A clubmate last year took put a small gas engine (weedeater or saw) on his 4*120 and relocated the elevator and rudder servos to the tail to assist in balancing. Sorry but don't know the exact size or weight of his engine. I'm flying a 4*120 with a Thunder Tiger Pro 120 engine (35 oz with muffler) and I was able to achieve balance by shifting my 1000mA nicad receiver pack up to the firewall and no additional weight. Total weight is about 10 lbs.