Not Ranked
The Victor Jr's have always been towards the middle of the pack for a good performance out of the box head. The ones that are towards the top (without going to a Cleveland style head) are the AFR heads, the TW-R, TFS High Port, etc. However, as you stated, the Twisted heads require a piston change.
You posed a situation earlier where you would use the Vic Jr's for the 306 and you would buy a higher end head for the AFR....that's where this idea stemmed from. The AFR 185/205 head would outperform the Vic Jr heads without really breaking the bank.
I would imagine that it is the shape of the port on the AFR. As you said, it's a smaller volume that flows exceptionally high. Low lift and mid lift numbers are high as well. I can't really say exactly what it is, I've never seen a port cross-section between the two.
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