I never have had a chance to play with any Square port heads in the past as they were either too expensive or I just never had an engine to use them on. I do have some 990's but I do not want to mess with them as they are virgin heads and my be worth something to someone some day. That being said I picked up 2 sets of 934 Gen 5-6 Big Port heads to see what they will go on the bench and may use them in a future project. What prompted me to get them is you can get head gaskets to use the heads on mark IV blocks. I was reading on Super Chevy that when they were using these heads on an old 454 HO engine that they tested 3 sets of heads. They used the factory ones, Merlin Sq. Port and a set of Merlin oval ports. Both sets of Sq. Ports were within 20hp of each other but when they bolted on the Merlin ovals you would think the low compression engine would shine, right?, well it lost "62"hp compared to both sets of the big port heads. Never saw that coming and Merlins are supposed to be better than 781's out of the box.