PeachParts Mercedes-Benz Forum - View Single Post

MB-Dude · #38 08-27-2007, 07:11 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