I don’t really like porting intake manifolds, so I don’t do much of it.

One thing I found interesting when testing manifolds is that they “usually” result in some amount of loss in flow from the head, when attached....... even when they are capable of flowing way more than the head they’re attached to.

Years ago I was messing with some std port Indy SR heads. I got them to flow in the 340 range, and was testing some manifolds on them.
An M1 single plane with very minimal plenum deburring and a gasket match would flow a little over 300cfm when attached to the head.
Or rather, the head would flow a little over 300 with the manifold attached.

So, we know the intake is good for over 300....... and yet when you bolt it to an unported 906 head that only flows in the 230’s...... the head still loses flow.

On those 286cfm 906’s in my previous post, that same M1 intake that was passing 300+ cfm when on the SR’s, knocked the 8cyl avg down to 263cfm.