I understand that there are some limitations to hemispherical combustion chamber compared to that of a canted valve wedge head while in a naturally aspirated application.
My research indicates that a tight quench and modern combustion chamber design produces a highly turbulent air/fuel mixture motion as the piston approaches TDC on the compression stroke. This turbulence increases homogenization of the air and fuel, increasing burn rate, producing a more complete burn, reducing timing advance requirements, and increasing resistance to detonation.
In a power adder application, that may not necessarily be true. Softening of the combustion chamber on BBC heads, making them more Hemi-like, apparently broadens the tuning window.
Hemi heads have some distinct advantages in the airflow department. For example, because the valves open directly towards the center of the combustion chamber, the valves move away from the cylinder walls as lift increases. Hemi valves are never shrouded. Line-of-sight port/valve layout, when you look down the ports of a Hemi, you can see right down into the combustion chamber. However, modern technology in canted valve wedge heads has surpassed the former advantage of the Hemi head.
Because of the shape of the Hemi ports, airflow tumbles into the combustion chamber like a waterfall. Swirl was a big buzzword back in the 80s and 90s. Now, head porters try to achieve quality flow. Some swirl is beneficial while the valve is open, and can continue in the combustion chamber after the valve closes. Tumble motion stops immediately after the valve closes.
I have always had a thing for Hemis. Even though there are limitations to Hemi architecture, at the level that I would be building my own engines at, I would still love to have one in my boat. I may opt for a set of Arias heads.
