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The stock 964 and stock 993 engines are different in that the 964 has a traditional barn-door restricted AFS versus the 993's hot wire AFS. Bruce Andersen said there were noted on his friend's dyno measured improvements of up to 8HP from freeing up the design of the air filter housing (by deleting the trunk and removing the top (or drilling holes) in a 993's airfilter housing but not in the 964's air filter housing (since the AFS is the main restriction governing the air flow). So the answer is don't bother with the drilling of the 964's but it's a cheap HP gain for the 993's.
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