in the how to book of the ls1/ls6 engines, they said they put a set of ls6 heads on a 5.3 and lost power, due to the compression. the 5.3 heads are 61 cc and the ls6 are 64cc. the cam made power, but lost a little on the bottom end. if you dont tow,that cam is a easy bolt on. everybody says that its not our best cam because our trucks are heavier.

 

I belive this is true, but no one to my knowledge has posted L33 head casting numbers to confirm it. But L33s have flat top pistons to keep compression up.