An interference engine is a type of 4-stroke internal combustion piston engine in which one or more valves in the fully open position extends into any area that the piston may travel into.

In a non-interference engine the piston does not travel into any area into which the valves open. Interference engines rely on timing gears, chains, or belts to prevent the piston from striking the valves by ensuring that the valves are closed when the piston is near top dead center. Interference engines are prevalent among modern production automobiles and many other 4-stroke engine applications; the main advantage is that it allows engine designers to maximize the engine's compression ratio. However, such engines risk major internal damage if the piston strikes the valve(s) due to failure or poor maintenance of the timing components and/or camshaft.

The basics of mechanical engine timing

Most cars today have 4 stroke gas engines. This is because the combustion process has an intake stroke, a[i] compression stroke[/i], a power stroke, and an[i] exhaust stroke[/i].
Throughout the 4 stroke cycle, the camshaft spins once and the crankshaft rotates twice. The relationship between the rotation of the camshaft and crankshaft is what’s known as “mechanical timing.” It’s what controls the movement of the pistons and valves inside your engine cylinders. The valves have to open at a precise time in conjunction with the pistons, and if they don’t, the engine won’t work properly, if at all.

Timing belts
The timing belt runs through a series of pulleys that have belt tensioners in place. As you might gather from the name, the function of a belt tensioner is to keep the proper tension on the belt at all times. They usually wear out at the same time as the belt, and are replaced along with a belt replacement. Most manufacturers and mechanics also recommend that you replace the water pump at this time as well. That’s because the water pump is also usually the same age, and will generally wear out at about the same time.
Now that you know the function of the timing belt, we probably don’t have to tell you that there is never any good outcome if you end up with a broken timing belt.

Timing chains
The timing chain looks exactly like a bicycle chain, and as you might expect, it’s noisier than a belt. The other problem with timing chains is that if they do break, they’ll usually cause a lot more damage than a broken belt. A broken chain will most likely result in damage that will result in a complete engine transplant being less expensive than the repairs you’ll need.

Pros of belts
1. Belts are quieter
2. belts tend to stay tight without damaged belt teeth
3. Timing belts are also less complicated
4. Lubrication not required

cons of belt
1. No protection against overloading
2. They are not economical (manufacturers make money from belt replacements interval)

Pros of chains
1. it can last the life of the engine
2. rarely requiring maintenance
3. reduce cost of change interval to the owner

cons of chains
1. chains stretch and loosen
2. cause a lot more damage than a broken belt

You can't retrofit a belt system to use a chain, you wouldn't want to do the reverse

sauce
https://en.wikipedia.org/wiki/Interference_engine
https://www.yourmechanic.com/article/what-is-the-difference-between-a-timing-belt-and-a-timing-chain
http://mechanics.stackexchange.com/questions/2711/timing-chain-vs-timing-belt

Timing belt all the way