What is a SUBARU non-interference engine?

What does it mean if a SUBARU engine is 'non-interference'?  Engines are designated as interference or non-interference.

In a non-interference SUBARU engine, there is no possibility of the pistons and valves coming into contact with each other.  In other words, there is enough clearance between the piston at TDC (top dead center) and the fully open exhaust and/or intake valves so that they do not touch (interfere). 

So on a non-interference SUBARU engine, if the timing chain or timing belt slips or breaks or jumps timing for some reason, the piston rising in the cylinder will not hit any valves, even if the valves are completely open.  (See also Subaru interference engine, where there can be piston and valve contact).

Also the intake valves and exhaust valves cannot contact each other in a non-interference SUBARU engine.

On certain SUBARU engine designs (namely certain DOHC (dual overhead cam) designs), it is possible for the intake and exhaust valves to interfere.  If for example the timing belt breaks or is installed incorrectly and the intake and exhaust valves in a given cylinder happen to both open at the same time, they may collide and hit each other.  Whereas in a non-interference engine, enough clearance still exists between the intake and exhaust valves that they will not contact each other.

Why does it matter if a SUBARU engine is non-interference?

With a non-interference SUBARU engine, if the timing belt or chain breaks, there is no possibility of engine damage from piston to valve or valve to valve contact.  (Whereas with the Subaru interference engine design, a timing belt or chain break could result in engine damage.)

Which SUBARU vehicles have non-inteference engines?

Generally earlier model SUBARU vehicles up through the mid 1990's had non-interference engines.  Most SUBARU engines manufactured in 1997 and later were interference designs.  SUBARU and many other manufacturers changed to interference designs for a various reasons, efficiency being one of them.