Your ignition system uses a ballast wire or resistor in the ignition coil harness. The engine starts on full system voltage and, as the ignition switch is released to the RIUN position, there's a voltage drop to the coil.
Normal for your car. My 1972 Olds 455 coil runs at 7.5 volts.
We have checked what you told, but we have only 8 volt when we try to start. The car don’t start. If we take and place the coil at the battery, it starts immediately. Can you tell us where the reley for the volt drop is? @Submariner409
Depends on your particular car's wiring diagrams. Many of those ballast systems use the ignition switch itself for the voltage change - START connects full system voltage to the coil; RUN connects the ballast wire/resistor to the coil.
Here's a quick generic Google search for ballast coil wiring diagrams.