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In thread http://www.allpar.com/forums/topic/146850-results-of-chrysler%e2%80%99s-bad-design-94-caravan-33l-installation/ some people are making questionnable claims about which spark plug is specified for a 1994 3.3L gasoline engine in a Grand Caravan, particularly whether or not those with a fine wire platinum centre electrode are suitable. I’m putting my rebuttal here as it is of general value.

The factory specified RN14MC5 spark plug for a 1994 3.3L gasoline engine is in Champion’s “Copper Plus” line, that’s what the C stands for. Their platinum lines would have one or more P letters in the “MC” group area of the part number (the 14 position is heat range, the 5 position gap – wide in this case). I expect platinum would be bragged about on the package.

(RN14MC5 seems to be more available than RN14YC5, both M and Y are projected nose, I don’t know the difference except Y is “standard”. N is for 14mm reach and ¾” thread, R is of course for Resistor to suppress ignition RF emissions that could interfere with radio reception.

There are now various _combinations_ of iridium (W character), platinum, and copper in Champion’s line. Wild.

Note that the location of material varies, Champion’s C type probably have a copper _core_ in the centre electrode to better conduct heat away, CC type probably also have a copper core in the side electrode. Use of gold, iridium, palladium, platinum, and other expensive materials varies – some have platinum centre electrode, others patches of the material on either electrode. I understand that it is desirable for igniting to have a small centre electrode, materials that conduct heat better facilitate reduction in its size, but I believe they will not extend plug life to the great extent some herein claim in switching from full size conventional electrode to fine wire platinum.

Champion’s Double Platinum has a different centre electrode than the troubled ones, according to Federal Mogul’s web site – a piece of platinum wire riveted onto the main centre electrode. (I know, I too would like to see details on how they do that and what the erosion is. OTOH the Champion “Platinum Power” plug design has a fine wire centre electrode.) The troubled plug had only a fine wire – Bosch had trouble with that wire falling out of the hole in the insulator in their early production.

Champion claim that some vehicle manufacturers vary the plug with cylinder position to save money.


The issue is whether a fine wire centre electrode of platinum material can withstand the erosion to give at least normal plug life in a DIS ignition system. Advice from experienced automotive technicians is that it cannot, due to erosion from the reverse direction of the spark in some cylinders. (A “DIS” ignition shares a coil between two cylinders, so one cylinder gets normal arc current direction while the other gets reverse direction. Which electrode erodes depends on arc current direction.)

OTOH it apparently was successful prior to DIS, in Chrysler’s old push to get extended service interval. Interestingly, I have the impression that Chrysler does not list any platinum plugs for flexfuel versions of the 3.3/3.8 engine, and that in general they are conservative with ethanol. Propane may be different again, as it burns differently.

An objection was raised by a person who pointed to polarity reversal due to oscillation from reactance in the circuit. But I say that significant oscillation only occurs after the arc across the plug gap has collapsed, thus is irrelevant to erosion. See:
http://www.picoauto.com/waveforms/Ignition
http://www.picoauto.com/waveforms/Ignition/Secondary/wave80.html
http://www.picoauto.com/waveforms/Ignition/Secondary/wave79.html

An automotive ignition coil is a transformer comprised of two wound inductors in close formation. A current spike in one creates a magnetic field spike which couples into the other, which will create a voltage spike that will result in an arc starting thus current flow across the spark plug gap during the time energy is available to do so. The coil does not need Alternating Current to work, only a rapid change of current. Oscillation only occurs after the arc ceases.

DC is fed to the coil pack in this vehicle, the ECM grounds the other side of the circuit for each spark plug. Does anyone think the coil pack deliberately creates an oscillation, as some hot-rod designs do? (“CD” is a common term for those, as some early ones were “capacitor discharge” designs. See http://www.ignitioninfo.com/cdignition.html.)


As for what Chrysler recommends, their 2003 document on spark plugs lists only the RN14MC5 and RN14PMP5 for 1994 3.3L and 3.8L engines, not the fine wire design which would be RN14PMC5. Observe the difference in number of P characters and my summary of Champion design information above. I take that as support for my statement “.... the fine wire ones will have short life.”

Chrysler’s 2003 document has good information on plugs in general, including colour and fouling plus a diagram of spark plug physical features with explanations (general) and re-re guidance such as loosening and torque. It includes Champion’s chart explaining their numbering systems, and cross-reference to MOPAR p/n and other brands. Even has racing plug numbers. Document number may be either 6715 0303 or 00PM4027.

BTW, the underhood label on a 1994 Grand Caravan 3.3L gasoline engine to US federal emission standards lists only RN14MC5, contrary to what someone claimed in the other thread.
 

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Keith, it seems you are a wealth of information. Please consider writing some articles for Allpar which could be posted on the main site. If you'd like to consider this feel free to PM me.
 

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Discussion Starter #4
Schreiber, does your vehicle have shared coils ("DIS" system) or individual coils (one per cylinder), are the plugs you use fine wire or thick centre electrode?
 

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I have the "fine wire" double Platinums in my 01 PT for 2 years now. Runs great.
 
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