Keep in mind that with an engine you still need to be aware that the prop needs to be able to let the engine reach the power peak. So beware of putting too large a prop on the model. I only add this due to the fact that a few posts up you were working with a 17x4 and an 18x6 is a big jump up for pitch and diameter all at once. Generally as the size goes up we like to reduce the pitch and vice versa.