Actually, neither diode is correct.
The rated voltage that you refer to is the Stand-off Voltage Vwm or sometimes Vrwm. The other voltage rating you're thinking off is the Break-down Voltage or Vbr.
Also, the Clamping Voltage Vc should be less-than-or-equal-to the max voltage that the device can withstand. In this case the Pertronix module can not take more than 16V. Ask Pertronix.
A Zener (TVS) diode with a Vc of 16Volts has a Vwm rating of 9Volts. You can see this is a real problem. Even if you choose a diode with a Vbr of 16 volts the Vwm will be 13 volts and that does not work because the Vwm should be greater-than-or-equal-to the Vop.
The bottom line is. The 16 volt zener will protect the Pertronix from most spikes up to about 19 volts and all spikes up to 26, but it doesn't matter because it will fry at 16. You could use a 15Volt rated diode, but it's not 100%. You don't get much current till the Break-down Voltage at 16.7V.
I still have the "external mechanical voltage regulator" on my truck and I suspect that is why the HEI module fried shortly after I installed it. I replaced the module and reinstalled the "ballast resistor wire" and have not had a problem for about eight years. I have 12 volts at the HEI, it runs great and the plugs are clean. And no, the ballast resistor will not give you 9 volts, because the HEI does not have a set of points in it shorting the supply voltage to ground three times every revolution.
A couple of years ago, Hot Rod magazine did a article on installing a Pertronix unit. They ask Pertronix, "Should you always take out the ballast resister?" They would not commit themselves. They were well aware that many of their units had gone up in smoke in the past.
Some of your small local stock car classes run without alternaters. They run high horsepower, high rpm, high compression engines for 40-50 laps. Do you think these racers would really go after that 10 hp if the distributor couldn't fire the plugs on 10-11 volts at the end of the race?
