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The spark plug is relocated to provide for better flame travel upon ignition of the charge. If the plug is in such a location that it is shrouded by valves,combustion chamber shape or piston crown shape, you get less than complete combustion (poor flame front travel). This is a serious matter in racing engines where you are trying to produce maximum HP. On a street prodified engine, you probably wouldn't notice the difference.
Larger valves are to overcome poor cylinder filling and/or increase volumetic effeciency. If a stock engine operates at 60-70% volumetric effeciency, addition of larger valves may increase it a few percentage points. A full on racing engine will approach, maybe exceed 95% volumetric effeciency BUT this includes modifications to the intake system and exhaust system. To get the greatest benefit out of any change, requires the entire system to be considered.
You also have to consider the shape of the back side of the valve head, stem diameter, valve guide shape and how they are placed in the combustion chamber(shrouding by cylinder walls, combustion chamber walls, etc)- Got a Flow Bench? You can make all the changes you want, but unless you have the equipment to measure increase you won't know if the change did any good. Generally, you butt isn't a good testing device (very subjective).
Take the heads, with the intake manifolds, to a competent head porter and let him do the "magic". It'll cost bucks, but you'll get the greatest benefit (and keep the American Economy green). The greatest horsepower gains in any engine begin at the air cleaner and end at the end of the exhaust pipe. All must be integrated to gain the most.
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