What happens if we inject jet fuel into normal cars?

Machines are designed for optimal performance under specific conditions. That's what engineering is all about. So changing the inputs won't benfit a system which is not designed for it - in some cases, even damaging it.

Separate engine design is required for jet fuels your normal engine wont have enough power to ignite the fuel but with additional modifications it is possible and has been done by many auto freaks to increase their car speeds although it does come with adverse effects,causing damage to your primary engine over a period of time