Danger: RESISTOR SPARK PLUGS - PeachParts Mercedes-Benz Forum

suginami · #1 08-25-2007, 02:05 PM

Anybody read this thread?

Why non-resistors plugs?

Also, relative to Platinum plugs, I did find this:

In the Technical & Restoration Forum in the March / April 2003 The Star Magazine, there is a question about platinum plugs and an interesting reply. It doesn't say who exactly wrote the reply, but it was probably Stu Ritter:

After I put Bosch Platinum +4 spark plugs in my 190E 2.6, a dealership advisor reprimanded me, saying they aren't recommended by Mercedes-Benz. He said that a service bulletin indicated that they should not be used. Is this so?

Some time ago a Mercedes-Benz service bulletin recommended against using fine single-wire platinum electrode spark plugs in their engines. The electrode on these plugs was so thin that the spark could be blown off the plug, causing misfires. The new Bosch +4 spark plugs are not covered by that old bulletin. Thin-wire plugs could cause poor idle quality and misfires at high rpm under full throttle. Mercedes-Benz has not recommended against the newer +4 plugs, which have four electrodes so are not about to have the spark blown off. You might tell your service advisor that serval versions of Bosch platinum plugs exist.

MB-Dude · #2 08-25-2007, 02:26 PM

Quote:

Originally Posted by suginami

Anybody read this thread?

Why non-resistors plugs?

Also, relative to Platinum plugs, I did find this:

In the Technical & Restoration Forum in the March / April 2003 The Star Magazine, there is a question about platinum plugs and an interesting reply. It doesn't say who exactly wrote the reply, but it was probably Stu Ritter:

After I put Bosch Platinum +4 spark plugs in my 190E 2.6, a dealership advisor reprimanded me, saying they aren't recommended by Mercedes-Benz. He said that a service bulletin indicated that they should not be used. Is this so?

Some time ago a Mercedes-Benz service bulletin recommended against using fine single-wire platinum electrode spark plugs in their engines. The electrode on these plugs was so thin that the spark could be blown off the plug, causing misfires. The new Bosch +4 spark plugs are not covered by that old bulletin. Thin-wire plugs could cause poor idle quality and misfires at high rpm under full throttle. Mercedes-Benz has not recommended against the newer +4 plugs, which have four electrodes so are not about to have the spark blown off. You might tell your service advisor that serval versions of Bosch platinum plugs exist.

Good finds! Is it possible through legend and folklore that the real issue – do not use fine-wire platinum plugs – evolved into ‘do not use resistor plugs’. Maybe the logic went like this… MB says ‘do not use fine-wire platinum plugs’. Now we all know that the vast majority of fine-wire platinum plugs are also resistor plugs. So maybe to keep things easy, people evolved the warning to include resistor plugs – if you do not use resistor plugs, you’ll never use a fine-wire platinum plug. Could explain a whole lot.

Cheers,
Jeff

suginami · #3 08-25-2007, 03:46 PM

Quote:

Originally Posted by MB-Dude

Good finds! Is it possible through legend and folklore that the real issue – do not use fine-wire platinum plugs – evolved into ‘do not use resistor plugs’. Maybe the logic went like this… MB says ‘do not use fine-wire platinum plugs’. Now we all know that the vast majority of fine-wire platinum plugs are also resistor plugs. So maybe to keep things easy, people evolved the warning to include resistor plugs – if you do not use resistor plugs, you’ll never use a fine-wire platinum plug. Could explain a whole lot.

Cheers,
Jeff

I am not an engineer of any sort, so I can't comment on whether or not using resistor plugs will damage equipment, but I don't see how everyone that has posted before me on this thread can argue that it is not a bad thing to increase the resistance by using resistor plugs.

300holst · #4 08-27-2007, 12:53 PM

This has been an interesting discussion. Because so many report poor running with resistor plugs, there must be a reason. Whatever this is, I'll add a few statements to the discussion.

1. I did a search in AllData for any Mercedes TSB relating to spark plugs. The only one I saw was for a change in the heat ratings. Nothing relating to resistor plugs in the older MBZ. I used my '86 300E as the search vehicle. This is not to say that there might not be a TSB on this listed for another model.

2. The comment that the spark coil is a form of transformer which has a fixed ratio of windings between the primary and secondary is correct. However, it is not then correct to say that the secondary spark voltage will always be the same. The sequence of operation is this:

a. A 'steady' current passes through the primary from the battery. This current is controlled by the ignition control device, computer, or in older cars, the points. This current builds a magnetic field in the windings of the primary.

b. When it is time for a spark, The current in the primary is abruptly cut off. The magnetic field in the primary winding collapses and this collapsing field generates a high voltage in the primary winding. It is more than 12v.

c. The voltage in the primary induces a voltage in the secondary. The faster the primary voltage collapses, the higher the voltage becomes in the secondary. Of course the ratio of the two winding affects this as well.

d. The voltage in the secondary rises until one of two things happens. Either the magnetic field in the primary has completely collapsed or the spark plug fires. The design is for the plug to fire as the voltage in the secondary reaches XX volts ( I don't know exactly but I think someone in this thread may have it).

Now, the major resistance to the spark occurring is the gap in the plug. The resistance in the wiring, the rotor, and possibly the plug adds to this. I would think that the added resistance causes the secondary voltage in the coil to have to rise higher before the spark occurs at the plug. This higher secondary voltage may then lead to breakdowns in insulation in the spark wires or internally in the coil. I don't know how big a safety margin the engineers build into the insulation but I have read that a too large gap in the plugs can lead to breakdown in the coil. The added resistance of a larger gap must be much greater than the few thousand ohms of resistance in a resistor plug, however.

The original purpose of the added resistance in the spark circuit was to reduce radio interference. The resistance dampens the 'ringing' or voltage oscillation which occurs because there is a resonant circuit in the coil which can briefly radiate an electromagnetic wave following each spark. This wave can be heard as spark noise in an AM radio. We used to add 'RF interference spark suppressors' to the spark plug connectors in the olden times. As far as I know, these never hurt the coils.

I wonder if the argument that the problem lies with the size of the center electrode may be worth following. I looked and I saw that the center electrode in the HR9DC and the H9DC seem to be about the same. I know the platinum plugs have a very small center electrode. The small electrode has advantages in initiating the spark but maybe the spark is more easily 'blown off' the electrode. I sure don't know.

MB-Dude · #5 08-27-2007, 08:11 PM

300holst - Excellent description of coil operation! You’re right on the mark with one small modification. In any given electrical circuit, regardless of in-series resistance, if there is an open in the circuit (such as the gap of the spark plug), the full voltage potential is always felt across the opening, until a spark is initiated.

What this means is that you can have as much resistance as you want between the coil and the spark gap – as long as there is a path for current – and whatever voltage is on the secondary will be felt volt-for-volt across the spark gap. This is because until the gap arcs, no current flow is generated through the high voltage cable. No current flow (until the spark arc) means no voltage drop across the spark plug resistance. This is basic Ohm’s Law.

However when the spark does occur and current flows through the high voltage cable, then there will be a voltage drop across the resistance of the spark plug. Undoubtedly, this would generate a weaker spark since not all of the energy of the secondary would ‘make it’ to the gap – some is lost in the resistance of the spark plug, along with cable loss, etc..

Thus in your beautiful explanation, as the secondary voltage rises – regardless of in-series resistance – the exact same voltage potential is felt at the spark gap – until the instant the spark fires. So again, the secondary coil cannot rise higher than what the spark gap is set for. Therefore, spark gap has a significant impact on how high the secondary voltage rises. But just the spark gap. This is why adding the old school ‘RF interference spark suppressors’ would have no impact on the secondary – as long as the spark gap remains constant. They are just like adding resistor spark plugs. And in the old days, engines only had simple points, rotor and distributor caps for engine timing – no electronics. This is what makes the alleged MB stance and elusive TSB so perplexing… there’s no electronics, etc. to burn-out in the older cars!?!

Further in your post, you describe the effect on secondary coil life if the gap is widened. This makes perfect sense, for the gap distance is directly proportional to the voltage needed to arc. Bigger the gap; higher the voltage needed. Again, regardless of the amount of in-series resistance.

Yes, this thread has brought a number of very valid points (no pun intended) to the table. There are two paths that I think deserve further study:

1. Center electrode size, platinum in particular, and the ability of them to remain intact. This seems to be an underlying item that comes up periodically. And makes more sense than not using resistor spark plugs.

2. Possible coupling back to the primary, from the secondary, when the secondary is collapsing. Is there a possibility that the use of resistor plugs – in SOME applications – causes ringing or abnormal pulsations that the Ignition Modules may not be able to handle? I do not worry about coils, caps and rotors, though it is likely their life could be shortened by having a longer duration spark.

This second item is pure speculation on my part, but electrically, it is possible. I only bring it up as a further discussion point and I look forward to other views/thoughts. Because there is no TSB to read directly – and I really wish someone could/would produce it – we have no alternative but to discuss the subject and piece together ideas.

Cheers,
Jeff

300holst · #6 08-28-2007, 11:11 AM

MB Dude, yes, you are right about the full secondary voltage appearing across the plug gap until the spark occurs.

About the plug points. Electrons will collect and jump between the closest sharp points on the plug electrodes. As the center plug electrode erodes though use, it becomes rounded and the spark can be less strong or sometimes no spark can jump ( a miss). We used to pull the plugs, file the plug points flat, and reinstall.

The platinum plugs have a very small center electrode. This should provide a good 'jumping off place' for the spark. Because the platinum is more resistant to spark erosion than the old copper points, the center electrode should provide a good spark longer. This also lengthens the life of the spark plug, a good thing with plug replacement so hard on many engines. The smaller point may also allow the spark to occur with the same secondary coil voltage but a larger plug gap. As I understand it, a longer spark has advantages in igniting the air/fuel mixture. All of this begs the question of why platinum plugs do not work well in some engines.

As I mentioned before, the HR9D plugs I was sent seem to have copper electrodes which are about the same size as the H9D plugs my 103 engine requires. No platinum. I haven't tried the resistor plugs since I was able to buy a set of non-resistor locally. Maybe these resistor plugs will work well, I don't know.

What I'd like to try is to rig up an inductive pick-up for my oscilloscope which would allow me to compare the spark waveforms on the plugs. Then I wold replace one of the regular plugs with a resistor plug and try to see the difference. I would not have a voltage calibration but I should be able to do a rough comparison the the two waveforms.

I don't think that the added plug resistance would produce any ringing in the coil. The resistance actually would serve to dampen oscillations. This is why resistor plugs reduce radio interference.

This all has my curiosity heated up. I imagine somewhere on the web there will be a good description of all of this stuff. There must be a reason why some engines, and their ignition systems, work well with platinum-resistor plugs and others do not.

MB-Dude · #7 08-28-2007, 02:15 PM

Quote:

Originally Posted by 300holst

... What I'd like to try is to rig up an inductive pick-up for my oscilloscope which would allow me to compare the spark waveforms on the plugs. Then I wold replace one of the regular plugs with a resistor plug and try to see the difference. I would not have a voltage calibration but I should be able to do a rough comparison the the two waveforms. …

Go, 300Holst, GO! Objective, real-world data – I love it! I really do not care whether non-resistive or resistive plugs should/could be used. What I am REALLY interested in is getting data so that intelligent decisions can be made on the subject; one-way or another. We will have to supply some theory, unfortunately, since MB seems to be taking an ‘arms length’ approach.

Quote:

Originally Posted by 300holst

... This all has my curiosity heated up. I imagine somewhere on the web there will be a good description of all of this stuff. There must be a reason why some engines, and their ignition systems, work well with platinum-resistor plugs and others do not.

Absolutely agree! I have spent much time searching, but have come up with nothing. In addition, JamesDean (seems funny typing that) reports ‘no joy’ in his inquiry to the MB Classic Center. His post is #15 of thread… Why non-resistors plugs?

We must separate the legend & folklore, and emotion, from the process and prove – or disprove – the thinking that resistor plugs are OK in certain applications. It simply does not make sense that they cannot - at least with the theory we know. Maybe in some ignition designs, such as the Waste Spark design (as written by Arthur_Dalton, post #5 of Why non-resistors plugs?) resistor plugs are a hindrance. But in the vast majority of applications, resistor plugs should be usable. 300holst, I eagerly look forward to your findings.

Cheers,
Jeff

pmoinot · #8 09-06-2007, 06:01 AM

Quote:

Originally Posted by MB-Dude

300holst ...However when the spark does occur and current flows through the high voltage cable, then there will be a voltage drop across the resistance of the spark plug. Undoubtedly, this would generate a weaker spark since not all of the energy of the secondary would ‘make it’ to the gap – some is lost in the resistance of the spark plug, along with cable loss, etc..

Thus in your beautiful explanation, as the secondary voltage rises – regardless of in-series resistance – the exact same voltage potential is felt at the spark gap – until the instant the spark fires. So again, the secondary coil cannot rise higher than what the spark gap is set for. Therefore, spark gap has a significant impact on how high the secondary voltage rises. But just the spark gap. This is why adding the old school ‘RF interference spark suppressors’ would have no impact on the secondary – as long as the spark gap remains constant. They are just like adding resistor spark plugs. And in the old days, engines only had simple points, rotor and distributor caps for engine timing – no electronics. This is what makes the alleged MB stance and elusive TSB so perplexing… there’s no electronics, etc. to burn-out in the older cars!?!

Except for one thing: The spark doesn't occur instantaneously. As the air/fuel mixture begins to ionize, it does draw current delaying the spark as the secondary voltage continues to rise and the dV/dt drops.

-Phil