Non-Resistor Plugs...Once More

[Cal Learner]

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...??

[whipplem104]

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.

[Chas H]

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.

[whipplem104]

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

[Chas H]

Originally Posted by whipplem104

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.

[Kestas]

Originally Posted by Chas H

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.

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.

[Chas H]

Originally Posted by Kestas

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.

[Arthur Dalton]

Originally Posted by whipplem104

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

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.

[NDP 2.6 190E]

Originally Posted by Arthur Dalton

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

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....

[whipplem104]

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.

[d.delano]

Originally Posted by NDP 2.6 190E

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....

what chap was that- I need to get some myself

[mak]

Originally Posted by NDP 2.6 190E

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....

were non resiistance leads for the 103 anywhere?

[Cal Learner]

Originally Posted by Chas H

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.

Waste spark engine? The M103?

[Chas H]

Originally Posted by Cal Learner

Waste spark engine? The M103?

The 103 is not a waste spark engine and I never claimed it was.

[pawoSD]

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.