Why pistons and valves get up-close and personal

Interference engines allow the piston to go further into the combustion chamber to increase compression, and the valves to go deeper so they open wider and for longer. 

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What is the difference between “Interference” and “Non-Interference” engines and what are the pros and cons of the choice? Marcus

Both pistons and valves move in and out of a space at the top of engine cylinders known as the “combustion chamber”.

In Non-Interference engines, the highest point reached by the piston crown is below the lowest point reached by the valve flange - there is a gap which ensures they can never touch each other (even if they get out of synch) but it limits the compression ratio the piston can achieve and limits how widely (and when) the valves can open and close.

Interference engines allow the piston to go further into the combustion chamber to increase compression, and the valves to go deeper so they open wider and for longer. 

In sum, Interference engines “breathe better”. This significantly improves performance and, potentially, economy.  But it means the piston crown and valves do occupy the same space – at fractionally different times.

One gets out of the way a micro-second before the other arrives.  The precision of timing is more crucial.  The choice is a question of design priorities.

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Note: The results are not exact but very close to the actual.