The limiting factor are the head gaskets for sure! I remember a thread that talked about this topic, but I can't quite remember what was said. What is the best way to go as far as head gaskets that are actually manufactured or are available?
Hypermax has "C" firerings and a matching headgasket for these IDI's. Those "C" rings have been used and are being used in pulling tractors making way more than 150psi of boost pressure. Eric has them on his IDI build, but like stated above, still waiting to hear how they hold up with the S400.
Heath