PCM also remaps fuel curves and timing tables. It's not a direct effect on KR but with the PCM your other mods work to their potential. Why put more air in and more air out if you can't get the extra fuel and timing to make a bigger bang to drive you that much faster? Remember there is a magic balance of air/fuel ratio if you put more air in to make more power you need more fuel. If your bringing in more air then the PCM can supply fuel for then you have a lean condition which leads to knock. This is more noticeable on carborated cars because there is no computer to control a good air/fuel mixture. That's why you had to tune a carborator.