Heck, when I had my Slowica w/ it's Kazuma exhaust, I got backfires w/ actual visual flame pops. It only happened when hitting on the gas w/ a little load then immediately letting off the gas or pressing in the clutch. Since the car was in the process of accelerating, there was more un-combusted gas in the exhaust that would ignite there.
This is also due to the fact that I was running no cats at the time.
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