<p>Scramjet External Nozzle (SEN) generally has a single expansion ramp wall in one side and the other side wall called cowl is fully or partially truncated. These designs are to aim large expansion area using rear part of a vehicle airframe with reduction in system weight, friction loss, and heat load. The present study experimentally investigated the effect of cell base width on the thrust performance of the SEN and computationally expressed those effects, based on the wind tunnel test. The cell base structure divides each engine module. Three test models were employed of which shape differs from each other. One is not clustered configuration, while the others are clustered and each of them has a different cell base width. The experimental study showed that the nozzle wall pressure distribution varies corresponding to the cell base width and optimum-expansion condition is most sensibly affected by cell base width. Additionally, the input correction is proposed to reflect the clustering effect to the computation model, which unifies the input physical quantities by solving mass, streamwise momentum, and energy conservation equations. The effectiveness of such a correction was confirmed using so-called Easy-to-Handle Prediction Model (EHPM) for SEN which is based on the wave method. Comparison between calculation and validation test results demonstrated that the prediction error could be suppressed under the threshold of 10% through the proposed correction.</p>