Porting without a flow bench can cause some major issues. To figure out the airflow per cylinder, the ecu divides the total airflow by 4, to calculate the injector duty cycle for each injector. The problem is when the ports don't all flow the same. If one cylinder flows 10% more air than the other 3, it just run's 10% leaner. Now with an aem, and 4 wideband sensors, this can be tuned around.