Champion Spark Plug RV15YC4.
V Series, 14mm thread, .460" reach, projected tip.
Conical seat, 5/8" hex. Resistor plug, wide gap. Very hot.
Replaces discontinued Champion plugs (R)BL13Y, (R)BL15Y, UBL13Y.
The Projected Tip style positions the spark an additional 1/8 inch into the chamber and, providing there is sufficient clearance to valves and piston, provides the ultimate in performance. Initiating the flame front closer to the piston top at a given spark advance has the same effect as advancing the timing. Therefore, maximum timing may be reduced, which helps to reduce detonation and octane requirement while still providing good part throttle response. For normally aspirated engines only; not recommended for turbocharged, supercharged, nitrous oxide or nitro burners.