I have had a K&B Twister on a scratch built Corsair A-7 that is 8.5 pounds and like it just fine. A friend has two of them and we have tached one at 12,600 rpm on 10% nitro with an 11 x 7 APC prop. That was with the ringed silver K&B short muffler and not the longer tuned muffler that came on ours. I understand that now that longer tuned muffler is not sold.

The picture shows a regular K&B .61, Jett .90, and Twister with tuned muffler on the planes from the bottom up.