Alquad, I've flown the Twist with a ST .45, a .51, an 82a Saito, and the OS .61. All of those engines do very well with the Twist. It's really a matter of what YOU want, the style of your flying. The .51 has plenty of power (use a 12.25 x 3.75 apc prop). If your ST 51 will handle it in your warm weather, try the Master Airscrew 13 x 5. Here, cold weather, we tend to use the bigger props for all our engines. But in warm weather they can get overheated. The general rule for 3D flying is to use the longest prop with the narrowest pitch your engine can handle.

My flying pal used an ST 51 through his first THREE Twists. Loved it--and he could make that bird do just about everything. Now he has an OS .61 FX on his Twist--and loves it more because of the long (20 plus minutes) flights we're getting, very efficient engine. The .51, if I recall correctly, got a bit over 12 minutes, safely, on the 11 oz tank.

What's REALLY nice about the SuperTigres is that much is interchangeable from .40 size through .51: carb, needle valve, head bolts, carb nuts and screws, and the muffler and stack.

If the Twist doesn't do well with the .51, it'll not be the engine's fault.

Best wishes, good flying.

Jack