Cross section is probably the most important factor.
The AFR 185 and the Twisted Wedge 170 have almost the same cross section. Very very close. But the AFR have a 5" long port and the TW has a 4.750" port. This alone changes the port volume. And many would say that the TW head is smaller. But in reality, its not.
No that's not what I'm saying. What I'm getting at is port volume alone is mis-leading. The TW has a smaller port volume than the AFR head, but these heads will perform similar. I wouldn't put to much thought in port volume alone.
Now if these two heads had the same port length at let's say 5", and the TW head was still rated at 170cc's. Then that would mean the cross section of the TW head would be smaller. And a smaller cross section would limit the potential of the heads.
A good example is the AFR 165 and 185 heads. Same port length, but different port volumes. And the 185 head out performs the 165.
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