I used to run 18/78 (11.3) on a 6.5 Novak using 3600 LiPo's.
21/81 is about 10.2ish, overgeared me thinks?
If you are running big fat 5000mAh LiPos you might find you need to sell that 6.5 for something a little softer?
Oh, and try and get some ventilation on the shell too
I agree with David about full EPA and a gentle finger. Throttle jamming cos of the expo wont help matters.