because plat. plugs require less voltage to fire (so I've read), would it be better to use them with a stock ignition system that's seen many, many years (eg: since the 80's) of service? This is with the assumption that the cap/rotor/wires have all been replaced sometime during its lifetime.

 

I'm not sure where you might have heard that, but platinum spark plugs don't have any reduced voltage requirement over plugs with plain steel electrodes. The voltage required to arc across the electrodes of the spark is determined by the amount of energy (EMF) that is required to turn the air space between the electrodes to a conductive plasma (ozone). That energy varies with the amount of air movement in the chamber, the pressure in the chamber, and temperature of the air/fuel mixture.

An ignition system will only generate the minimum voltage required to overcome the wire resistance, rotor/cap clearance gap, and spark plug gap. Nothing more - nothing less. Ignition systems that advertise the ability to generate 50,000 volts usually don't operate at any voltage level above a stock system, or about 18-23KV, unless the conditions warrant a higher voltage to complete that circuit. Generally, if the secondary voltage has to be higher than that, something is wrong in either plug selection or gaps.

The major benefits of platinum spark plugs are that the electrodes don't erode as quickly as plain steel, maintaining a correct gap, and the capability to operate at a slightly higher temperature to prevent deposit formation. Basically, they last longer and are more consistent throughout their useful lives. For that reason, a set of platinum plugs at 20,000 miles may have less tip erosion, and therefore require slightly less firing voltage. However, a new/clean set of platinum plugs will be no different that a new/clean set of plain steel plugs.

Actually, platinum has a higher resistance to electrical flow than carbon steel (about 27% higher resistance), but the difference is so minute that it is insignificant at a level of over 15KV. Still, the argument could be made that new steel plugs have a lower voltage requirement than platinum, which would have more basis in fact. In any case, the consistent performance and longevity of platinum electrodes usually makes them a better choice for spark plugs in a daily-driven vehicle.