I am using an FP40 run in a moderate 2 stroke setting. I guess I could tack it. Using a real fat setting left the plane without enough to get it over the top on the initial wing overs. This engine can put out more power than needed for the PF, but set as a I use it, it proves a very solid match. With an FP20, I believe the plane would be under powered except in an ideal situation, with a constant breeze that does not change direction. Mike Pratt recommends a 25 as possible power, perhaps on 55 foot lines. Some have recommended similar power for a Twister sized plane. At our club's field, where the air can be tricky and treacherous, I doubt that a 25 would be reliable on either plane. I've seen many underpowered planes plow in when hit with sudden gusts and changeable wind direction. The Primary Force I fly weighs 37 ounces and was built from a kit. It was given to me as a stunt trainer, already assembled, having been flown many times. It is predictable and easy to fly. Nice plane. When equipped with a tongue muffler the FP40 does not create a nose heavy condition.