Here and there I am reading that it is best to use a plug one step colder than the "book" says. I am looking at getting NGK iridium IX, and wonder if I should go one step colder. My engine is in stock condition. I would like to hear from people who have done this with success.

Also, I called NGK because I wanted to understand the difference between the "iridium" plug and the iridium IX. Well the plain irridium is a longer lasting plug, and uses platinum on the ground electrode. But the IX was developed for performance, and does not last as long. The IX is also less expensive. Gotta go for the performance !!!!!!

I had great results with Iridium IX'*, until I put a 2.25" pulley on. I now have Autolite 5245'* as per Jeffery10x2'* suggestion and they reduced KR.

Any idea why the autolites reduce KR? Are they just a regular plug, or some premium type?

Also, please tell me your thoughts on going to that 2.25 pulley, I am considering something close to that size. I don't want to go too small. Did it make a small change or ?

Thanks

The 5245'* are 3 ranges colder than stock and that helped a lot, but not needed unless you go to a smaller pulley, like 2.25". I would recommend that combo. You'll gain a bunch of low end torque.

Wow three ranges lower seems quite a bit. Do you think I would see any difference if I went to just a 2.4" ? I think the stock is 2.5

Iridium'* would probably be fine then.