Bob's does have different cones, but I am pretty sure they are primarily based on the type of lower unit you have (you can check their website). I am no mechanic, so this is my layman's understanding of the dynamics, one of the more mechanical guys on the site can probably explain this better, but here is my understanding and what I have noticed: Porting the exhaust - like shortening the pipe on a car's exhaust - reduces the distance for the air to flow and allows additional gas to escape more easily which can improve engine efficiency and power. Your exhaust tub competes with both air and water, allowing the gas to escape sooner and more easily reducing some of the competition, as well. Additionally, you will blow less carbon monoxide out of the hub, thus less around the prop blades if you don't have a through exhaust hub-ed prop, like a chopper. Though these props are designed to have a little air mixed with the water to free them up some, to much causes them not to bite as much on the hole shot. I found that by reducing the gas around the prop I have improved my hole shot w/these types of props - giving the prop a little more bite coming out of the water. The biggest thing you will notice though is the sound. My E-rude was a quite little motor, but she sounds like a big dog now. By adding the bigger holes I found that I picked up 100 rpms on my last run (yesterday) w/my 29p ET style 4 blade. I have also noticed a difference on my 3 blade 29p SRX, as well (my last Concordia trip w/arkiepowercrew). In the running vid. above I had smaller holes (4 half inch on each side - eight ports - a little smaller than OMC recomended) {it was the second lower unit}. I went back to their recommendations of 5/8th. The sound difference was amazing and horse power improved, by 100 rpms, as noted above.