From recommendations I've always used NGK BR7EFS spark plugs - with heat range 7.
After a rolling road session I was told to use a cooler spark plug as the insulator nose was chalky white all the way up after a full throttle sweep.
A too 'hot' spark plug has less cooling, causing the plug to run hotter. On a road engine in traffic this is preferred but on full throttle it can get too hot and become a glow plug instead of a spark plug. Car manufacturers compensate for this with more fuel that cools the plug. On a race engine we can prioritise differently.
As 'cooler' plugs transfers more heat to the engine top it instead lowers the temperature of the tip of the spark plug and allows a more optimum air/fuel ratio on wot without overheating, and a overheating spark plug can cause pre-ignition which is disastrous for an engine. On the flip side are possible troubles starting when cold and carbon fouling when the engine is used at low speed. Very extreme engines even use two plugs with different heat ranges. If any of this will be a problem for me we'll see, but too cold is better than too hot in order to avoid engine damage.
As NGK doesn't manufacture a similar but cooler BR8EFS plug, I had to switch to Iridium. As a bonus iridium plugs require less voltage to fire and therefore allow wider plug gaps, which in turn gives a more stable combustion and less risk of miss fire.
Denso IT24 was cheaper than NGK TR8IX, and Denso is also what SBDev recommends. So I ordered a set from eBay.
Sources:
- Four-Stroke Performance Tuning, 4th edition, A. Graham Bell
- http://www.sbdev.co.uk/Duratec/Electrical%20components.htm
- https://www.ngk.com/Spark-Plugs-NGK-c593.aspx
