First off as far as I know the Z1 headers will not fit on your current berk HFC. I believe the flange configuration allows for the DE cats/ test pipes to bolt up. When you open up restriction there are improvements ie. headers exhaust etc. Have you looked at the Z1 dynos with their headers and components? This will give you a good idea to ballpark gains. Yes, headers will make your car louder as you are reducing restriction along with your berk HFC. From what I have read BERK cats have some rasp to them so I am sure this will be amplified. As far as weighing between the Z1 manifold and motordyne I say research, research, and research. The Z1 will be less cost wise and they have comparative dynos on their website with motordyne. Either way both companies have been around for sometime and supply quality parts. I personally have Z1 manifold and love it. I am not a VQ guru just observations I have made from reading the forums for quite some time.
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