Fox and K&B sport engines were and still are set up to use 5% nitro.
Actually that was only true of the engines since the early 80's. Before that the stunt engines used mostly low nitro and the high performance engines were set up for the 25% nitro Missle Mist. Many of the early K&B engines were set up for 20-25% nitro also, though they ran nice on 10%, not worth a darn on 0%.