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As a general rule, most 6 port center irons have less available area for porting than the 4 port center irons. If you look at the 6 port center iron, and compare the stock ports to a 4 port iron, you would see the 4 port has larger ports to start with. This tends to support the idea that a 4p center can be ported more, since there was less casting needed around the ports on a 6p center.
This is not a "Hard" rule, but is the general findings of many engine builders. Even RB makes mention of this on their website, in the porting area.
It is entirely possible that *some* 6p center irons can be ported to the same extent as a 4p center, but most cannot. The 6p center didn't need the ports to be as large, since the secondaries were so much larger total area compared to stock 4p secondaries. This also allowed for an improved idle quality on the 6p engines, in spite of the more aggressive overall port design, and resulting higher HP.
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