I know it's not fire suppression, but on that Eagle-540-powered Lancair IVP I've been working on, I installed a string of 368*f thermal fuses around the upper perimeter of the firewall, on which the opening of any one of them triggers a master annunciator alarm advising us of excessive under-cowl temps that could only be associated with a fire.  Should the alarm go off in flight it would dictate declaring an emergency, shutting off the fuel to the engine and making an immediate landing.  Like I said, it's not suppression but it's better than doing nothing about the issue.  FWIW

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