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My last set went about 2000 miles. They were about 3 years old but only 2000 miles. I was out driving today and got a little stutter while cruising. The same indication that happened last time. These also have around 2000 miles and are about 2 years old. I have a Crane HI6 so I have plenty of fire power they just seem to go away. They seem OK on light acceleration (roads did not allow to really get on it. Just cruising I have a slight stutter. Off to the store for some plugs tomorrow.
On my blown bigblock I am on the same plugs for 4 years and they still look good. Plug should not wear out that quickly. Properly jetted your plugs should still look new, Mine do after 4 years,
I have over 20k and about 2 years on mine and they are in fine condition. Lightly browned and hold a gap very well. That is definitely excessive wear. What exactly did the bad ones look like when you removed them the last time?
My LT-1 is pretty radical and I have 10K miles on my Autolites but recently changed to A/C Rapidfires because I was feeling guilty about how long they have been in there. I have a Petronix Ignitor triggering a Accel 300+ digital ignition (like it much better than the MSD it is way less voltage sensitive than the MSD!) You need to have a proper idle mixture and lower end fuel curve for good plug life, it is possible!