The first series head had the spark plug by the exhaust, going to the center of the chamber. This meant the exhaust port was offset. It caused a few problems, couldn't have an entirely machined chamber, caused a water jacket bump around the exhaust port that caused poor circulation that resulted in high temps, longer spark plug wires and harder compression ratio control due to machining problems. This was corrected with the series 2 which is the production head. This comes from the SAE paper. The second series head did make more power but there are a few factors as to why. The porting was improved, the cr was improved, cooling was improved and the lift was increased .05". All this will have a decent increase, around 20% from the tech writeup.