Leading the 250cc-water category
The launch of this engine with its 36CV has significantly contributed to an evolution towards smaller wings, as before engines were usually of around 25 to 29 HP with respective thrusts, limiting the size of glider to 18m2 in the most radical cases. Today we see the major international Slalom pilots flying the Thor 250 with gliders sized 16m2 or less.
We can say that this Thor opened the 250 class and today several manufacturers are developing engines of this displacement.
The price of power
The 19 Kg of weight of the most complete version with electric starter (+battery) makes a paramotor powered by the Thor 250 heavy, but after four steps and once you are airborne this sacrifice is fully offset by the feeling of power and smoothness in acceleration. Compression is high, and so we must be precise and careful with the kickback when starting it manually.
The hydraulic performance is faultless, no leaks or drops in joints or hoses.
The cylinder is made of aluminum with Gilnisil coating, and the piston has chrome/silver rings to decrease friction.
The carter maintains the golden look of the Thor series and differs from the rest of the range because it houses the radiator pump. The carter also includes the helical reduction gearbox of 2.8 to 1 ratio and the centrifugal clutch, both in oil bath.
The exhaust bends in a beautiful L shape and ends up in the muffler of the Thor series. We can say that the body is very compact and the surrounding air flow is optimized.
The carburetor is a PWK, the same used in the 200 but with a different setting.
Easy start but requires precision
After priming the carburetor you can start it manually using its Flash Starter. You will have to pull smoothly and progressively a couple of times so the spring gets in, at the third time keep a touch of throttle and do the final pull; this way the engine will almost always start immediately. Attention with the recoil since if the starting is not done well the kick back can snatch the handle away from your hand quite hard and even hit the radiator.
With the electric starter the process is simpler; just connect the battery, accelerate a little and simultaneously press two buttons that are at the base of the Polini throttle.
The Thor 250 has a vibrant and strong sound, of pure adrenaline! When it is new the sound is more acute and then it softens but keeping the sporting nerve. On the other hand, the muffler works well and the noise is not too loud, very endurable for a pilot equipped with a helmet with good ear protection, especially because the exhaust is located far from the pilot's head.
In its element. The Thor 250 was born to propel in the sporting arenas. The world of pylons lets it display its full potential: its thrust is strong, gradual and precise, allowing the pilot to fly the glider fast and smoothly in tight turns.
Break-in is done with 2.5% mix for three hours on the ground, changing the revs from time to time, and then 6 hours flying gently. Then we move to 2% in the mix, and off to fly! The mixture works well with 95 octane fuel and 100% synthetic 2-stroke oil.
The Thor 250 can be brutal in terms of thrust but at the same time it delivers power smoothly, which most pilots will appreciate.
Foot-launched and paratrike
Most of the over 100 flights I've done with the Thor250 have been with the Dudek Snake 22, a fantastic paraglider with great qualities for flight, especially Slalom-type. When I take off on foot loading 145 kg it is a fast and very maneuverable wing, but still easy to fly and with good performance to navigate.
When I fly on paratrike with a load of 165 Kg, the potential of the glider rockets: turns are immediate so the 100 Kg of thrust of the 250 are very welcomed. With this combination I can afford to accelerate the 100% exceeding 72 km/h, and climbing! In fact I think I could even go down to a 20m size of glider.
I like to fly the Thor 250 on foot, but I prefer it on paratrike as there is power to spare, the torque is counterbalanced, and also I don't have to carry the weight on my back...
The Thor 250 is perfect for light dual paratrikes in combination with performance wings, although it can also handle reflex gliders well. We test flew it fitting the paramotor on an Airfer Diamond trike, and with different gliders, like the BiGolden 3, the Niviuk Takoo 2 or the ITV Bulldog 38. The first two are performance wings and we flew very comfortably with lots of power. With the Bulldog we were faster and more agile, but also with more limited power.
Its power and stability can let you fly with loads from 150 to nearly 300 Kg, depending on the glider you use; for example, the Bulldog asked a lot of power to gain speed on the take-off run, but once in flight we could keep level flight at half-throttle. With this type of glider the Thor 250 is in its element and we can use it without crushing it.
Above: Light tandem paratrike from French manufacturer Macfly, designed by Paco Díaz.
We believe that this engine is ideal to motorize light and transportable tandem paratrikes and for a moderate to medium use. If you want to make a very intensive use or go with small or reflex wings I would recommend a bi-cylindrical engine from 50 HP up.
The smoothness in level flight and the low RPM are remarkable when flying intermediate performance gliders, like the Gradient Golden 4 (EN B).
Vibration and comfort
At idle the noise is contained and vibrations are not appreciated, you could even not realize that the Thor 250 is running. In flight, the same: revolutions rise gradually without bumps, and thanks to the power of the engine and the performance of modern gliders, level flight is easy to manage, which is achieved at low RPM - between 4,000 and 5,500.
The comfort in navigation in terms of torque depends largely on the paramotor frame and also on the qualities of the glider to counterbalance.
After the first flights the cap of the clutch and the reduction gear should be checked and retighten if necessary. The aluminum base holding the propeller to the engine to a steel gear must be well tightened otherwise it could lose grip.
Starter and high compression: As mentioned before, we should try to master the manual starting technique in order to avoid that the rope recoil pulls the handle back violently, as it might hit and damage the radiator. We had two Thor 250 engines flying for 2 years and the summary of breakdowns can be cut down to a starter that failed and the loosening of the aluminum piece supporting the propeller, as mentioned above. Both parts were replaced by Polini and we can say that the Polini customer service and warranty work well.
Conclusion: The reference
The best Polini engine, European and World slalom champion, is the strongest single-cylinder engine with the most vibrant and exciting sound we have tried so far. Its main argument is the liquid cooling system, but it also has reduction gear and centrifugal clutch, manual and electric starter, and even dual ignition for the most demanding pilots.
The best of it is that it works effortlessly and can be kept in the optimum operating range. It is very robust and with a high degree of reliability, so we can ensure a long service life. But of course all this has a cost: on the one hand in the weight we have to carry before taking off, and on the other, the price as we are talking of about €4000 for a single engine, i.e., nearly twice the price of most engines - although this Thor 250 also has nearly twice the power. In spite of that, we are sure that this engine will seduce paramotor connoisseurs; and those who can afford it will probably pay each euro gladly.
The +: Smooth, linear power delivery, perfectly controllable
The -: It is heavy to fly it foot launched. High torque
+ Info: www.polini.com
Winner star at the first Slalom world championship in 2013 and also at the Europeans 2014, where many pilots flew it in combination with small wings of up to 15m2, the most powerful Thor from Polini goes like lightning also on solo paratrike with bigger gliders of between 18 and 22m2.
Also, it is being used to motorize lightweight tandem paratrikes, minitrikes, moto-gliders and other small aircraft.
It is currently the most successful engine at sporting level, dominating the main competition podiums.
In the picture, the Thor 250 with dual ignition. Its total weight including radiator, coolant, starter and battery may exceed 25Kg. On the other hand, the set is pretty compact as the exhaust clings to the engine, and the air filter -although bulky- has rounded lines. The set can be easily installed on any paramotor or paratrike frame.
The Italian company Polini boasts one of the widest engine ranges and the Thor 250 is, undoubtedly, the King of the House for flight, not only for the power and sophistication of the engine. With 244cc it has the greatest displacement and a power of no less than 36 HP at 7,500 RPM.
Then, liquid cooling makes it very stable in terms of temperature operation, allowing us to crush it at the throttle.
You can also order it with dual ignition, electric or manual starter, or both, and it comes with pre-installation for temperature sensor.
The Thor 250 shares many features with its smaller siblings of the Thor series, like the counter-rotating axis to eliminate vibrations or the centrifugal clutch.
Installing the Thor 250 in a frame is similar to other engines; the only difference to take into account is the need of a base to place the carburetor and the water charging bottle.
We must also leave space for the coolant hoses which should not touch the net or any other surface as micro-vibrations may damage them.
Some people think that the radiator is too big and that the water charge bottle should be integrated in it. The truth is that in fresh environments it is best to cover part of the radiator with a sleeve or some duct tape; otherwise the motor does not reach the optimum temperature and stays cold. On the other hand, the entire surface of the radiator is necessary during hot summer days.
The radiator and its system work well and after about 100 hours flying the Thor 250 solo, in tandem and in competition, we have not suffered any breakdown.
Polini offers a number of accessories to install the carburetor sideways, getting a different configuration for the Thor 250 that makes it easy to install in different types of trikes or ULM.
This engine is ideal for lightweight tandem paratrikes of +/-70kg empty.
Best with manual & electric starter
For leisure pilots who fly three or four times a week, the manual start will be enough. For more sportive pilots who make heavy use of the starter several times a week, with sessions of 10 or more takeoffs, we recommend having both starters and use them alternately to lengthen their life.
Also, if one day one of the two fails you can use the other.
The Airfer Thor 250 paramotor we tested has a 2-cell Paracell battery, very small and light, which goes in a side pocket. This allows for some 10-15 consecutive starts and it is charged when the engine is running.
|Technical data Polini THOR 250
Engine: 2 stroke monocylinder
Cooling: Liquid cooled
Bore for stroke: 72 x 60
Displacement: 244 cm3
Power: 36 HP at 7500 R.P.M.
Cylinder: Aluminum with Gilnisil coating
Compression ratio: 11,5:1
Piston: Two chromium plated rings 1mm
Intake: Reed valve in the crankcase
Air filter: Air box
Ignition: Electronic and with battery charger possible
Battery charger prearrangement
Output power 80 W at 5500 RPM
Spark plug hood 5k Ω resistance
Fuel type: Lead free petrol with 2% synthetic oil
Gear reduction unit: Helical teeth in oil bath with 2,8 reduction ratio
Starting: (Electric starter optional)Pull start with self winding cable FLASH STARTER
Clutch: Centrifugal in oil bath
Muffler Expansion with oval silencer
Engine weight 18 Kg (19kg with electric starter) without radiator
Propeller rotation: Clockwise.
The 36 HP declared by the manufacturer can give around 95-100 Kg of thrust, using a 130 cm 2-blade wooden propeller from Aerobat. With this prop the engine offers a linear power curve, a good "punch", and fine performance to cruise. Besides, the paramotor is relatively small.
It is possible to get up to 110 Kg of thrust from it using propellers of 140 cm or more, an interesting option for fitting in a tandem paratrike.
Disregarding the radiator, this 250 has practically the same size as the Thor 200 and other smaller engines. However, the increase of power is so notorious that makes it worth carrying that extra weight, especially if you like the sportier and more extreme side of paramotoring.
The original Polini throttle slides smoothly and fits well in the hand. The touch is pleasant, and it has two buttons on the base which you must press simultaneously for the electric starter.
Before flying, the Thor 250 has to be heated up to its optimal temperature of between 60 and 80°C; this takes about 10 minutes, more or less the time you will need to prepare the glider and get ready to fly.
The hardest part is to stand up from the ground with the paramotor; undoubtedly, it feels heavy even with little fuel in. In our first take-off we noticed the high torque and that it is best not to squeeze the throttle until the glider supports our weight and the anti-torque system is working. If you start the takeoff run and the glider has no tension, the torque will turn you to the left.
The Thor 250 is the engine chosen by most paramotor manufacturers for their Slalom pilots. In the picture, Pierre and Nico Aubert, of PAP, during a training session.
The most powerful engine at the time is ideal for sportive and ambitious pilots who want to have that extra power to go fast, turn at full speed, or simply to make long navigations at a touch of gas.
All this power requires expertise, especially at the time of take-off, so it is not recommended for newbies or low-airtime pilots. There is also the issue of weight: if you suffer from some kind of injury, have trouble carrying weight, or are simply not in shape, you better look for an alternative or consider launching on wheels.
The Thor 250 was born to be fitted on paratrikes: All its beauty and power without the handicap of the extra weight is perfect!
Tested by Daniel Crespo
Daniel is an experienced Paraglider & Paramotor pilot and competitor; PPG World Champion 2007 (Paratrike) - Bronze Medal PPG Worlds 2012 (Paratrike).