So, what did I do to my engine? (bang!)

[zlexiss]

Car is a 2001 5-speed Firebird. Build has about 100 miles on it, pretty much the works - Started with a Series 3 block from a FWD with about 18k on it, added Abbott Racing Stage 3 heads, ARH road racing cam, ARH ported intakes, double roller kit incl. zzp milled oil pump cover, pacesetter headers, 30% upsized injectors, and more..

Was doing wideband tuning (FAST unit through EIO). Fuel was tracking commands pretty well, still some nagging knock I hadn't gotten to yet on WOT but cruising well. Just coasting down in prob 4th gear, 2400 rpm foot off the gas, when bang! rattle rattle rattle (metal noise). Pulled over and killed the engine. See 1:11 in the attached log or look at the jpg:

Frame ~615 end last WOT run
Frames 620-640 2 upshifts
Frames 675 car goes DFCO lean as my foot is off the gas
Frame 700 something just went wrong
Frame 725 drop to neutral, bump the gas a couple times (not best idea, but I'm new to the engine metal clanging noise thing)
Frames 900+ I'm idling

What's to see? Manifold vacuum gone, but no airflow since the throttle is closed.
So I figure I broke something in the valve train, prob intake side, causing gas to get back into the intake.
Pulled both valve covers - nothing out of the ordinary
Pulled all 6 plugs - no visible mashing or damage

Before I take more stuff apart, looking for some good educated guesses. What's making the noise and where did my intake manifold vacuum go?


Bang.jpg


bang.hpl

[5FDP]

No glitter in the oil?

It might not even be motor related if none of the plugs are smashed, maybe the clutch blew apart??

Or even the harmonic balancer broke, those make some bad sounds.

[zlexiss]

Would either of those explain the intake manifold vacuum going bezerk? Would love to not take as much apart!