Non-Resistor Plugs...Once More
 

I'm almost reluctant to even start on this topic again, but I've read so many threads here and at MBCA that I'm no longer sure I know anything about the correct plugs to use on my M103 motor. Here's my starting point:
1) Almost everyone says non-resistor plugs b/c of the resistance already built into the plug wires and cap;
2) EPC lists Beru, Bosch and Champion numbers, all non-resistor types;
3) WIS says that MBz no longer approves platinum plugs;
4) Many enthusiasts swear by NGK plugs.
So I did a cross-reference check using the Bosch H9DCO part number from the EPC to ID the correct NGK plug, and it coded to TR5. But the NGK website says TR5 is a resistor plug. Numerous posters have said that they ordered H9DCO plugs and HR9DC plugs were substituted by the vendor. Some other posters say they regularly use platinum plugs and like them better than copper.
I guess the easiest thing to do is to just go the dealership and let them supply me with the plugs they have and not worry about it, but I'd really like to know why there is such a debate raging over something that seems like it ought to be one thing or the other. But which one...??

These debates are often filled with myths and wive tales. I actually called ngk and talked to them about the differences. I agree that if you are using a copper plug that it has to be a non resistant plug. Platinum plugs should be non resistant to. Iridium on the other hand have to resistor plugs. The reduced tip size brings the arc voltage down to a much lower kv. The spark jumps the gap easier. I run iridium in my m103 and m104 and m112. I have had no problems with them. The problem with adding resistance with a copper plug or platinum is that you are creating in essence a harder path to the ground. With platinum plugs you could gap down and some platinum have a smaller tip and lower arc voltage also. So regardless of what you are using, copper, platinum, iridium, they all need to be correct for the application.
My preference at this point is the iridium plug. They last forever and the idle is better. The small tip platinum plugs wear down over time, but work well.

Using platinum or iridium plugs in a waste spark engine is a waste of money. Half the plugs are sparking from the outer to the center electrode.

The mercedes waste spark does not work this way. Spark is actually out to both plugs and maximum current to the plug under compression.

Sure it does. The current flows out the center and across the gap of one spark plug, through the block, and then from the side electrode across the gap to the center of the other plug. It's a big circle.

Series circuit on Waste Spark..second plug fires from ground source.
And Plats in a 104 misfire right out of the box...scope one sometimes.

For orig poster..you want to use BP5EFS NGK...plugs.

Chas, I believe you and whipple are both correct. Though the current flows in a big circle across two plugs, 80% of the spark energy is used in the compressed cylinder. It has to do with the difference in environment around the gap.

That's not my argument.
One spark plug of the pair will have the spark jumping from side to center electrode every revolution, half of those sparks will happen under compression.

Agreed. Just received 12 of these from a chap in the UK. Gapped 6 of them to 0.032" and installed in my 190E yesterday. She purrs like new....

The way I understand the spark cycle of the wasted spark you guys are talking about if on plug is not connected the spark would not occur. Ford for instance if one spark plug is not connected then both do not fire. This is not the case with the mercedes. Either plug will fire with the other disconnected. The spark arcs to ground, which is the engine. Thus there is no way that the spark is coming back from ground into the opposite plug. I have done many a diagnostic on the m104 system. In fact if you create enough gap on one plug to pull more load to that plug the computer will think that the misfire is the wrong cylinder. Thus a bad plug wire can cause a misfire on the opposite cylinder.

what chap was that- I need to get some myself

The Bosch plugs you get at the dealer for about $20 for a set of 6 are the right ones....I have them in my M103, and it runs smoooooth. It formerly had junky champion Resistor plugs in it, and ran horribly.

The spark arcs to ground on the first plug, not the second.
The first plug is pos polarity and the second plug is reverse polarity...which is why the electode wears faster on the first than on the second. If the spark arced to ground on BOTH plugs, they would have to be wired in Parallel, not Series. And they would then both have the SAME polarity.
But they don't b/c it is a series circuit.


The coils secondary winding has a Pos and a Neg tower. The bottom tower one [ TA+ ..that goes directly on the plug ] is the pos terminal. The other one [ TB-] is the neg terminal and has the return plug wire from the second plug .
The flow goes from pos TA+ to the first plug, arcs to ground, from ground to the second plug, arcs back to the center electrode, and completes the SERIES circuit back to the neg side of the coils TB- secondary winding. A closed / completed circuit, using the two plugs in series. That is the complete secondary loop..completely isolated form any other circuit/s.
You can get a code for the first misfire with a bad wire on the second b/c the first plug can't fire with a bad plug wire to complete the circuit.
Each plug fires to complete the series circuit, but the one under no load/compression [ Waste Spark] only needs 2-3 K to jump...and these coils have a 40K output to overcome that and still get enough ramp to work the EVENT plug. The coils output is dependent on what is needed , and that is
what is monitored on the primary winding trigger amp draw.
So, to say both plugs fire to ground is not correct on a Series Waste Spark system, which simply means that the plugs are out of phase, polarity wise with each other b/c it is a series circuit. And being a series circuit, the flow has to return to the neg terminal of the coil thru the 2 plug gaps.
One an EVENT firing, and the other a Wasted firing, with the combined load on the coil being the total of both plugs series circuit.
Which was CTH point to begin with.....

Here - http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=260334931615&ssPageName=ADME:B:EF:GB:1123

It would be great if Phil could stock these....

A Dalton, I know what you are saying but the simple fact is that either plug will fire without the counterpart. Therefore it cannot be a series circuit. If you completely unplug the wire from one of the two companion circuits either will still fire. If it was a series circuit both would stop firing. Something else is going on. Also the engine is grounded. The path of least resistance. Am I missing something here.
I have run platinum before without trouble. I used the plugs from a m113. I know that they are not correct but they worked just fine.
I know this is way off topic also. We could discuss this somewhere else if you would like.