+1 with Raptureboy.

Rcexl ignitions are known to operate well up to 7V since day one! Maybe with shortened lifespan, but surely not 25 flights. I operate the old types on 2S LiFe battery to be sure of a strong spark. Others have done the same without ill effects.  I use either NGK CM6 (used in Honda generators, not a Honda plug) or RCexl ICM6 iridium plugs. Both are good. The Iridium plug is slightly colder than NGK CM6.