It is not possible to increase the torque without increasing the force applied, F$.$

The torque would increase by using a wrench that is half the length of the original one $($decreasing it to $4\text{'}\text{'})$ and applying the force F to the end of the shorter wrench.

The torque would increase by using the same wrench $($with a length of $8\text{'}\text{'})$ but applying the force F at a $45$ angle to the end of the wrench.