[KingLT1]

6509 is a LTR6IX iridium. It's a 6 heat range.

6510 is a LTR7IX iridium. It's a 7 heat range.


Heat range is the biggest factor here. The colder plug will foul easier with a lot of low rpm street driving. But it shouldn't be misfiring with very little use or mileage. They will need changed more often is all. I am running 6510's in mine right now being that is what Whipple recommends. I would check the plugs for cracks in the porcelain ….I would suspect that is the issue. A 7 heat range shouldn't misfire with low mileage imo. I suspect Ted uses a 6 heat range for cars that are daily driven opposed to a weekend warrior. With proper tuning it probably can be done with proper timing and fueling adjustments.