What can I expect from premium spark plugs?

Dear Car Talk

Dear Car Talk | Aug 01, 2001

Dear Tom and Ray,

I'm wondering if you have any firsthand experience with the latest in spark-plug technology, the not-yet-famous "Iridium" spark plug. The Iridium plugs are supposed to be more efficient than their platinum counterparts. I just ordered a set of Denso Iridium spark plugs for my Audi A4 Avant 1.8T engine. I will be installing them this weekend. I don't really expect to see an improvement in performance, but better gas mileage and smoother power delivery would be a big plus in my book. What do you guys think? --Adam

TOM: We think it's good that you're not expecting improved performance. And you'd do well to not expect anything in the way of gas mileage or power delivery either (whatever that means).

RAY: We have not had any firsthand experience with Iridium plugs -- my brother's still getting used to the changeover to metal plugs from the old stone ones -- but we understand the theory.

TOM: The theory is that the harder the metal -- and Iridium is harder than platinum -- the longer a spark plug will last. Nippon Denso -- which, by the way, is a very reputable maker of ignition parts -- claims that its original-equipment Iridium plugs can last as long as 120,000 miles, versus 100,000 for most platinum plugs.

RAY: What they don't tell you is that after 120,000 miles in the cylinder head, those plugs are going to be practically welded in there, and you're going to have to break them off and scrape out the remains with garden tools. So when you use any kind of long-life plug, we recommend that you take them out every 30,000 miles or so, just so you CAN get them out when you need to.

TOM: Denso is also marketing a very thin-tipped version of the Iridium plug for super-high-performance cars. The company claims this plug requires less voltage to fire than a standard plug, and therefore it will improve mileage (whatever that means), smooth out power delivery (whatever that means) and bring peace to the Middle East. That might all be true (we'll ask Jackie Stewart the next time we see him). But I doubt it would make a noticeable difference in your four-cylinder Audi wagon.

RAY: So, in general, improvements in spark plugs have been in the area of useful life, rather than increased performance. And barring any overwhelming evidence to the contrary (Denso, just send the evidence along WITH the lawsuit papers), I'd assume that's the case here, too, Adam. So don't expect any miracles.

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