RE: Top-end questions
most people change the valves when they have a turbo or a supercharger. larger valves allow engine to cool faster and allow hotter air to leave the combustion bay faster. also people upgrade valves not in size but in material. some use titanium valves, because the last longer and are more durable and more resistant to heat and pressure. adjustivle cam allows to advance or retard the timing, meaning allowing the intake valves to open earlier or later or allowing exhaust valves to open earlier or later. it can also be used to adjust the valve overlap (meaning the time period when the both exhaust and intake valves are open at the same time). what this does is produces more torque, but decreases your gas milage and also could make your car illegal (by not passing smog).
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