What are the advantages and disadvantages between open chamber and closed chamber big block heads? I want to build a reliable 500hp 454cid motor. I'm getting ready to send out my heads to be rebuilt. What modifications should I do to the heads (ie valve size, porting, spring tension, etc.) The head casting number is 353049.I believe it is a semi-open chamber heads with 113cc, 2.06 in, and 1.72 ex. I'm open to ideas!!