Brain-Teaser 683: Powerless

From The Sunday Times, 18th August 1974 [link]

I have here two positive single figure numbers, each less than 9. Neither is a factor of the other. I add the larger number to the smaller.

Then, to that total I again add the original larger number, and to the new total I again add the original larger number and may, if I like, continue this process indefinitely, but never shall I obtain a total which is a “power” of any whole number whatsoever.

What are my two numbers?

This puzzle was included in the book The Sunday Times Book of Brain-Teasers: Book 1 (1980, edited by Victor Bryant and Ronald Postill). The puzzle text above is taken from the book.