H&S are excellent. I have had three race exhausts made by them and their work has been complimented by my engine builder. I have given them quite stringent specs which they were able to meet despite being well beyond what most would use. E.g. Very low included angle for collectors and long primary pipes.

That said - I believe Sifab may have a design for Aero S1/2 exhaust based on his fluid dynamics analysis for a GT Aero from years ago.

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