I don't know what was written in the thread that precipitated this one, but...

The purpose of resistance in the secondary (high voltage) side of the ignition system is to reduce the current in that part of the system. Reason for this is to reduce the strength of the electromagnetic field around the secondary parts of the system (primarily the wires). This is to reduce the amount of EMI (Electromagnetic Interference) that is radiated to the surrounding area. And this is often mandated by government agencies in requirements that the auto manufacturers have to follow. And it wasn't just to reduce interference in the radio of the vehicle, but to reduce interference in the environment at large.

Putting any resistance, anywhere, in the secondary side of the system will reduce current and therefore will reduce the energy of the spark.

Now a lot of series of Mercedes have been mentioned that I don't have experience with. But with all the Mercedes that I have had experience with (450SEL, 500SEL, 420SEL and a few others), the spark plug wire ends had the suppression resistor built into them. Any more resistance for any reason (bad wires, cap, rotor, etc.) will reduce the temperature of the spark. And may reduce it's effectiveness. These vehicles were designed to run non resistor plugs. And they ran solid, non-resistor wires.

By having the resistor contained in the plug wire end, provided a much more stable and consistent resistance than say resistor wires (which was the cheaper and favored method chosen by the U.S. auto manufacturers). Which vary by type of wire and length of wire.

For instance, new carbon impregnated core resistance wires have about 5,000-10,000 ohms of resistance per foot. And this is per foot not per wire. So a short wire might have say 10,000 ohms of resistance, but a long wire might have as much as 30,000 ohms or more. Pretty uneven with the plug with the longer wire getting maybe less than 1/3 the spark energy of the plug with the shorter wire. Think that might cause some unevenness in how the engine runs? Then, as those wires age, that can go up quite a bit. So a long plug wire that has some age on it may have over 50,000 ohms of resistance. That means a pretty weak spark at the plug gap.

Now another style of resistance wire, Monel conductor wires (which use a solid wire wrapped around a flexible core) will have about 1000 ohms per foot. These are also known as "spiral wrap" or "magnetic suppression core" wires. They provide excellent EMI suppression with much less resistance and therefore allow much more energy at the plug. And being real wire are much more stable over time.

But this really doesn't concern us as this is primarily for American cars. I just wanted to include this as part of this discussion to illustrate why Mercedes (and BMW by the way) chose to use resistance ends on their wires instead of resistance wires. Choosing instead to use just plain old, very low resistance (and impedance) solid conductor wires. Doing it this way, provides for a very stable, uniform resistance for ALL the plug wires in the system. That way the engineers knew that they could rely on relatively consistent spark energies at ALL of the plugs in a vehicle.

Now the issue of using resistor plugs in a vehicle designed for non resistor plugs, welllll, first of all, it WILL result in less spark energy than the engineers counted on when the vehicle was designed. Will that cause a problem with YOUR vehicle? I suppose a lot of it will depend on the condition of your exact vehicle. How is the compression? How are the other parts of the high voltage side of the ignition system? How well is the fuel being metered?

Will it damage your ignition system components? Ignition coil secondary voltages can be VERY high. And if there is excessive resistance in that secondary path, well it's possible that that high voltage may find a different path to ground. Either partly or fully. And if that is back through the coil primary wire and into the ignition control module, well, that can't be good. No, it might not ruin the module. At first. But it won't do it any good either. And by increasing the resistance in that system at the plug, that means that since less energy is going to be dissipated by the spark at the plug gap, that the wires, coil, cap, rotor are going to experience more voltage across them and thereby make their condition more critical.

Will resistor plugs "slow down" the spark. Well, yes, and no. So yes, but it makes NO difference. As the voltage builds in the coil secondary (it doesn't happen instantly, but it is very very very fast) the less resistance to ground through the high voltage ignition system, the lower the voltage that the spark will occur. So with ALL solid, NO resistance components anywhere in that part of the system, the spark is going to occur at the lowest voltage, and therefore the earliest part of the coils secondary output curve where that voltage occurs. But as resistance is added, the point where there will be sufficient voltage to bridge the gap will occur later in the coils output cycle. But as I said, this happens, while not instantly, it is a very very short period of time. At least probably sub milliseconds and maybe even down to microseconds (did I say that it happens really, really fast). And considering that the point of highest resistance in the secondary systems is the plug gap itself, across which the resistance is tens or hundreds of MILLIONS of ohms, an additional 5,000 ohms, isn't going to matter in relation to the time that the spark occurs. Just how hot that spark is once the spark is initiated.

Personally, I'd prefer to run non resistor plugs in any vehicle. But certainly in any vehicle where the manufacturer specifies non resistor plugs. However, it seems that there is some debate about the availability of non resistor plugs in some plug types and some markets. I know the last time I put plugs in my 96 M120 powered C140, yeah, it took a couple of hours for the local import specific auto parts store (BAP) to get them rounded up, but it wasn't like there was a lack of availability. Maybe I was just lucky? At least I didn't need to go to the stealer to get them.

As to plug construction materials, I have heard a few things here and there. The local Mercedes store has one tech that is not only one of those super sharp Mercedes techs, he is also a Mercedes enthusiast that personally owns several older Benzs himself, personally. His recommendation to me was to NOT use platinum or other "fancy" plugs in the older series Mercedes that don't require it. Just use regular old copper core plugs. He related to me that one of the primary causes that he's seen of cars that were in otherwise very good condition, running rough, and high fuel consumption, was some boutique shop putting in platinum plugs. So, take that for what it's worth, and decide for yourself. But why would I pay almost as much per plug for platinums as a whole set of the right ones cost unless it is going to make an noticeable difference in performance or gas mileage. Thanks, I'll save my money for a nice prime rib at the "nice" steak house.

But if anyone has done track or dyno testing and found that something DOES give a noticeable performance advantage, PLEASE relate your experiences.

Karl M.