I broke a rod while breaking in my V-max7. That was using Norvel 15% fuel with some 4 or 5 washers under the head.

My problem, at least, was not fuel related. The wrist pin had walked out of the piston and lodged in one of the cylinder transfer ports. The engine instantly stopped, of course, and snapped the conrod. It is my understanding that this was the usual source of broken rods for the Vmax7.

As far as fuels go I have run a wide variety of brands and nitro content. I have always found the engine runs better using higher nitro and richer needle settings. It seems you have to run rich no matter what you do, but if I run 10-15%, it has hard to get enough power running that rich. I usually run 30% and this allows me to run the needle safely rich while still having plenty of power. The motor stays nice and cool and is easily hand holdable after flying.

For what it is worth,
Spiro