What is the difference between an interference engine and a free-running engine?

Answer 1

In an interference engine, the valves drop down past where the piston was at top dead center. This means, if the timing chain/belt breaks, the pistons could impact with the valves doing very expensive damage to the engine.

In a free-running engine, even with the timing off, the valves cannot come in contact with the piston.

Answer 2

You would know once the timing belt or chain breaks. On an interference engine, if a valve were down and the piston up, you would have a bent valve. On a free-running, there may be indents in the top of the piston to allow for a valve to be down and not be bent when the piston comes up.

Answer 3

if the timing belt breaks or skips a free-running engine will just need a new timing belt, an interference engine will probally have bent valves and need major repairs. if your engine is an interference engine it is best to replace the timing belt enery 50,000 miles or so just to be safe.

Answer 4

if you break a belt on one, you find out. Most of them run timing chains so that doesn't happen. It cost lots more to repair a interference engine. They bend valves and break pistons, eg Fiat and Mitsubishi.