The voltage will dictate the power and the mah will determine the run time. If you are breaking in a new engine or have a tight pinch on it, it would be a good idea to warm the engine with a heat gun before you start it. You might have to loosen the glow plug a touch to ease the compression as well.
People spend their lives worrying about death. I'm sliding in broadside h ollering "Wow, what a ride"