If you look at some of the pictures you will see that the openings are smaller than the gaskets. By porting them out and making them the same size as the gaskets you get more air into the heads. I took a lot of material out of the port on the lower intake where it attaches to the heads. Probably about this much II the space in between the I's. I also smoothed out any casting flash in the ports to make air flow smoother. Porting is probably not something you want to take your engine apart to do, but if your doing a rebuild it would be silly not to. Granted this is not a race port or anything close but it definately cleaned things up and opened up things up with no rough lips or edges.