I'm currently trying Champion stock # 526, RD15Y on my RV-8 Parallel valve O-360, 8.5:1 . It's an 18mm resistor plug like the Autolite 386, but also has a projected tip. I'm hoping it's slightly cooler, but I only have a few hours on them, trying both high power, and very low power flight. I won't know much until something happens
or it's oil change time and I pull the plugs for a look.
3.99 ea thru Summitracing.com
I also have successfully used Autolite 386 on my Thorp T-18. They stay clean and do not show heat stress on my O-290 with 7:1 compression.
EAA 668340, chapter 326 & IAC chapter 67
RV-8 N89SE first flight 12/26/2013
Yak55M, and the wife has an RV-4
There is nothing
-absolute nothing-half so much worth doing as simply messing around with Aeroplanes
(with apologies to Ratty)