Piston Slap: Platinum performance with Pertronix persuasion?

Niterra North America, Inc

Jim writes:

Regarding classic car engines that have a Pertronix electronic ignition inside the distributor, can they use platinum plugs after removing the points and adding electronic ignition?

Robert writes:

I have a stock 1948 Plymouth six-cylinder and will be using Pertronix ignition in my distributor. Can I use platinum plugs, and what plug would you suggest?

Bill writes:

I have a 1970 Dodge Charger with Pertronix, and I’m trying to figure out if I should use platinum, iridium, or just stick with copper.

Sajeev answers:

Well, this is clearly a question on the minds of many a Hagerty Community member! But first, let’s get on the same page. Pertronix (and others, to be fair) allow classic car owners to retain the external look of their stock distributor, but with solid state internals like the ignition systems ushered into automobiles starting in the mid-1970s. And with it there was no more fussing with points of dubious quality, increased voltage, more reliable performance, and plenty of good vibes going forward.


Spark plugs are a little harder to describe, as their design is most differentiated by their longevity. When placed into a vehicle, a host of variables (ignition coil output, compression ratio, etc., and maybe even heat range) ensure they behave radically different. Most 1996-up vehicles are designed for platinum or even iridium plugs, and their ignition systems are sized up to provide the correct amount of juice to make them happy. I’ve personally seen a modern (modern-ish; it was a 2006 Crown Vic) vehicle absolutely murder a set of copper plugs in less than 30,000 miles. But I doubt the reverse is true, that older cars with a modern Pertronix ignitions must also use platinum or iridium plugs.

The answer, as per usual here at Piston Slap, is that it depends on your unique situation. The Pertronix Ignitor III puts out way, way more voltage than the baseline Ignitor system. And no matter the ignition system, platinum and iridium plugs may fail to perform to their maximum potential on an engine designed for copper plugs. So here’s what I recommend:

  1. For Bill: A 1970 Charger may need platinum/iridium plugs, if it has an aggressive ignition system upgrade, a built 426 Hemi with something like a 13:1 compression ratio, or forced induction, etc. If not, stick with copper.
  2. For Robert: This Plymouth is stock and likely has a baseline Pertronix Ignitor setup. Given that and the engine’s mild state of configuration, copper plugs are likely best.
  3. For Jim: This concern is more generalized, and there’s no better general advice than to stick with whatever the motor came with from the factory.  (Which is usually copper.)

Considering most vehicles this age are not used for primary transportation, and since many have easy-to-reach spark plugs, sticking with copper spark plugs really isn’t a big demand to place on people. Their lower asking price is the textbook definition of an “added perk” too, but Piston Slap isn’t about my thoughts being final conclusions. This is only a means to encourage comments from readers.

So, as a member of the Hagerty Community, what do you think each of these guys should do?

Have a question you’d like answered on Piston Slap? Send your queries to pistonslap@hagerty.comgive us as much detail as possible so we can help! Keep in mind this is a weekly column, so if you need an expedited answer, please tell me in your email.



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    In fact, this question wasn’t “on my mind” because, prior to reading this column, I didn’t even know there was a question! But after thinking on it, I believe that Sajeev’s suggestions are pretty valid. Unless you’ve REALLY upgraded ignition output far beyond the stock specs (and just installing most aftermarket base modules aren’t going to do that), and for a weekend driver, OG plugs (e.g. – copper) are going to be fine. If you’ve built a monster with high-output coil, etc., you’re probably better stepping up. As with most things, I like to try the lesser (read: CHEAPER) option first, and if results are unsatisfactory, then it’s time to step it up a bit.

    Can’t quite put my finger on it, but something tells me that Shereeldony hasn’t provided a legitimate response to the topic – and in fact, I’m guessing that Shereeldony isn’t even a legitimate member of this community and should be purged. Oh yeah, I know: This isn’t a help wanted column. Whaddaya think, Sajeev?

    This is a replacement for points not an HEI system.

    You just need to stick to the stock plugs as you are not high voltage and you are not high compression.

    People confuse these systems for high voltage HEI systems and they are not. You just removed the mechanical points.

    Personally, I would keep the points. The primary purpose of high voltage ignition systems is to light off lean mixtures – the kind found in fuel injected engines under cruise conditions. If you are still running a carb, you will not see a difference. I have also found some of the current electronic ignition systems out there to be less reliable than points. My neighbor had a Mallory system installed in his GTO, and the ignition module failed around annually, and this was a car probably driven 200 miles per year. I have a Pertronix unit on one of my boat engines, and it has been relatively reliable.

    I did a deep dive into this about a year ago when it was time to get new plugs for my mildly modified Triumph TR250. I got big sticker shock when I looked at plug prices, and I was also curious about the various plugs now available. Bottom line is that platinum and especially iridium are specced for long, consistent performance over time. Platinum electrodes maintain the gap longer than copper (or erode slower if you prefer), and iridium slower still. So put on these new fangled plugs and you don’t have to take your new fangled engine halfway apart to change the plugs every year or two.

    But, platinum is a much worse conductor than copper, and iridium is somewhat worse. So if you have a weak or marginal system the copper is more likely to limp you home.

    As stated, on an old collector car with easy access to the plugs, and a sheltered, low mile life, copper is the easy choice. A copper plug in spec is a better plug in most cases, it just doesn’t stay in spec. as long

    I was very surprised when I decided to change the plugs in my Porsche Cayenne. Porsche recommends new plugs every 40,000 miles when practically everyone else goes for 100,000 miles. Porsche put copper plugs in my 4.8 L V8. I did some research and yes, copper plugs are a better conductor of electricity. So it comes down to longevity or performance. The Cayenne plugs are easy to change so I’m sticking with copper for that application. The problem I then had was what to do with my 356? I did install an electronic ignition. It is different than a Pertronix in that the advance etc are programmed into it. Set the static timing once and your good for life. It can only be changed by telling the distributor that it is now a different distributor via a switch on the side. Then I installed platinum plugs. I went for longevity for two reasons. Two of the four plugs are hidden behind the intake manifolds and are changed using braille wherein it is not unusual to lose one in the sheet metal or not start it in correctly. The second reason is that there is just no way that I’m going to experience more performance from a 74 HP, 96 Cubic inch engine by using different plugs. The performance base line is barely detectable to begin with. Porsche put copper plugs in the 356 because that’s all there was.

    Greg has it right. Platinum and iridium plugs just last longer. Period. There might be some minor advantage if the finer point center electrode allows less shrouding than a larger copper center electrode, but it’s probably not going to be significant. Replacing points with electronic ignition does not require a spark plug change. In addition, if you are dwelling the crap out of your ignition coil, and creating high secondary voltages, you are likely to damage ignition cables/spark plugs/coil. Unless your boosting the engine, or running very high cylinder pressures, 30KV is all that is required and 50KV will just cause punch through issues.

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