needing a certain amount of back pressure is a myth. the best headers on the market are designed to scavenge as much back pressure away from the exhaust ports as possible. think about it: if there is a significant amount of back pressure trying to force itself back up the header and into the stream of exhaust gasses being pushed out of the combustion chamber, the pistons pushing the spent exhaust gasses will have to push harder therefor reducing power overall.
look at all motor dragsters, they always use open headers and large dia exhaust systems. just my $.02
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