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Dan, You mention that they would'nt offer a performance benefit?
Right. For any given gap, the fine-wire centre electrode lowers firing voltage requirements across the lifespan of the plug, and that's good, but doesn't translate to better performance except in situations where standard plugs' firing voltage requirements exceed the capabilities of the ignition system, which only occurs in cases of weak ignition systems and/or extremely high combustion chamber pressures. Neither is found in most slant-6 engines. Of course, we can also keep the firing voltage constant relative to standard plugs if we open the gap on the Iridium plugs, and wider gaps can improve driveability, but we hit the limit above which there's no improvement pretty quickly. Much above 0.045", and the benefit levels off; around 0.050" you're starting to work against yourself unless you've got a distributorless ignition system.
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What brand and plug# of iridiums have you used on the slants or other engines?
NGKs, in a 225 and a 2.2 turbo and a 2.5 TBI and a 318 and a Volvo B230 over the years. They worked fine, the engines started and ran normally. Back to back emission tests on the 225, the 318 and the 2.5 with new NGK Iridiums and new standard plugs gave essentially identical results, and since (on the 225 at least) there was no catalytic converter gobbling up CO and HC, that's a reliable indication the Iridium plugs weren't producing any significant improvement in combustion. The emission test on the same day in the same bay with the extended-electrode NGKs (the ZFR5Ns I'm always
babbling about), on the other hand, did show a drop in CO and HC, indicating more complete combustion. (In case you're wondering, no, I didn't shell out $24 for each of six emission tests just to satisfy my curiosity. A friend of mine worked at an emission testing station, and after hours we did "unofficial" tests, wherein the normal test protocol is used, but the results are not reported to the central computer).
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I knew there would be someone who would mention the "Dyno" issue.
That's 'cause facts and data trump guesses and opinions.
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Theres clearly two camps: ardent doubters/cynics and believers.
Dude, we're talkin' spark plugs, here, not religion and beliefs. The laws of physics apply, whether you or I or anyone else "believes" in them. Either a particular spark plug
really improves combustion, or it doesn't. There is no magic spark plug!
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OK, you've tried these plugs and say they work better. That's good, but it's not good enough. Not because I think you're a liar (I don't), and not because I necessarily distrust you (I don't), but because of simple human psychology. We humans are very poorly equipped to perceive patterns and phenomena accurately just based on what we see (or think we see, or want to see). That's why scientists and engineers base their research on
data. The Slick-50 effect is very real ("Of
course my engine runs smoother and better and I get more mileage and have more power; I just spent $39.99 on a bottle of Slick-50!"). Substitute spark plugs, headlight bulbs, etc. etc. and the effect still holds.
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And a dyno won't show that there is better throttle response, better idle , and starting.
There are controlled, scientific tests that can be done for all of these aspects of performance you say are invisible to a dyno, but an emission test is a pretty good proxy for all of them. The only way spark plugs can improve starting, throttle response, and idle is by allowing more complete combustion. If there's more complete combustion, there'll be less CO and HC in the exhaust. There'll also be more power, visible on a dyno.
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I'll just assume that you think these plugs are pure marketing
Why assume anything? Why not just ask me?
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The NGK Iridium plugs are very well made indeed, and they last a very long time due to the ultrahard electrode metal, and if my priority were ultra long spark plug service life, I'd probably use them. If NGK were to make an Iridium version of the extended-electrode plug, I'd almost certainly use them. But when my tests show that moving the spark plug electrodes further into the combustion chamber (w/the extended-electrode plugs) gives measurable improvements not shown with other "different than stock" spark plugs, and the extended-electrode plugs cost under $2 apiece and themselves have a nice long life, that pretty much wraps up the question of which "different than stock" spark plug is most cost-effective...for me, in my applications. Your applications and cost-effectiveness calculations may well vary.
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