The IJN evaluated the capability of the "Shiden Kai" to redesign it for replacement by the Zero Fighter. After a number of test flights and improvements, the upgraded version, N1K3-J Model 31, was armed with Type 3 13.2mm machine guns and the engine was moved six inches forward. The model 32, N1K4-J, was given a 2,000hp NK9H-S Homare 23 radial engine. There are no exterior differences in these two models from the original Shiden Kai, except for the double cooling flaps.