In Makuta's Guide to the Universe, page 89, it gives the thrust of the T3 engine as 2.5 million pounds, and that of the T9 as 7 million pounds. The thrust of the T6 is not given. Since it appears that the amount of thrust increases with the engine number, would it be safe to assume the T6 is in the middle, at around 4.75 million pounds of thrust?
Also, why 3, 6, and 9? Are these the only T engines, or could Artakha have made, for example, a T5 engine?
EDIT: You've said in an answer to another question that "it may be that there were, say, five prototypes for the T6 that were scrapped before one was made that worked correctly" (https://greg.thegreatarchives.com/2008-2010/page67#post2674), but this is more of a conjecture than an answer. It also implies the T3 is a scrapped prototype of the T6, even though the fact that the Rockoh T3 exists seems to disprove that.