Over the last 36 years running 14mm without inserts detachable head motors with various compression ratios up to 15 to 1(Full squish head).
I have found that NGK B6HES(lohg reach) with spacers, or B6HS(short reach) with the combustion chamber modified so that the electrodes slightly enter the combustion chamber work very well. I have used these for racing,sprinting and with nitrous oxide injection. I do however run less advance on all my engines, around 25deg.
Ian Pearce used the same grade of plug(eg a NGK 6) when racing and uses an “adjusted” B5HES for the road with excellent results. I’ve never bothered to run a 5 as I’ve had no trouble on a 6(I use a mag, Ian is on points with his road bike). Ian also uses less advance than std.
There does seem to be a power penalty if a short reach plug is used .
I have only run one engine with inserts, but only for a short time. I suppose you might get a hotter running plug and perhaps in that case you’d need to run a B7HES.
I wish you luck!
