5 inches in the exact distance for any motor, due to the cowl. It's not really based on balance, as the battery and such will easily keep it on target. I have a Saito 120 mounted in mine and it balances perfectly with 1 oz. lead just behinf the tailwheel. The 91 will nto need extra weight. As my battery is as far back as it can go in the tail, yours will be much closer to the actual servo compartment. I am anxious to hear how the smaller 91 performs. Keep us posted [8D]