There are hat companies that make ones that flow well. I think the biggest issue is the filters. The cone type filters have some pitfalls in design.
Not sure what's considered 'significant' horsepower, but have heard some say in personal testing that 20-30hp ranges have been seen.
Personally, I would think it would be best to value in percentage, but believe after X, it can drop off faster. So larger displacement engines would see a bigger drop compared to smaller displacement engines.
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1970 GTO Judge Tribute Pro-Tour Project 535 IA2
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