Here's an excerpt from my Hemi book:

It’s interesting to note that a Hemi provides a unique set of problems because the piston to head clearance is not a flat surface so quench is hard to control. Manufacturers do their best to conform the shape of the dome to the chamber thereby creating some quench. I like calling this “squench” because it’s different from a wedge but the process is the same. Because of this when I use clay to measure the piston to valve clearance I also measure the “squench” distance too.

If the desired compression ratio is high, the stroke is low, and the valve lift is high, the resulting piston can end up with deep valve pockets and a peaked top. This limits flame travel. I try to specify that I want the top of the dome clipped slightly and I’ll live with a little less compression.

Notice too that as the bore size increases the piston is left with a flat perimeter around where the head’s chamber dome would end. This becomes a secondary quench area where you must make sure you have adequate piston to head clearance.


So the bottom line is - Hemis are tough to get a high CR while controlling all the other issues.