- How much it costs.
Obviously important in any purchasing decision.
- Fuel requirements.
Some engines take a different fuel mixture than others. For example, some manufacturers recommend a higher nitromethane requirement than others. Nitromethane isn't cheap, so you might want to take that into account.
- Engine reputation.
Some engines are absolute dogs to get running correctly. Make sure you ask around and get some experienced opinion before purchasing.
- Local knowledge.
If you're getting into your first engine, you'll probably need all the help you can get to get optimum performance from your power plant. Try and get one of the more popular engines at your field.
- Two Stroke

Two strokes are by far the most popular helicopter engine. Most aren't too hard to set up and produce good, reliable power.
- Four Stroke
Four strokes aren't very popular just yet. They often require a lot of tuning and can be somewhat underpowered. However, having said that, there's one here in New Zealand that I know of that can keep up with the hot 60's. It sounds like a tractor but it can really haul.
- Electric
Electric powered helicopters are slowly becoming more popular as battery technology advances the capacity of batteries allowing for extended flight times. Electrics are popular for indoor flying and are often installed in small helicopters. However, there are larger electric helicopters which can easily do 3D aerobatics.
- Petrol
Petrol engines are based upon chainsaw or weedeater engines. They take a mixture of petrol and two stroke oil as used by chainsaws. Petrol's have tonnes of torque at low speeds which means they can often hover at around 30% power (depending on your setup). They're very economical to run both because the cost of fuel is low and their fuel consumption is also very low. For example, most petrol helicopters will fly for about 30-40 minutes on a tank of gas.

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