That is not true, A ported 530 will outflow a standard 531
Erland Cox takes the intake on a 530 to 200CFM, stock 531 measures 175CFM but he can get a 531 to 225CFM but with further work more is possble.
For a turbo engine the biggest issue is the exhaust side and that is the same same with both the 530 and 531 with almost identical flow.
The site you linked to is nice to read but a lot of the info is out dated and coming from the US. You need to look to Sweden for good information
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