I dont really know what kind of signal to expect here
I would be tempted (in this unspecified situation) to add more resistors, a couple of gas discharge tubes, and a couple of MOVs, and perhaps be prepared to replace the Arduino.
a couple of gas discharge tubes,
That will kill the signal altogether, once triggered the impedance is very small and will swallow the signal.
About 100 to 200V.The capacitor does little to protect the Arduino pin it is the resistor and diodes that do the protection. You should use normal diodes not Zener ones.
I don't see how the circuit in the OP offers any protection.See here for some diode clipping circuits which clip both halves of the cycle:https://www.electronics-tutorials.ws/diode/diode-clipping-circuits.html
I don't know about the capacitor, and the value of the resistor is unspecified.
Isn't there somewhere else you can get a more usable signal - coil pack low tension side, crank sensor , ECU
How do you estimate the 100 to 200V?
I connected the wire round the coil and there was enough voltage to fire a miniature neon lamp directly. Which you need over 80V to do.
I connected the wire round the coil