I've been reading up on LS1 heads and found some information that might be valuable to some of the members here. GM uses three different casting facilities for their heads. One uses sand casting and the others use SPM (Semi-permanent mold) during manufacturing. The SPM process is very smooth with consistant ports and chambers which actually make 4 to 7 HP more than the sandcasting procedure. To identify the SPM heads, there are tooling marks under the spark plug hole in the shape of a triangle known as "chills". Hope this helps someone out.
Vortec383
