Understanding Two-Stroke Exhaust Systems

A two-stroke exhaust system is designed to optimize the performance of a two-stroke engine by managing the flow and timing of exhaust gases. Here’s a detailed look at how it works:

1. Exhaust Port Opening

Exhaust Gas Release: As the piston moves down the cylinder during the power stroke, the exhaust port opens. This allows the hot, high-pressure exhaust gases to escape from the combustion chamber into the exhaust system.

2. Pressure Wave Creation

Pressure Wave Formation: The sudden release of exhaust gases generates a pressure wave that travels through the exhaust system. This wave is a result of the high-speed ejection of gases from the cylinder.

3. Expansion Chamber Dynamics

Initial Expansion: The pressure wave travels into the expansion chamber, which is a specially designed part of the exhaust system that is wider than the header pipe. The expansion chamber's design allows the pressure wave to expand and decrease in pressure as it enters.

Reflected Wave: The expansion chamber features a baffle or cone that reflects the pressure wave back towards the exhaust port. This reflected wave can interact with the exhaust gases still in the cylinder.

4. Scavenging Process

Wave Timing: The timing of the reflected wave is crucial. Ideally, the reflected wave arrives at the exhaust port just as it is closing, creating a low-pressure area that helps draw out any remaining exhaust gases from the cylinder.

Enhanced Intake: By pushing out residual gases, the reflected wave helps in drawing in the fresh air-fuel mixture more effectively, improving engine efficiency and performance.

5. Exhaust Gas Exit

Stinger Pipe: The remaining exhaust gases exit through the stinger, which is a narrower section of the exhaust system. The stinger helps control back pressure and ensures that the exhaust gases are expelled efficiently.

Performance Influences

The performance of a two-stroke engine is heavily influenced by how effectively the exhaust system manages gas velocity, pressure waves, and the scavenging process. Proper tuning and design of the exhaust system are key to maximizing engine power and efficiency.

Gas Velocity and Pressure Waves:

Temperature of Exhaust Gases: Hotter exhaust gases possess more energy, causing them to travel faster through the exhaust system. This increased speed directly impacts the formation and behavior of pressure waves when the exhaust port opens.

Pressure Wave Dynamics: The timing and strength of these pressure waves are crucial for the scavenging process. Higher temperatures result in faster and more robust pressure waves, enhancing the efficiency of scavenging.

Impact on Yamaha 650-760 Platform

Exhaust Duct Size Issues:

The Yamaha 650-760 platform features an overly large and long exhaust duct, with an exhaust duct size at 110% of the exhaust port area. This oversized design creates a scenario where exhaust gases move sluggishly, making for a 'lazy' exhaust flow. The ideal port duct size for a single exhaust port engine is about 90% of the exhaust port area, so the Yamaha's design significantly hinders efficient exhaust gas evacuation, leading to reduced performance.

Wax Racing Manifold Solution: By using the Wax Racing manifold to keep the exhaust gases hotter, the velocity of the gases increases, somewhat mitigating the issue caused by the oversized exhaust duct. This adjustment improves the flow of exhaust gases, enhancing engine performance.

Kawasaki Platform Comparison: Interestingly, the Kawasaki platform does not suffer from this issue, even though the Kawasaki manifold is water-cooled, due to a better port-to-duct ratio. A Yamaha cylinder with a Kawasaki-style exhaust duct would potentially offer superior performance.

Tuning Implications

Hotter Exhaust Gases:

A hotter manifold means the exhaust gases are hotter, which results in the engine pulling harder in the mid to top RPM range. This increased performance requires more fuel to prevent the engine from running lean.

Retarded Timing Effects: If the engine timing is retarded, particularly at higher RPMs, this will add more heat to the exhaust gases, further impacting the tuning process.

Engine Temperature Management: When using dual cooling with the Wax Racing manifold, it's important to maintain the engine temperature between 120°F and 130°F. This temperature range is ideal for ensuring that the engine operates efficiently while benefiting from the hotter exhaust gases. Proper water routing through the manifold and header pipe is essential to achieve this balance and avoid overheating the engine.

Free Ride Ski Considerations

Cold and Wet Pipes:

Free ride skis often face performance issues with cold, wet pipes that reduce throttle response and power delivery. Keeping the exhaust system hot improves throttle response and maintains power. While it might be tempting to run the pipe cooler, thinking it will make the ski pull harder at low RPMs, this approach often results in the engine going flat at higher RPMs, preventing it from reaching full revs. Instead, it's a far better solution to use a lower-pitched impeller, which can enhance low-end performance without compromising top-end power.

Water Injection Concerns: Water injection systems can cool the exhaust chamber too much, leading to a delay in reaching optimal temperature and power output. A dry, hot exhaust system provides better throttle response and overall performance, avoiding the pitfalls of excessive cooling.

Setup Recommendations

Dual Cooling Preference:

The Wax Racing manifold is designed with large water channels, allowing for more effective dual engine cooling. Dual cooling is preferred for optimal engine temperature control, especially in high-performance applications.

Water Routing:

For setups with an aftermarket head, it's recommended to use the back water outlet on the head to route water to the bottom of the header pipe. The water should flow into the bottom and exit from the top of the header pipe.

From the top of the header pipe, direct the water straight overboard. Use a T-fitting to split the flow, with a lower flow directed to the stinger. A restrictor can be used here, with a drilled needle and seat serving as an effective option (4mm for high-speed applications like SuperJets, or 2mm for free ride hulls).

This approach results in snappy, punchy power, whether you’re setting up a race ski or a free ride machine. If you need further clarification or additional tuning advice, feel free to ask!