Spark Plugs

[robbobster]

I think several posts above hit it: Platinum is great for cars where it's a PITA to change the plugs.

They are great for longevity, not so great for performance. Platinum is not a good conductor.

Copper is a great conductor, but they don't last. No way I'm gonna change out plugs every 15K miles when it takes 4 hours to do it on my Expy.

davefr - Platinums should not be used in blown engines because it is a poor conductor, especially under high cylinder pressures.

I'm not sure about OEMs, but as far as the performance aftermarket goes, platinum plugs are never recommended in a blown/turbo motor.

While I'd like to run copper for the performance, I don't have the time to do a plug swap all the time so I stick to iridiums.

Remember that a blown engine also needs a very tight gap compared to a NA motor.

 

[y_p_w]

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Originally posted by robbobster:
I think several posts above hit it: Platinum is great for cars where it's a PITA to change the plugs.

They are great for longevity, not so great for performance. Platinum is not a good conductor.

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They're fine for performance. Electrical resistance of metal plug parts is of little concern when there's intentional use of a resistor and/or resistive plug wires. I calculated the resistance of a .7mm wide (round) by .7 mm long piece of pure platinum. With a resistivity of 1.59*10^-7 ohm-m, that tip contributes to a total of 0.00019 ohm. Compare that to the 5,000-10,000 ohm resistor that's in the plug, and you get the insignificance of the electrical conductivity of metals on plug performance. Copper has a resistance about 1/5 that of platinum.

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Copper is a great conductor, but they don't last. No way I'm gonna change out plugs every 15K miles when it takes 4 hours to do it on my Expy.

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Copper is not a good plug tip material. It's a good core material because it conducts heat better. It'll erode and oxidize quickly in the combustion chamber. Most "standard" plugs use a nickel alloy tip fused to a copper core. Most platinum/iridium plugs use a small iridium or platinum tip fused to a wider nickel-alloy tip which is then fused to a copper core. The internals of a standard plug are generally going to be identical in materials to a platinum/iridium plug.

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davefr - Platinums should not be used in blown engines because it is a poor conductor, especially under high cylinder pressures.

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What? I'm not sure if you're talking about electrical or thermal conductivity here. In either case, the larger nickel-alloy tip the platinum (or iridium) is fused to is generally the same size as a standard plug. The Bosch design is the only one that's vastly different. It's microthin wire of platinum.

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I'm not sure about OEMs, but as far as the performance aftermarket goes, platinum plugs are never recommended in a blown/turbo motor.

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No disagreement there. However - I think the main concern has been whether the fused platinum is going to stay there. I've heard of them coming off in some cases.

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While I'd like to run copper for the performance, I don't have the time to do a plug swap all the time so I stick to iridiums.

Remember that a blown engine also needs a very tight gap compared to a NA motor.

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Not necessarily. Some go to a more powerful ignition.