318 rebuild/upgrade

The Performer is Not suggested for use on a 273,

I think the Performer will not flow well at low cfm,
speeds,whereas the LD4B is a Chrysler approved,and sold part,
that was made for the 273,and 318.

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Why would you think a performer would flow poorly on a 273 and or be out done by a older discontiued intake. Is it a C.I.D. thing?
I seldom if really ever see a company, like Edelbrock, go backwards in intake manifold design.