The stock spark-plugs are NGK Platinium based plugs. As most know, Platinium might have excelent “bling-bling” factor. However they are not well suited for turbocharged applications due to its higher heat-range. Colder plugs are needed for elevated boost applications. I have tried 5 different types of plugs and in the end I settled on the NGK-BKR7E plugs. They are copper, and 1 heat range cooler than the stock plugs.
They are much cheaper than Irridium or Silversonte plugs, and work better in my opinion. With the super-rich nature of the stock injectors / ECU of this car you really need to replace these quite often. I change them about ever 5k or so. If you wait any longer, hesistation will result due to heavy carbon buildup.
an email from NGK on the matter:
–snip–
“Hello,
All NGK Spark Plugs are copper core. It’s the firing end design that’s different. Remember NGK engineers work closely with Mazda engineers to produce a spark plug that’s best suited for the application. The reason they use the fine wire platinum design are as follows.
1. Lower voltage required to fire the spark plug. Improves ignition
efficiency.
2. Longer service life
3. Enhanced ignitability
4. Lower emissions
5. Less quenching
6. better anti fouling
The BKR7E is a nickel alloy spark plug and will wear very quickly. You won’t obtain a complete burn of the A/F mixture as with the double platinum series. In addition to this the PZFR is an extended projected design. This means the A/F mixture will be ignited towards the center of the combustion chamber. This means stable idle and improved combustion. If you use the nickel spark plug the ZFR series is a better choice. I would highly recommend using the new iridium IX series. BKR7EIX if you need cooler spark
plugs.
–/snip–
I have been using the BKR7EIX for about 15k miles now, they work great, runs smooth idle, only downer is they cost $40. But, as I was spending $10 for a set of BKR7E nickle plugs it may turn out to be worth it as I was changing them every 5k (or sooner) miles.
I would use nothing other than the BKR7EIX for the mazdaspeed protege