The 125cc 2-stroke engines used in KZ karting categories (KZ1, KZ2) are among the most advanced in motorsport. These engines stand out for their ability to deliver high power at very high RPMs while maintaining a compact and relatively simple structure compared to 4-stroke engines. Let’s look at a technical overview of the main features of a 125cc 2-stroke engine for KZ karts.
1. Displacement and Engine Architecture
A KZ 125cc engine has a displacement of 125 cm³, achieved through the relationship between the bore and stroke of the piston. In most KZ kart engines, these measures are optimized to achieve an ideal balance between torque and engine speed.
- Bore: Generally around 54 mm.
- Stroke: Around 54.5 mm.
This combination allows for a square or nearly square engine, favoring both maximum piston speed and good torque delivery at medium-high RPMs.
←--- Picture. Motori Seven L9A. A state of the art KZ Engine with a stunning 52 HP output
2. Power and Torque
These engines are typically capable of producing between 40 and 50 horsepower (HP), depending on the model and tuning, at an RPM range between 12,000 and 14,000. The power-to-weight ratio of a kart equipped with a KZ 125 engine is very high, making the performance truly remarkable in terms of acceleration (0 - 100 km/h in about 3 seconds) and top speeds up to 180 km/h (with a long gear ratio).
- Maximum power: Usually delivered in the RPM range between 12,000 and 14,000.
- Torque: Values are around 25-30 Nm. Torque is especially important for corner exits and maintaining speed on straights.
3. Induction: Reed Valve
One distinctive feature of 2-stroke engines is the absence of a traditional valve system like in 4-stroke engines. Instead, 2-stroke engines use a reed valve system located at the crankcase intake to control the flow of the air-fuel mixture.
- Reed valves: Open and close automatically based on the vacuum in the crankcase, optimizing cylinder filling. This system ensures more efficient intake, especially at high RPMs.
4. Fuel System and Carburetion
The 125cc 2-stroke engines for KZ karts typically use high-precision diaphragm carburetors, with a diameter of 30 mm, capable of managing large quantities of fuel and air, especially with a highly turbulent mixture flow at high RPMs.
- Carburetor: Regulates the air-fuel ratio very precisely, which is particularly important for optimizing power and efficiency and requires fine-tuning to maximize performance based on atmospheric conditions.
These engines use a gasoline and oil mixture (usually at 4%), where the oil lubricates the moving parts and prevents seizure.
5. Exhaust: Expansion Chamber
The exhaust is a crucial component in 2-stroke engines for optimizing the evacuation of burnt gases and promoting the return of pressure waves for cylinder scavenging. The KZ 125cc engines use expansion chamber exhaust systems, characterized by a trumpet shape with a reduction cone at the end.
- Expansion exhaust: Uses pressure waves to help empty the cylinder of exhaust gases and improve the filling of the combustion chamber with fresh mixture. This system significantly boosts power, especially at high RPMs.
A precise exhaust setup is essential to maintain an optimal torque curve and reduce the “power dip” (lack of power) at low RPMs.
6. Ignition
The 2-stroke engines for KZ karts typically use internal rotor ignition systems with adjustable spark advance, synchronized to optimize ignition timing based on RPMs.
- CDI (Capacitor Discharge Ignition): Controls spark plug ignition and adjusts the timing based on engine speed, optimizing combustion and power. The system must be precisely synchronized to achieve the best balance between torque and high RPM power.
7. Lubrication and Cooling
Unlike 4-stroke engines, the lubrication system of a 2-stroke engine is much simpler since the oil is mixed directly with the fuel. The air-fuel-oil mixture lubricates the piston, bearings, and other moving parts internally.
- Lubrication: Is premix, so the oil is mixed with the gasoline before combustion.
- Cooling: The KZ 125cc engines are liquid-cooled. This system is essential for dissipating the heat generated by high engine speeds and maintaining optimal operating temperatures.
8. Transmission and Gearbox
A key feature of KZ kart engines, compared to traditional 125cc single-gear engines, is the presence of a six-speed sequential gearbox. This allows the driver to better manage the power and torque on different parts of the track, improving acceleration efficiency and increasing top speed.
- Sequential gearbox: Characterized by fast shifting without the need to use the clutch after the start. This offers a significant advantage in control, especially in corner exits.
Power is transmitted through a chain connecting the gearbox sprocket to the rear axle sprocket.
9. Final Considerations
A 125cc 2-stroke engine for KZ karting is a technological powerhouse designed to maximize power and response at high RPMs. Its main features—such as the reed valve, expansion exhaust, and sequential gearbox—make it extremely high-performing and suited to experienced drivers who can handle the aggressive nature of these engines.
Despite the conceptual simplicity of a 2-stroke engine compared to a 4-stroke, the high performance and regular maintenance required to keep the engine efficient make this type of engine particularly fascinating and challenging for KZ kart drivers.