Just some info about how critical it is to have the correct plug for the correct application re spark plugs. . . .
I thought I'd experiment with fitting Iridium CR9EIX plugs to my 99 Busa, they are not listed as recommended for this model, CR9E is the only one listed by NGK as being correct fitment for this model.
I went for a 140 mile ride today and noticed a brief misfire occasionally at low rpms in the lower gears (2nd,3rd) as I was opening the throttle smoothly.
With the CR9E plugs this has NEVER happened before, so I can only put it down to the Iridium plugs and they were gapped at 0.9mm, the CR9E is gapped at 0.8mm.
Anyway, I,m changing back to CR9E's tomorrow!
Regular CR9Es are always gapped tight in any blown configuration. Boost pressures
can snuff out spark, and you eliminate that possibility when the gap is close . . . .
I worked at the Subaru Dealership for many years in the service workshop and the WRX turbo always needed the plug gaps to be less gap than a non-turbo (N/A) for the very reason Frank from Powerhouse stated, the boost pressure air/fuel charge can snuff out the spark on a wider gapped plug.
And the turbo Subarus always ran standard NGK plugs, NOT Iridium, and all the N/A Subarus ran Iridium long life plugs (100,000kms)