I have got a project to research on performance of si engine by varying voltage of spark plug....how to do it?

If I were you, I would determine and collect together all performance related parameters first like rpm, torque, fuel consumption. Next arrange the measurement of these parameters and log them while you gradually change the primary voltage of ignition transformer in both direction from the default value. A good result evaluation, for example a diagram will show you the effect clearly.

Perhaps it would be useful to monitor the ignition angle too and prepare a2D result matrix for data evaluation.