You would start looking for one when your engine is making useful power beyond 5800rpm with a 231ci engine. So pretty much high 9's with a typical small hyd roller. Even going to a larger blade which is common and easily done up to 62mm will give a decent increase in the airflow that it will support. You can use various calculators online to determine the engines airflow and figure out the cross sectional area of the tb inlet so you can calculate the flow potential. The major contributors to increasing airflow through an engine are to increase displacement or increase the operating rpm. Other things have very little effect on airflow. Mass flow and cfm aren't the same.
