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Message: <blockquote data-quote="wytstang" data-source="post: 868470" data-attributes="member: 7973">Ford Racing and Mac both sell P specific headers (I've read Mac l/t also clear the spark plug), the spark plug angle on the P heads get in the way of the typical header tubes. Also the P's would still make more power then gt40's due to the revised spark plug location (improved combustion). Also the gt40p's have 58cc's whilst gt40's have any were from 62.5 to 65.5cc (bit more compression power). If I remember right the exhaust valve on gt40p's are smaller yet more efficient then gt40's.With these 3 advantages there are disadvantages to them, having to do with emissions. P's used an external egr (we use internal), missing the smog ports on the rear of the heads (not 100% they are all like that). Keep that in mind.</blockquote>