The head (on the right) is a KR head(No. 7 late style). The spark plug is located over the intake valve. You can see where material has been removed at an angle, making the space above the valve larger(more area for air) and it improves the flow and power. Originally, builders kept the combustion area "tighter" and smaller to increase compression, thinking it would add power. Later testing proved where "improved air flow" gained power over higher compression ratios. Matching the aluminum head to the cylinder eliminated "lips and steps" . . . interrupting air flow. Jerry Branch(there's that name again) did extensive testing, as did Tom Sifton. I read a story about the H-D Racing department paying Sifton $1000 per horsepowergain he could squeeze from the Harley KR engines. You can see the deeper pockets allowing more flow over the top of the valve heads. The idea is to get the most fuel and air in there - get it to mix and burn rapidly - then get the exhaust out - and be ready for the next batch of fuel and air - and hopefully the exhaust with help you along the way creating a favorable environment to make it all happen quickly and efficiently. One wrong part of the equation (carb size, jetting, air cleaner, exhaust pipe diameter and length, valve size, intake/exhaust port size, compression ratio, fuel quality, air quality, combustion chamber shape/size, spark timing, valve timing, valve seat shape, valve angle, valve timing (you get the idea) can contribute to the difference in poor performance or improved performance . . . or " that sucker really runs !"