Exhaust gone mad!!!

[White Hazard]

Finally my turbo is operational and kicking some major butt on the streets since we installed it about three days ago, my engine, blowoff valve and exhaust sound awesome so we wanted to know the amount of noise produced by my exhaust.We managed to borrow a db meter from a friend that works in a tune-up shop, so we were testing it and my buddy was hitting the pedal like crazy while i was behind the car with the meter.I was sitting behind the car and when i stood up my buddy put the pedal to the metal one more time, and at that point there was this major bang and a freakin flame of about 30-40 cm.As i was wearing shorts my lower legs are now completly hairless hahaha, it really scared the crap out of me but my mechanic told me the flames are normal, now i was wondering if it is my exhaust or something else???The exhaust is a open 56 mm exhaust with a HKS blast with a 110 mm tip hanging at the end of it, but it seems to me that it needs a spark somewhere to shoot flames that big.Does anyone have an idea??It's not that its a problem, because i think it's awesome, but i'm just curious.

(flame2) (uhm)

 

[Matthew]

if you are running really rich the unburnt fuel will ignite in the exhaust under certain conditions, creating flames.