Polini Thor 303 EVO

38 HP | 281cc | Liquid Cooled | Electric + Flash Start

The Polini Thor 303 EVO takes Polini’s largest-displacement Thor engine and gives it a major update focused on reliability, durability, smooth power delivery, and thermal management.

Producing 38 HP at 8,000 RPM and weighing approximately 47.4 lbs including the radiator, the 303 EVO delivers an excellent power-to-weight ratio for lightweight aircraft applications.

This isn't simply a renamed Thor 303. Polini redesigned many of the engine’s key components for the EVO, including the cylinder, cylinder head, exhaust, ignition and starting system.

What Changed With the EVO?

The new Thor 303 EVO features:

• Completely redesigned cylinder

• Enlarged exhaust port and transfer passages

• New 5-bolt cylinder mounting flange for improved sealing

• Revised combustion chamber and squish geometry

• Improved combustion and thermal management

• New optimized exhaust system for smooth, linear power

• Redesigned spherical exhaust joint to improve sealing and isolate vibration

• New ignition system

• Dual Spark ignition available

• New electric starter motor

• Electric start now standard

• Polini Flash Starter backup starting system

The result is a smoother, more refined version of Polini’s high-output 281cc engine built for demanding light-aircraft applications.

Power Without the Weight

The Thor 303 EVO produces 38 HP from only 281cc while maintaining a complete engine weight of approximately 47.4 lbs including the radiator.

With the appropriate propeller and reduction configuration, Polini reports maximum static thrust of up to approximately 265 lbs.

For builders looking for strong two-stroke performance without the weight of a larger four-stroke installation, the 303 EVO offers an impressive combination of power, compact size and simplicity.

Two Reduction Ratios

Choose the reduction ratio that best matches your aircraft and propeller installation:

2.8:1 Reduction
Designed for installations using smaller or higher-RPM propeller combinations.

3.2:1 Reduction
Allows the engine to turn larger propellers at lower propeller RPM.

Propeller selection must be matched to the aircraft, reduction ratio and intended operating RPM.

Standard or Dual Spark

The Thor 303 EVO is available with Polini's standard electronic CDI ignition or the optional Dual Spark ignition system.

For aircraft builders looking for additional ignition redundancy, the Dual Spark version provides an additional level of redundancy within the ignition system.

Electric + Flash Start

Unlike earlier configurations of the Thor 303, electric start is standard on the EVO.

Polini also retains its mechanical Flash Starter, giving the engine a secondary method of starting without relying solely on the electric starter.

Specifications

Engine Type: Single-cylinder, 2-stroke
Cooling: Liquid cooled
Displacement: 281cc
Bore × Stroke: 76 × 62 mm / 2.99 × 2.44 in
Power: 38 HP @ 8,000 RPM
Maximum RPM: 8,400 RPM
Maximum Static Thrust: Up to 120 kg / 265 lb with 160 cm / 63" propeller
Compression Ratio: 9.5:1
Cylinder: Aluminum with Gilnisil coating
Piston: Two trapezoidal chrome-plated rings
Intake: Crankcase reed valve
Carburetor: Dell’Orto VHST 28mm
Air Filter: Airbox
Ignition: Electronic CDI with variable advance
Dual Ignition: Dual Spark version available
Charging Output: 40W @ 5,500 RPM
Starter: Electric Starter + Polini Flash Starter
Clutch: Centrifugal, oil bath
Reduction Drive: Helical gears in oil bath
Reduction Ratios: 2.8:1 or 3.2:1
Exhaust: Expansion chamber with aluminum silencer
Engine Weight: Approx. 21.5 kg / 47.4 lb including radiator
Propeller Rotation: Clockwise
Manufacturer: Polini Motori
Country of Origin: Italy

Fuel & Oil

Polini specifies:

• E5 unleaded gasoline, 95 RON with no more than 5% ethanol, or

• 100LL aviation gasoline

• Mixed with 2.5% fully synthetic 2-stroke oil

Always follow the current Polini operating manual for approved fuel, oil, mixture and operating requirements.

Fuel Consumption

Polini reports approximately 3 liters/hour / 0.8 gallons/hour while producing 30 kg / 66 lbs of static thrust under its test conditions.

Actual fuel consumption will vary with propeller, aircraft installation, RPM, atmospheric conditions and operating power.

Carburetor Options

The Thor 303 EVO is offered by Polini with both:

Dell’Orto VHST 28mm
Traditional float-style carburetor configuration.

Tryton Diaphragm Carburetor
Polini also offers the EVO with a Tryton diaphragm carburetor, designed to provide stable carburetion, optimized fuel consumption and easier transportation.

Availability may vary. Contact Mini-Max USA if you would like a specific carburetor configuration.

Recommended Applications

The Thor 303 EVO is designed for lightweight aviation applications where strong power-to-weight performance is important.

Potential applications include:

• Experimental aircraft

• Amateur-built aircraft

• Three-axis ultralights

• Lightweight sport aircraft

• Trikes and weight-shift aircraft

• Paramotor and powered-paraglider applications

• Custom light-aircraft projects

For Mini-Max builders, the Thor 303 EVO is an excellent engine to consider when your aircraft and mission call for approximately 38–40 HP of two-stroke power.

Engine suitability ultimately depends on the aircraft model, completed aircraft weight, center of gravity, engine mount, cooling installation, propeller and reduction ratio.

What's Included

Your Thor 303 EVO is supplied in the configuration selected when ordering.

The engine package includes the major factory-supplied engine systems applicable to that configuration, including the engine, reduction drive, starting system, exhaust and cooling components.

Propeller is not included unless specifically stated with your order.

Because Polini periodically updates engine packages and supplied components, contact Mini-Max USA if you need confirmation of exactly what is included with your specific configuration before ordering.

Manufacturer Warranty

Polini states that the Thor 303 EVO is covered by a 24-month manufacturer warranty.

Warranty coverage, exclusions, maintenance requirements and eligibility are governed by Polini Motori's current warranty terms and conditions.

Ordering & Shipping

Polini engines are typically special-order engines and may ship directly from the manufacturer or through Mini-Max USA depending on availability and the configuration ordered.

Lead times vary.

International freight, customs fees, duties and import tariffs may apply. If an import tariff or duty is assessed on an engine, that charge is the responsibility of the purchaser and may be collected by the carrier or billed by Mini-Max USA.

Need help choosing your ignition, reduction ratio, carburetor or propeller combination?

Contact Mini-Max USA before ordering and we'll help you configure the engine for your aircraft.

Important Engine Notice

Polini Thor engines are intended for appropriate recreational, ultralight and experimental aviation applications. Engine selection, installation, propeller selection, fuel system, cooling, weight and balance, maintenance and operation are the responsibility of the aircraft builder, owner and operator.

Engine failure can result in serious injury or death. Always operate within the manufacturer's limitations and maintain a safe landing option whenever practical.

Shopify Short Description

38 HP from just 47.4 lbs including the radiator. The new Polini Thor 303 EVO features a redesigned cylinder and combustion chamber, improved exhaust, updated ignition, standard electric start plus Flash Starter, and optional Dual Spark ignition. Available with 2.8:1 or 3.2:1 reduction.

Product Highlights

38 HP
@ 8,000 RPM

281cc
Single-cylinder 2-stroke

47.4 lbs
Including radiator

Up to 265 lbs Thrust
With 63" propeller*

Electric + Flash Start
Standard

Dual Spark Available
Optional ignition redundancy

2.8:1 or 3.2:1
Reduction options

Liquid Cooled
Optimized thermal management