EF

According to the OS manuals, a 450 cc tank is recommended for the 91, while a 350 cc is for the 120.

Is the 91 a relatively thirsty engine? and/or is the 120 relatively more economical to run?

MinnFlyer

My Feedback: (4)

I think OS was just pulling numbers out of the air. A 350cc tank should work well with either of them. You could go with 450cc on the 120 if you like really long flights.

Gray Beard

My Feedback: (-1)

I run both of these engines and just figure about 1.5 oz. per minute of flight. That seems to work out OK.

Jetdesign

My Feedback: (8)

I have a 14oz tank on my 120AX, and get something like 10 minute flights? Never really timed, and never really ran out of fuel. Basically the 14oz lasts as long as my attention span while performing aerobatics.

I would not go smaller, I would consider going bigger if I wasn't concerned about weight.