Fel-Pro 9627PT unusual failure - potential causes and limits of the HG

[Jacob5se]

Hey guys,
I believe I experienced quite an unusual failure to my Fel-Pro 9627PT headgasket. Long story short car significantly overboosted (from normal 14.5 psi it was tuned for to 21.5 psi), white smoke came out of exhaust, when slowed down it died and didn't want to start. Towed it home and finally got around to disassembling the engine. All nuts on ARP headstuds were tightened the same, not one was loose. After taking the head off this is what we saw. The cylinder bores on the headgasket are not round, they have become oval-shaped, especially on the intake side. We haven't found any marks of detonation on pistons (Arias 8.8:1 made out of 2618 alloy), there's some small pitting on the head though we don't think it's detonation marks and we're not sure if it was present before when we assembled the engine. Coolant was present in cylinders 2, 3 and 4 (number 4 looks clean because we wiped it while searching for detonation marks, it looked the same as 2 and 3). Any ideas what could cause this type of HG failure? This is first time we see something like this.

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Another question is what would you say is the limitations for 9627PT with regular 7 bolt ARP headstuds? I've read many posts from people running it with way more power than I'll ever be, but I've seen they're commonly used with L19 studs. Are regular ARP headstuds my problem? I plan on running around 22 psi from my TD05H-20G in the future, so there should be plenty of room for this even with regular ARP headstuds right?

 

[Jacob5se]

Forgot to add head pitting photos.

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[bastarddsm]

Have to see the plugs, but I see that when I let it rattle. What fuel, AFR and Timing? Intercooler?

Also check the surface finish and flatness of the cylinder head.

What were you torquing the head studs too? IIRC ARP reccomends some comically low number like 75 for the 6 bolts. I go like 105 on a 6bolt with 12mm studs, and that's good enough with a 9627pt for a 3000+lb car to go 145mph in the 1/4.

Shooting from the hip I'd go like 85 or 90 on your setup, but proceed with caution. Maybe some of the other 7bolt guys have a better idea.

 

[Jacob5se]

This is what plugs look like. Unfortunately I do not remember what I were torquing head studs to. I bought a new set and I've read that people torque them to 85 with ARP lube. ARP recommends 80 for 7 bolt.
I'm on 93 octane, won't tell you my AFR and timing since I'm not the one who tuned the car, though it was tuned by a reputable tuner.

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