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What is the difference (besides price) between normal, platinum, and iridium spark plugs?

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5 Answers 5

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Copper conducts better and is generally used in higher-performance/modified engines. In dedicated race cards resistor-less copper plugs are used.

Iridium and platinum plugs are chosen for their longevity only. You shouldn't gap iridiums because of potential damage to the tips. For that reason and their inferior conductivity, they aren't used in modified engines. Keep in mind their price, as well.

Any claims of more power or fuel efficiency of one type over another are pretty much baseless unless you were using the wrong plugs to begin with. Stick with what your owner's manual calls for unless you have a reason to upgrade/modify. For example, if you have a turbocharged car but are running more boost you might gap your plugs a little smaller to accommodate. The claims by parts stores to upgrade to the Triple-Spark Ultimate Unobtainium plugs are just upsells.

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Yep. I would argue that the plug that gives best performance would at any rate give the best economy because they cause the most efficient burning of fuel. –  Juann Strauss Jul 22 '13 at 7:26
    
Resistor less plugs can be a little dangerous. On my Eclipse I had resistor less plugs once (because that's all the local AZ had in stock). Somehow it confused the ECU and it was not setting timing appropriately. It was allowing full timing advance (42 degrees base+advance) instead of the 16-19 degrees I should have been seeing under boost. Resulted in wicked high EGTs. Swapping back to resistor plugs fixed it. Note that with higher boost I need to run iridium now as the copper plugs seem to fail at the higher powerlevels. –  Brian Knoblauch Jul 22 '13 at 16:43
    
Well, going resistor-less is something you have to do with consideration of all the extra EMI you'll get. I would think that higher boost levels might require smaller gap, not different plugs. Did you try that? –  Nick Jul 22 '13 at 18:33
    
Not only are they iridium plugs (with stock preset gap), 3 heat ranges colder than stock, but also was switched from protruding to non-protruding style to keep everything happy. I'm now at the limit of what this particular engine/turbo build can do on pump gas. Some excellent tuning by a local shop didn't quite get me the peak HP I hoped for (false knock due to engine NVH), but I'm getting significantly more torque than expected! :-) –  Brian Knoblauch Sep 17 '13 at 19:55
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The main difference is the material that the 'tip' of the spark plug is made of - normal ones are usually copper, whereas the other two have tips made out of platinum or iridium.

Platinum and iridium tips tend to last longer - copper tends to erode over time so the spark plugs wear out - plus they're slightly better conductors and can produce a spark under more adverse conditions (like under high cylinder pressures on turbocharged cars that run high boost).

On an unmodified car that specifies normal plugs, platinum or iridium plugs won't make much of a difference apart from cost; I'd stick with the manufacturer's recommendation and change the plugs as per schedule. On an engine that has been modified with higher compression or more boost, it might be worth springing for platinum or iridium plugs.

Of course, on a car where those plugs are specified (usually due to the extended service interval for the plugs) you'll have to use them anyway.

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Slight disagreement. Copper is the better conductor, but due to rapid erosion under adverse conditions it's not selected by manufacturers for use in turbo cars. Racers typically use copper and just have to replace them frequently for best performance... –  Brian Knoblauch Sep 23 '11 at 15:32
    
Brian's comment is in line with other things I read on the internet anyway. @Timo Geusch: Do you disagree? –  krumpelstiltskin Sep 26 '11 at 21:42
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For the most part, Timo is correct, but I have a few disputes with his answer... 1) As Brian said, copper is a better conductor. 2) On an unmodified car that specifies normal plugs, platinum and iridium are excellent because though they are less conductive, they last MUCH longer. If you aren't looking for performance, and just want to worry less about changing out your plugs, iridium plugs can sometimes last over 100,000km. Many people don't even keep a vehicle that long. Provided you clean them up and gap them, they are fairly worry free parts. –  Sivvy Nov 1 '11 at 15:49
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Back to O.E.M (orignial equipment manufactured). The four types of plugs are copper, platinum, double platinum, iridium. The type of ignition system should be your first variable to the question. Old ignition systems had one coil that povided spark to 8 plugs. They survived off the copper plugs, they would require to be replaced at the 2 yr interval, sometimes one year for excessive driving. Today they are called coil packs, some packs serve 2 plugs or 4 plugs depending on cylinder configuration. Newer ignition systems have 6 or 8 indiviudal coils for each plug-some of these types require ididium tips. None the less, go with your O.E.M. Natural gas engines run better with copper even some manuals specify otherwise. It is wise to use anti-seize to prevent plugs from "welding" themselves to your block. Some engine blocks are aluminium some are iron, some are steel and when they are mixed with a different metal, they tend to add challenge for removal. I'm not even going to talk about Ford engines, they are known for blowing out their plugs while driving-what a nightmare. Extracking a broken plug without proper tools and potentially damaging the plug thread. Spend the 1.00 and buy a small pack of it.

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I have used all three types of spark plugs in my cars (all Suzukis) and found quite a difference between the Iridium/Platinum and the Copper ones which are much cheaper. The reason being that the iridium and platinum tips are more resistant to erosion due to the sparking. So overall they provide better performance over their lifetime, and don't degrade in performance that rapidly, than copper ones. Invariably, the platinum ones worked best in terms of price and fuel efficiency did improve marginally. The iridium ones were no better than the platinum ones in terms of performance but provided better durability (about 15% more life), but not that much so I can't really justify the price difference. Do try the platinum ones once, I think you'd be happy with them.

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I gained a significant increase in economy and power when I switched to platinum plugs. They warm up much faster and don't burn out like copper.

That's my experience.

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