What is a non-interference engine?

June 11, 2011. Author:

What does it mean if an engine is 'non-interference'?  Engines are often differentiated by whether they are non-interference or interference designs.

In non-Interference engines, the pistons and valves cannot come into contact each other.

In a non-interference engine, there is no possibility of the pistons and valves coming into contact with each other.  For example, 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 interference engine, where there can be piston and valve contact).

Also in non-interference engines, the intake valves and exhaust valves come into contact.

On certain engine designs such as DOHC (dual overhead cam), it is possible for the intake and exhaust valves to end up all being fully open at the same time if the timing belt or chain breaks or timing jumps.  In a non-interference engine, there is still sufficient clearance between the valves so that they will not contact each other.

Why does it matter if an engine is non-interference?

The primary benefit of an engine being non-interference is that if the timing belt or chain breaks, there is no possibility of engine damage from piston to valve or valve to valve contact.  (As opposed to the interference engine design, where a timing belt or chain break could allow this type of engine damage to occur.)

Which vehicles typically have non-inteference engines?

Generally earlier model vehicles up through the mid 1990's had the greatest chance of having a non-interference engine.  However many manufacturers switched to interference designs for efficiency and other reasons.

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