turbocharged 535i blown head gasket

I have recently installed a turbo kit (my own design) on my '91. 535i. The boost was set to 7psi. Although everything seamed OK to me (AFR was 12:1
during boost condition), my head gasket blew. Any ideas what could be wrong? The car had less than 100 miles after turbo instalation when this happened, but prior to failure it ran OK. We had it connected to LM-1 air-fuel meter from Innovate Motorsport and all readings were fine (boost, AFR). Blow-of valve also worked. The most curious thing is that this head gasket failure happened when the car was slowing down from the high speed test run on the track. I know a lot of people mentioning something like "6-cilinder BMW's head gasket is a weak link" but also know that these same cars also run turbochargers with greater boost than me without failure.If anyone has any idea about the possible reasons, thanks!
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You can get a better head gasket for this car to use with a turbo. I think they're solid copper(?). Tuoma Perfection in Finland do them and I think www.bavauto.com or www.bmpd.com had one.
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The most curious thing is that this head gasket failure

Many automotive failures have nothing at all to do with what transpired at the time of the failure. Lots of stuff fails over time, and the immediate prior behavior is merely the straw that broke the camel's back.
I would suggest that perhaps your head gasket was previously compromised, and increasing compression by 20% was a bit more than it could take.
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