Keep it slightly above room temperature.
Rationality is cool; passion is warm. Rationality provides logical justification for a position, while passion provides a human connection to it. Both are needed to advance an argument; an abundance of one will not compensate for a dearth of the other. An argument may be extraordinarily rational, but its correctness alone is unlikely to compel others to care enough to right the wrongs behind. it. An extremely passionate argument may initially attract sympathy, but unmitigated displays of emotion at the expense of rationality will wear thin and eventually prompt others to tune out your message.
Rationality makes an argument worthy. Passion makes it worthwhile.