Sorry mate, allow me to explain.
You car has been tuned for a re-circulating BOV. When you have a BOV that vents to the atmosphere (opposite to your stock one) your car will run rich when you change gears and your exhaust/gases are hot enough to ignite the extra fuel that is not being used, hence the reason you will get a pop or explosion in your exhaust. There are two causes of this.
1. A BOV that is tighter than the stock one will usually makes the turbo flutter, which is called reversion. Reversion is where air bounces off the butterfly (throttle body) back through your piping and out of your airfilter past your AFM (Air flow meter) which cools the sensor and therefore making your fuel mixtures rich. This happens as the BOV is too tight to allow small boost pressures to make it open.
The other is
2. When the BOV opens to the atmosphere the ECU has already recorded this air passing through the AFM and thinks the engine has used this air when really it hasnt there for the fuel for this air has been added to the mixture but the air has escaped = rich condition.
A SAFC is a Super Air Flow Converter made by APEXi which taps into your AFM which enables you to alter the fuel maps at different RPM/Load levels.
Fouling your plugs means your spark plugs are coated with unburnt fuel and carbon which makes it harder to get a clean ignition spark.
Hopw that helps,
Ben