Okay, here’s one example.
It’s a nicely ported 906 head with a 2.14 valve, tested @.700 lift.
All 4 ports tested, then averaged. Then the intake was bolted to the head, and both runners of the manifold that corresponded to that position of the head were tested.
1&8, 3&6, 5&4, 7&2.
A 1000cfm Holley carb main body and baseplate were mounted to the manifold.
All eight results for each manifold runner were averaged, and that number can be compared to the average flow for the head with only the radius plate.
The 8 cylinder avg for the head alone for this test was 286.7cfm.
The 8 cylinder average for a stock 383 intake #301, with the 1000cfm carb attached was 212.8cfm(-73.9cfm)
The 8 cylinder avg for an ootb low deck RPM, with the carb attached was 250.6cfm(-36.1cfm)
The 8 cylinder avg for a mildly gasket matched RB M1 single plane with carb attached was 263.1cfm(-23.6cfm)
The manifold installed nearly always causes a reduction of flow through the head(unless it helps fix a turbulence/separation issue).
The very M1 manifold that knocked this 906 head from 286 down to 263......... flowed well over 300cfm........ when attached to a ported std port Indy SR head......... that flowed 340 with a radius plate.
So, it will pass 300cfm....... yet still shows a flow reduction when attached to a 286cfm head.