Not Ranked
Steve, I got my information from a chevy high performance book, that had a 3-5 page article, actually interview with the engineer that designed the mark IV BB, and he talked about the designed aspects, the reasons why the Mk II and MK III BB engines didn't make it into production, and then finally the MK IV, and reasons why it was designed the way it was. If I can find the article, I'll either tell you the source, or maybe send you a copy of the article.
Just from visual comparison, I can tell the volume difference caused by the difference in stack height is much greater than the volume difference between the small and larger ports in the head. In addition to the volume difference between the ports, I believe there was also, and I think more importantly a flow difference that affected VE and maybe fuel distribution, but Maybe I'll look that up, not that it makes a big deal to me.
In other words, I think the reason was for the fatter torque curve as others have stated, and not so much for the slight difference in port volume.
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"After jumping into an early lead, Miles pitted for no reason. He let the entire field go by before re-entering the race. The crowd was jumping up and down as he stunned the Chevrolet drivers by easily passing the entire field to finish second behind MacDonald's other team Cobra. The Corvette people were completely demoralized."
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