Wow, I love this site!! No bull.. here. We'll not much anyway. I started waxing so first: I just replaced 12 plugs on a E320. Single electrode Bosche, 88k miles on plugs, engine running poorly, so I guessed and got it right. I didn't notice if resistant or not. But the new plugs were dual prog Bosche. Old plugs had the fine point and single plated wedge ground. Inspecting them I could see worn multiple tiny bumps on the electrode near the ceramic and much ware on the wedge. I wish I kept them to look again. I remember tiny nodules on the electrodes, too. You need a magifier to see them well. Rounder surfaces and wear giving wider gap requires longer time for the voltage to build before it can jump and harder to sustain = shorter spark.

Why Two plugs? A clue is two coils, one for each plug. I'm guessing the computer can fire the coils independently and the best use of that would be to give a longer spark.

We used to file the old plugs to give sharpe surfaces at the edges for the spark to jump from and to, reducing the needed voltage and increasing the time the spark continues. The lower the voltage the longer the gap sustains the spark, given equal losses (gap mostly) accross the gap. The multiple electrodes allows for more wear points for the gap. Spark jumps the shortest distance first. I've never looked to see if multiple sparks occur, but my thoughts are if multiple jumps occur, the gap is broader and can distribute the spark leading to a longer duration spark in the chaotic environment of ignition and more surfaces allow longer plug life. My assumption is the 4 prong plugs don't all fire at once, but share the duty maybe unpredictably. I'd love to see the process captured in a video.


Waxing on ignitions: Excellent explanations of the spark / coil circuit and the issues of getting a spark and sustaining it. Nothing better than presenting factual knowledge as opposed to what people think. Its amazing that people believe its true just because they think it.

I've been in electronics since the 50's and saw the first transitor ignitions come in. Incredibly, mechanics put ice picks through the insulation to measure dwell!! I worked in a ford dealership (1965-66) and the mechs routinely pulled the new transistor point units because they didn't understand them or how to measure new low current dwell signals with old equipment. But they were sure they were doing the right thing without trying to understand.

Keep the high level of technical knowledge flowing, Please!