THOR 303 EVO – 303 EVO D.S.

Evolution, power and reliability

Polini presents the THOR 303 EVO, the largest-displacement engine in the Thor family, developed to deliver high performance, reliability and safety, even under the most demanding operating conditions.

The two-stroke, single-cylinder 281 cm³ Thor 303 EVO produces 38 HP at 8,000 rpm and develops 120 kg of thrust with a 160 cm³ propeller.

Despite its high performance, particular attention has been paid to efficiency. Under standard level-flight conditions, fuel consumption is approximately 3 litres/hour at 30 kg of static thrust.

The Thor 303 EVO is the result of extensive development work on every component. The cylinder, combustion chamber, muffler, ignition system and starter have all been developed to provide smooth and consistent power delivery, controlled operating temperatures and high long-term reliability.

The engine is available with either a Dell’Orto VHST 28 or Tryton carburetor, and with either a single- or dual-spark ignition system.

Cylinder

The heart of the Thor 303 EVO is a cylinder designed specifically for aviation applications.

With a displacement of 281 cm³, the engine has a 76 mm bore and 62 mm stroke. The cylinder is made from aluminium with a Nikasil coating and is fitted with a piston featuring two chromed rings.

The cylinder has been completely redesigned, with improved transfer ports and an exhaust duct optimized to improve gas flow and contribute to increased performance.

The connection to the muffler has also been redesigned, incorporating a new five-screw flange to provide improved stability and sealing of the exhaust system over time.

The cylinder head features an advanced combustion chamber developed through a dedicated study of the compression ratio and squish parameters. The new geometry promotes fast and complete combustion, helping to maintain low operating temperatures.

The compression ratio is 9.5:1.

Muffler

The THOR 303 EVO is equipped with an aluminium muffler developed to work in synergy with the cylinder.

Its geometry has been optimized to provide full, progressive and smooth thrust throughout the entire operating range, while keeping power output under control even in demanding conditions.

Particular attention has also been given to the spherical exhaust connection, which has been redesigned to improve sealing and isolate the exhaust system from vibrations, contributing to long-term reliability.

Fuel System: Dell’Orto or Tryton

The Thor 303 EVO is available with either a Dell’Orto VHST 28 mm or a Tryton carburetor, providing two different fuel-system configurations.

The Dell’Orto VHST 28 mm provides precise carburetion, smooth power delivery and a wide range of adjustment options. Its design helps reduce variations caused by atmospheric pressure, humidity and altitude, thereby reducing the need for carburetor adjustments.

To facilitate transportation, the carburetor is equipped with a dedicated valve that allows the fuel contained inside it to be drained. This makes it possible to position the engine horizontally while reducing the risk of fuel leakage through the vents.

Alternatively, the Tryton carburetor is available, developed to provide high operating stability under changing atmospheric conditions and at different altitudes. Its uniform and consistent fuel flow promotes regular combustion and optimized fuel consumption. The sealed configuration also makes it easier to transport the engine even in an inclined position.

Air Box

The intake system is completed by an air box developed through fluid-dynamic analysis, with the aim of optimizing the speed and pressure of the air supplied to the carburetor at every engine speed.

The special internal S-shaped air path stabilizes turbulence and contributes to smooth, linear and consistent power delivery.

The air box can be completely opened and inspected and uses a 30 PPI open-cell polyurethane foam filter element, supported by a dedicated frame that keeps it securely positioned over time.

The air box housing can be separated into two halves using just two screws, making routine inspection and maintenance operations quick and easy.

Reed-Valve Intake

The THOR 303 EVO uses a reed-valve intake system with direct induction into the engine crankcase.

The reed valve assembly, developed by Polini’s R&D department, uses six carbon-fibre reeds with calibrated flexibility. They have been designed to ensure a consistent response at different engine speeds and contribute to smooth power delivery.

Liquid Cooling

The thermal management of the THOR 303 EVO is entrusted to a liquid-cooling system developed to keep the temperature of the engine assembly under control, even during prolonged operation and under high loads.

Coolant circulation is provided by a 30 mm mechanical pump housed on the countershaft, while the system uses a compact radiator and an expansion tank.

The engine is also equipped with a thermostat, which manages the temperature of the cooling circuit during operation.

Engine Crankcases

The crankcases are manufactured from high-silicon aluminium die casting, a material selected to combine mechanical strength with low weight.

The entire production process, from casting and painting to CNC machining, is controlled to ensure dimensional accuracy and consistent quality.

The crankcase design has been developed to maintain a compact structure, with reinforcements concentrated in the areas subject to the highest loads.

Crankshaft

The crankshaft is manufactured using selected steels and specific heat treatments.

Particular attention is paid to crankshaft centring in order to minimize vibrations resulting from any imbalance and reduce stress on the components during operation.

CNC machining and precision measuring instruments ensure that dimensions remain within the specified tolerances, while high-quality bearings are used for the connecting rod and in the engine crankcase.

Countershaft, Reduction Gear and Clutch

The front crankcase incorporates the centrifugal clutch, mechanical reduction gear and balancing countershaft, with the entire system operating in an oil bath.

The centrifugal clutch allows the propeller to remain stationary while the engine is idling. This is an important feature for increasing safety during ground operations and pre-flight procedures.

The balancing countershaft is designed to counteract vibrations generated by the reciprocating movement of the piston, contributing to improved comfort during flight.

The reduction gear uses helical gears operating in an oil bath and is available with two ratios:

• 2.8: suitable for propellers up to 140 cm in diameter;
• 3.2: designed for propellers with a diameter greater than 140 cm.

The system reverses the propeller’s direction of rotation relative to the engine crankshaft, helping to reduce reaction torque.

Electronic Ignition and Dual Spark

The THOR 303 EVO uses an electronic CDI ignition system with variable advance, developed to automatically adjust ignition timing according to engine speed, providing smooth operation and prompt, linear power delivery.

The new ignition system has been developed with particular attention to spark consistency and operating reliability.

For those seeking an additional level of redundancy, the Dual Spark configuration is available. It features two independent ignition systems designed so that a potential failure of one system does not compromise the operation of the other.

This solution has been developed with a specific objective: to further increase operational safety in flight.

Electric Starting and Flash Starter

The THOR 303 EVO is equipped as standard with an electric starting system, combined with a new starter motor developed to ensure quick and reliable engine starting.

This is complemented by the Polini Flash Starter, a manual starting system designed to make starting simple and immediate.

The two systems therefore provide both electric starting and manual Flash Starter operation.

Mounting System

Like the other engines in the THOR family, the THOR 303 EVO uses the common mounting system shared across the Polini THOR range.

This allows manufacturers to develop frames compatible with different THOR engine models, simplifying integration and enabling pilots to switch from one engine model to another without necessarily having to modify the frame.

The THOR 303 EVO is supplied fully assembled and ready for installation.

Performance and Applications

With 38 HP at 8,000 rpm and 120 kg of static thrust with a 160 cm propeller, the THOR 303 EVO is an engine developed for applications requiring high power output.

It is therefore particularly suitable for single- and two-seat trikes, delta-wing trikes, single-seat three-axis aircraft and UAVs (Unmanned Aerial Vehicles).

When used on a single-seat paramotor, its high performance makes it suitable for experienced pilots and for applications where power and rapid response are key requirements.

Polini Quality and Safety

The THOR 303 EVO is the result of Polini’s experience in the design and manufacture of two-stroke engines, combined with a development programme that includes bench testing and flight testing.

Every engine undergoes inspections before delivery, with particular attention paid to the components and systems that affect reliability and operational safety.

The THOR 303 EVO is covered by a 24-month warranty.

With the THOR 303 EVO, power, reliability and safety evolve together in an engine designed to handle the most demanding flight conditions.

THOR 303 EVO

*Values measured at sea level under standard atmospheric conditions. Thrust and fuel-consumption values may vary depending on air temperature and density altitude.

928.000.150  THOR 303 EVO ENGINE – DELL’ORTO – SINGLE IGNITION – 2.8 – ELECTRIC STARTER

928.000.151  THOR 303 EVO ENGINE – DELL’ORTO – SINGLE IGNITION – 3.2 – ELECTRIC STARTER

928.000.154  THOR 303 EVO ENGINE – DELL’ORTO – DUAL IGNITION – 2.8 – ELECTRIC STARTER

928.000.155  THOR 303 EVO ENGINE – DELL’ORTO – DUAL IGNITION – 3.2 – ELECTRIC STARTER

928.000.156  THOR 303 EVO ENGINE – TRYTON – SINGLE IGNITION – 2.8 – ELECTRIC STARTER

928.000.157  THOR 303 EVO ENGINE – TRYTON – SINGLE IGNITION – 3.2 – ELECTRIC STARTER