[lecture]
The difference between interference and non-interference is often the performance and efficiency of the engine. There is a direct correlation between how much the intake valves open and how much air/fuel mixture can be pulled into the cylinder, as well as how much burned mixture can be pushed out through the exhaust valves. More intake charge, and less leftover burned mixture contribute greatly to how well the engine runs.
If the belt (or chain) is replaced at an appropriate interval, the risk of engine damage on an interference design is negligible. Conversely, you're probably paying for your non-interference engine with reduced operating efficiency.
It's not like someone woke up one morning and decided to roll the dice to design the piston-to-valve clearance, there's a lot of engineering that goes into it depending on what the goals are for the engine design.
[/lecture]
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