractor wave from the exhaust system can pull a fresh charge through the engine. But early snowmobile engine tuners found that if they cut the straight pipe to a specific length, the extractor effect could be used without pulling too much of the fresh charge out the exhaust port before it had the chance to ignite and make power.

Four-stroke race engines of the era were using megaphones on their exhaust systems to amplify the extractor effect, and it didn’t take long for megaphones to show up on racing snowmobiles. The megaphone amplified the negative pulse in the exhaust system, but the snowmobile was almost impossible to be near when it was running because it was so loud.

The megaphone is still used in some four-stroke racing applications because the exhaust and intake cycles are distinctly separated. In the complicated breathing pattern of a two-stroke cycle, however, a megaphone can result in such a strong negative pulse that it will draw much of the fresh fuel-air charge right out the exhaust port, especially on engines with long exhaust port durations.

For two-strokes, a baffle or converging cone is used to reflect the sonic wave back to the exhaust port. In front of the reflected wave is positive pressure that can be used to push the fresh fuel-air mixture back into the cylinder just before the port closes.

Timing and amplitude of the waves is controlled by working with the length and angles of the diverging and converging cones and the lengths of the head pipe, center section and tail pipe or stinger. There are mathematical relationships that dictate the basic design of a tuned pipe, but the final design of any pipe is always developed on a dynamometer.

A tuned exhaust system for a two-stroke engine is a large and complex shape. Fitting that shape into an engine compartment requires the touch of an artist. Once the pipe is fitted, further tuning is generally required to restore the losses imposed by all of the bends necessary to make it fit. And when the pipe is finally fitted, the system must be silenced with an after muffler, often referred to as a silencer.

Building an effective tuned exhaust system is no easy feat. Constant monitoring of the engine’s fuel consumption is necessary to make sure the pipe doesn’t add power by simply leaning the engine at some point. This is one common mistake of amateur pipe builders. Changing the carburetor jetting or fuel injection system fuel delivery could actually have accomplished what seems to be an increase in power from the pipe.

A Precise Design Is Key

Today’s high-performance snowmobiles all have tuned exhaust systems. Often the stock tuned exhaust system is so well designed that it is impossible to gain more power by changing to an aftermarket pipe without making other major changes to the engine or the engine’s operating speed.

The design of an exhaust system must be based on the bore and stroke, port timing, desired operating RPM, desired width of the power band and the general flow characteristics of the engine. One could work up a pipe that really scavenged the cylinder well, but if flow restrictions from the airbox, carburetor or intake system were too high, no additional fresh charge could be flowed and there would be no gain with the system.

Often the gain from an aftermarket exhaust system is achieved by turning the engine at a higher RPM. Turning up the engine requires an ignition system that has the proper advance curve for the new peak power speed. Digital ignition systems will usually require a new chip with the proper advance curve mapped on it. In some cases complete ignition systems will have to be changed.

When the power output of an engine is changed, the clutch has to be re-calibrated to follow the power curve of that “new” engine. Springs, weights, cams and often chaincase gearing needs to be altered. The power gain must be significant in order to justify the increase in engine speed because the increase in belt speed of the transmission results in lower overall efficiency.

A tuned exhaust system is critical to the output of a two-stroke snowmobile engine. While there may be aftermarket pipes that can improve a machine’s performance, that benefit in power may require many other changes in the machine’s set up and there can be sacrifices in fuel economy and trail-ability.

The years of simple modifications to accomplish major power gains are gone. Today’s high performance sleds are well-researched, well-designed packages and the all-around use intended for these machines is often compromised when the high performance addict starts to work on them. When specialized uses such as drag racing, radar runs or some other form of competition are intended, an engine modifier can make some worthwhile improvements.

High performance trail riders can also benefit from the use of a properly designed, aftermarket tuned exhaust system. Just make sure you understand that special tuned exhaust systems are not for everyone. Evaluate your needs and spend time with a knowledgeable racer or tuner before diving into the world of specialized tuned exhaust systems.


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