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Sparkplug Design

New 10/02/2004

I've read in previous emails about you specifying AGSF32FM plugs.
The plugs that I pulled from the car are AWSF32FM....do you know what the difference is (which is better) ??
I've gone to the Motorcraft site, and they list both, but they don't give me any more info.
Enlighten me if you would.

Stan Phillips


These are the current plug AGSF32FMF4.

Some snippets about the new plug design......

...I meant to tell you, when my wife and I were at SEMA week before last, I visited the Motorcraft booth, and had a lengthy discussion with a Motorcraft rep. with my experience with the Motorcraft double platinum plugs for my 3.0L FFV (I'm sure you have seen my posts on using AGSF12PPs, instead of the AGSF22PPs, a hotter plug that Ford is now recommending). The Motorcraft rep. recommended a new platinum plug, a "fine wire" plug (AGSF12FMF6). Sometime ago, I got the e-mail address of an engineer at Motorcraft (James Power), so I sent him a note asking him about the new Motorcraft "fine wire" platinums. He talked very positive about the new plugs (I'll forward his e-mail to you...).

Here is the email from the Motorcraft engineer that he is referring to:

...Fine wire means a .030" to .040" dia platinum nickel alloy rivet welded to the end of the center electrode. There is about three times the precious metal than what is found on a 'P' level enhancement. The P's are made by crushing and welding a small special lump of Platinum nickel so that it is a disc about .003" thick.

The effect of the point on the center electrode is reduction of required firing voltage and that aspect slows down the erosion of the sidewire. Consequently, we can use a single 'finewire' enhancement to replace the double 'p' designs for service and get the same durability.

Some auxiliary advantages of the reduced firing voltage is less stress and therefore better life of coils and wires, etc. and cold foul misfire is improved. Don't go crazy getting some because our systems are robust without it and the improvement is modest and mostly unrecognizable except for special cases. We use it in production as a durability measure for those engines that wear out plugs fastest.

Your SHO forum friend brings out a point ....that has been the argument all the time for using DP plugs with EDIS!

I raised the same question with the Motorcraft engineer, and I'll try and explain his response. One thing to understand first, when a coil "discharges" (due to the induced field from the coil primary into the coil secondary), the voltage will only build up until the spark occurs, and will not increase any more. There are several parameters that effect the voltage required for the plug to spark. In addition to the gap, and pressure/composition of the cylinder charge, the physical structure of the plug electrodes also determine the spark voltage. A "blunt" electrode requires a higher voltage to spark, than a "sharp" electrode. With the "fine wire" center electrode being sharper than the platinum bead on the older DP plugs, the spark voltage is lower with the new "fine wire" platinum's. With this lower spark voltage, there is less energy, so electrode erosion is reduced. Because of this, platinum is not needed on the outer (GND) electrode. I'll keep an eye on mine to see how they do!!

I hope this answers the question on why only the center electrode is platinum on the new "fine wire" plugs. As explained by the Motorcraft engineer, there are several side benefits to the lower voltage:

1) lower misfires
2) longer coil pack life
3) longer plug wire life

These points alone are reasons NOT to increase the plug gap wider than the recommended gap. I have seen many posts about gapping plugs wider than the recommended gap!

Paul Nimz

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