Originally Posted by Ford F834

I try not to go overboard with making decisions based on anecdotal evidence. There are just too many variables involved with old, rebuilt engines. Many people out there are running 90cc pumps with various turbochargers and have not had HG issues. And failures can arise from non-flat heads or deck, contamination at installation, defective gasket etc., the main reason I would pay attention to this particular anecdote is that Justin knows the guy it happened to, and we have verification that this was a fresh rebuild, (done correctly), and the alalysis of the failure is accurate (that the HG failed, no underlying root cause). This example along with orhers pretty much forces the conclusion that with 90cc's of fuel and mid 20's psi of efficient boost, we are at the fringe edge of what the 7/16 ARP studs will hold. Decompression does help, but one point to 20.5 is likely not enough. RacinNdrummin was running with .060 shaved pistons, .010 decked off the block, net .050" decompression which is around 18.5:1. His gaskets failed spectacularly but he was running 190cc's and an adjustment to his fuel inlet pressure shot his timing way advanced, and is likely the root of his failure in spite of the decompression. However, that too demonstrates how thin the margin of error you have with those head fasteners. NMB2 ran his pistons shaved .80" which is around 16.5:1, and his saw a lot of abuse in the 40+ psi boost range. So there are the examples that we know of. If you are just wanting to run the 90cc and S256, I would shave .040" to .060" and if possible intercool it. But this is just an educated guess. Shaving pistons more than about .010" is its own controversy. Thinning the crown and adding heat to the top rings are the concerns, so ceramic coating is probably wise. Maybe this sheds more light on Justin's statement about using a 7.3 for any serious performance build up.

Originally Posted by tbehrens1

Good info. Thanks. Blew hg with just 90cc with studs?? What c/r and turbo was he running?

Originally Posted by hairyboxnoogle

Stock compression 6.9, BW s256 22-23psi i believe. Think they said it had almost perfect drive pressure ratio.

F834 said it perfectly. Other than the early 6.9 they have 22:1 CR. If i bothered to decompress it i would go to 18/19:1 ballpark. 7.3 is 21.5:1, not like half a point makes much of a difference, just throwing it out there.

Originally Posted by hairyboxnoogle

Dont sweat it, thats what the board is for. Early 6.9 is 20:1 or 20.5:1 or something like that iirc. Difference is in the pre-cups. The early blocks arent as beefy in the front and by the block heater, but other than that are fine. Its not so much the headgaskets, but the 7/16 fasteners, just not tough enough to get wild with. 7/16 headbolts/studs, 20+ to 1 compression, and lots of boost is just bad combo. Unfortunately no easy way to convert to bigger fasteners.

Originally Posted by tbehrens1

I'm sorry if relaying this information is redundant for you guys. I'm new to these engines. I have an early 6.9l which C/R is the same as the 7.3l correct? Also other than the idit block being more rigid, the only advantage to the 7.3l N/A block is the larger head studs? I'm understanding HGS are the main weak link in the setup.