must be enough fuel to burn in the exhaust i guess, it wasnt just flames it was like a violent rev limiter with no fuel cut,was a big bang out the exhaust. revs right up to 6500 at less than 9 psi, the more boost the lower the "violent rev limiter" could be a spark problem that shows its face at high boost/rpm i suppose. spark plugs not meant for boosted applications maybe??
eom spark plugs- bpr6es 11
in car - bpr6ey 11