Essence of Japa: it can lead you to the same goal as the 'Who am I?' enquiry. Japa means clinging to one single thought to the exclusion of all other thoughts.

Sadhu chanting japa

(from "Conscious Immortality"): 

Bhagavan: Japa must be done until it becomes natural. At first it requires effort; later it happens automatically. When it becomes natural, that is realization. Japa may be done even while engaged in other work. Bhakti, vichara and japa - all finally resolve themselves into that one single Reality. They are only different tools to keep out the unreality. 

The unreality is an obsession at present. Reality is our true nature. We are wrongly persisting in the unreality, namely, thought and worldly activities. Cessation of these will reveal the truth. Our attempts are directed towards keeping them out. It is done by thinking of the Reality only. 

Although it is our true nature, it looks as if we are thinking of Reality. What we are doing really amounts to the removal of obstacles for the revelation of true Being. The practice of vichara is thus a reversion to our true nature.

Question: Is there any harm in my continuing japa, or is it essential that I should do only the bare 'Who am I?' enquiry?

Bhagavan: No, you can trace the root of any thought or japa or mantra, and continue to do so until you have an answer to your question. That is a meditation which will lead you to the same goal as the 'Who am I?' enquiry.

What is mental repetition if it is not meditation? Vocal japa becomes mental which is the same as meditation.

==========

(from "Talks with Ramana Maharshi):

D.: Is not mental japa better than oral japa?

M.: Oral japa consists of sounds. The sounds arise from thoughts. For one must think before one expresses the thoughts in words. The thoughts form the mind. Therefore, mental japa is better than oral japa.

D.: Should we not contemplate the japa and repeat it orally also? 

M.: When the japa becomes mental where is the need for the sounds thereof?

Bhagavan: Japa, becoming mental, becomes contemplation. Dhyana, contemplation and mental japa are the same. When thoughts cease to be promiscuous and one thought persists to the exclusion of all others it is said to be contemplation. The object of japa or dhyana is the exclusion of several thoughts and confining oneself to one single thought. Then that thought too vanishes into its source - Absolute Consciousness, i.e., the Self. The mind engages in japa and then sinks into its own source. - Talk 328

You are always repeating the mantra automatically. If you are not aware of the ajapa (unspoken chant) which is eternally going on, you should take to japa. Japa is made with an effort. The effort is meant to ward off other thoughts. Then the japa becomes mental and internal. Finally, its ajapa and eternal nature will be realized. For it will be found to be going on even without your effort. The effortless state is the state of realization. - Talk 312

Mr. K. S. N. Iyer, a railway officer, asked about japa.

M.: The utterance and then remembrance and later meditation are the successive stages finally ending in involuntary and eternal japa. The japakarta (doer of japa) of that kind is the Self. Of all the japas, ‘Who am I?’ is the best. - Talk 72

Japa means clinging to one thought to the exclusion of all other thoughts. That is the purpose of japa; it leads to dhyana which ends in Self-Realization. - Talk 413