Honda GX25 and GX35 are 4-stroke, overhead cam single cylinder engines designed for garden equipment, agricultural equipment, water pumps… however they can also be used as powerplants for fixed wing and VTOLs drones and it have benefits over commonly used RC engines.
- They burn pure gasoline, without oil.
- Better fuel consumption.
- Reliable.
The fundamental difference from other four stroke engines for UAVs is that the Honda GX25 and GX35 engines have oil in the crankcase. The vast majority if not all 4T RC engines that convert to UAVs do not have oil in the crankcase. They use a much more basic blowby lubrication system. Blowby lubrication consists of lubricating the engine with the oil that seeps between the cylinder and the piston. Blowby lubrication engines must carry oil in the mixture just as if it were a two-stroke engine.
In an engine with oil in the crankcase it has many advantages and some disadvantages. On the one hand, gasoline does not need to carry any oil, this has important advantages: there is no need to mix oil in the gasoline and the risk of forgetting or making mistakes when making the gasoline-oil mixture is eliminated. On the other hand, you don’t have to carry the weight of the oil mixed with the gasoline. For example Saito recommends a 20: 1 ratio for its 4-stroke engines. For a UAV that carries 5 liters of fuel, this is 250ml of oil, which is no small feat.
As a disadvantage of Honda GX35 engine, it could be said that it weighs a bit more due to the fact that it has oil. However, we must also think that there is no need to add oil to the gasoline and part of this weight is compensated.
The Honda GX35 engine is not intended to be used as a powerplant for unmanned aerial vehicles, and the power is less than equivalent RC engines. Some RC fans have worked on increasing power with great success. For example, the user “oipaloi” has published a dyno chart of the original Honda engine and a modified one at https://honda-tech.com/
Engine modifications consist of, among other things, mounting the piston of an older engine called the GX31, the GX31 piston has a greater distance between the pin and the piston head and therefore generates greater compression, installing this piston requires some knowledge on engines because you have to check the “squish band” and “valve to piston clearance” and adjust it by removing material from the GX31 piston.
The GX35 has a Walbro K10-WYB carburettor with a venturi diameter is only 9.53mm, I am aware of some modification in which an 11.1mm carburettor has been successfully installed. The suitable body for EFI would be 12mm.
Conclusion
The GX35 engine is a great engine for a UAV, it is a quality engine, and a very proven one. Although it does not have much power, it can be suitable for many applications.