[blykins]

Pretty much any trick flow head, inline or twisted wedge, some AFR, there are others. All your modular ford heads are way down there, your ls heads, etc.

Chamber design is a lot of it, compression ratio is a lot of it. My last sbf with high port TFS heads was at 28, last sbf with track heat TFS was at 30, some of my fe stuff likes 32-34.