Complex sentence: The 1957 heads are the most popular for performance due to their large valves and the unique stacked intake runner design flows very well and ported ECG-Z castings have flowed up to 235 cfm on the upper port.
Split into simpler sentences: The 1957 heads are the most popular for performance due to their large valves and their unique stacked intake runner design, which flows very well. Ported ECG-Z castings have flowed up to 235 cfm on the upper port.