Iridium plugs on an FJR?

I'm splitting this off of another thread that was seriously hijacked. I admit that I was one of those participating in the jacking. :blink: Mea culpa! That thread got me thinking about the use of the NGK iridium spark plugs.

When a spark plug fires, as the negative electrode gives off electrons to spark across the gap, the metal electrode is subject to erosion. Iridium plugs are supposed to last longer because the center electrode is made from iridium, a very dense, rare earth metal. Sounds good right?

Below is a picture of one of the iridium plugs I recently took out of my daughter's 2003 SAAB after 120k miles of use. Notice that there is a little build-up of white metal on the underside of the side electrode. This is metal that eroded from the iridium center electrode and was deposited on the side electrode. All four of the plugs looked exactly the same. Not bad for 120k miles. Could'a probably used 'em for a while longer!!



However, when you look at the wiring diagram of the FJR ignition coil, you'll see the problem. Unlike my daughter's car, which has "coil over plug" technology (a dedicated coil for each plug) the FJR plugs are operated in pairs, wired with 2 in series off a common secondary winding of the ignition coil. Picture below:



The primary of the coils are fed by a pulsed DC input. The Red/black wires are a constant +12V and the other side (Orange or Gray/red) is grounded and ungrounded by the ignition system. This means that the output of the coil, the secondary, will always be the same polarity. Current will flow from the negative side of the coil secondary through one plug (from center electrode to ground electrode) then via the metal head to the side electrode of the 2nd plug where current will flow from the ground electrode to the center, and finally back to the + side of the transformer.

Well, I know this is fascinating and all, but the main point of my mentioning all of this is that for 2 of the 4 plugs, sparks will jump from the non-iridium side electrode to the iridium center. Therefore the iridium on those 2 plugs is more or less wasted. Since the side electrode isn't made of iridium it will wear at the normal rate, whatever that is.

Maybe you guys need to rotate those iridium plugs every 8k miles?

While I fully understand and appreciate that there are complex engineering aspects with regard to our ignition systems, I let my actual, real-world experiences form my opinions of spark plug performance/longevity.

I can find no meaningful difference in plug performance between a new set of the garden-variety NGK copper plugs that came with my bike, and a new set of equivalent Irridums. If there is a difference, my highly-calibrated butt-dyno can not pick up on it.

But the tune here changes after around 10,000 miles, or so.

After 10K or so, the bike has a slight but noticeable decline in its pulling power on the copper NGKs, where the Iridiums are still pulling just as strong as the day they were installed. With the Iridiums, I don't start to detect the slight loss of "snort" until around 16K-17K, and I'll generally replace them before 20K.

In my experience and through direct observation, the Iridums are worth it primarily because of their longevity. A long spark plug life is quite valuable on bikes where it is a COMPLETE ass-pain to remove/replace plugs (such as the GTR1400, or the 2nd-generation Hayabusa). On bike like the FJR, where it is a relatively trivial exercise to replace plugs, I could go either way. Yet I use the Irridums on the FJR as well, which allows me to typically ride most of the summer without having to swap out plugs.

YMMV, etc.

Not quite. . . . . .

The coil is not a circuit, it's an inductor. The pulse on the primary induces a high voltage on the secondary coil. The whole coil. At both ends. It's not a circuit that flows from one end to the other, it's a charge induced in the coil whose only escape is the ground on the other side of the spark plug. Both spark plugs. The plugs are parallel to each other, not in series, and the effect of the arrangement is to dry fire one plug when that cylinder is on the exhaust stroke and the other is on compression.

The ground is the ground, and the center is the center. On both plugs.

I think all the guesses are wrong. A transformer has a voltage induced in the secondary when the magnetic field on the primary changes, meaning when the input current starts or stops. When the input turns on, the secondary will spark, and then when the input turns off it will spark with the opposite polarity. The on transition will happen at TDC on one cylinder, and the the off transition will happen at TDC on the other.

But that's just my guess....

BTW, I was skeptical that FJR plugs would really be worn out at 10K miles, but my butt dyno detected loss of smoothness and throttle response at about 12K. New plugs made a noticeable improvement.

wfooshee said:

Not quite. . . . . .
The coil is not a circuit, it's an inductor. The pulse on the primary induces a high voltage on the secondary coil. The whole coil. At both ends. It's not a circuit that flows from one end to the other, it's a charge induced in the coil whose only escape is the ground on the other side of the spark plug. Both spark plugs. The plugs are parallel to each other, not in series, and the effect of the arrangement is to dry fire one plug when that cylinder is on the exhaust stroke and the other is on compression.

The ground is the ground, and the center is the center. On both plugs.

Click to expand...

Sorry, but you are wrong. The secondary circuit of the coil (it's actually a step-up transformer, not an inductor) has no center tapped ground reference. The secondary voltage is floating, not referenced to ground.

You are correct that the whole coil secondary gets a voltage induced into it, but that results in a positive voltage on the center of one plug and a negative on the other one (referenced to each other, not ground). The ground is of no significance to the coil's secondary voltages.

I don't know what you guys are talking about. I changed my stock plugs at 29,000 miles. They looked fine to me. Those Iridium things should go 60, k easily.

You are right Fred you are all wasting money throwing away good plugs.

Send em to me and I will use them.

And yes I really did change my stock plugs at 29k miles. I did not realize they needed to be changed so often. I was just out enjoying the ride.

Warchild said:

I can find no meaningful difference in plug performance between a new set of the garden-variety NGK copper plugs that came with my bike, and a new set of equivalent Irridums. If there is a difference, my highly-calibrated butt-dyno can not pick up on it.

Click to expand...

I too noticed nothing new after changing my plugs to the pricey Iridiums. I have not put on more then about 6K since I did the change out, so we'll have to see how it goes over time.

I would think that since the ground electrode has so much more mass than the iridium center electrode, erosion from metal deposition would be somewhat of a moot point.

I think the real advantage lies in the small diameter of the center electrode causing a higher concentration of electrons allowing them to arc over at a lower voltage and therefore more reliably.

The PO had iridium's in mine, and since I had already bought new ones I put them in, although after looking at the iridium's that came out, I'da never changed them, I still have them.

I'm treating my FJR like your SAAB, they're not coming back out.

Fred W said:

wfooshee said:

Not quite. . . . . .
The coil is not a circuit, it's an inductor. The pulse on the primary induces a high voltage on the secondary coil. The whole coil. At both ends. It's not a circuit that flows from one end to the other, it's a charge induced in the coil whose only escape is the ground on the other side of the spark plug. Both spark plugs. The plugs are parallel to each other, not in series, and the effect of the arrangement is to dry fire one plug when that cylinder is on the exhaust stroke and the other is on compression.

The ground is the ground, and the center is the center. On both plugs.

Click to expand...

Sorry, but you are wrong. The secondary circuit of the coil (it's actually a step-up transformer, not an inductor) has no center tapped ground reference. The secondary voltage is floating, not referenced to ground.

You are correct that the whole coil secondary gets a voltage induced into it, but that results in a positive voltage on the center of one plug and a negative on the other one (referenced to each other, not ground). The ground is of no significance to the coil's secondary voltages.

Click to expand...

FredW is correct according to the schematic symbol as drawn for the coils. In order for current to flow in the secondary both spark gaps have to be bridged, the secondary voltage is floating until at least one gap has been bridged, and one will always have to go plus with the other going minus. I wonder if they didn't do this to try and keep the spark in the compression stroke more consistent, as I suspect that the potential required for the spark between exhaust and intake is far more consistent while requiring a higher potential to spark(compression stroke has compressed air and fuel mixture)? Of course, this could just be the way it is drawn in the schematic, and maybe these coils are more conventional in nature with the plugs in parallel? I bet that's it, just an easy premade symbol in the schematic editor! If that is the case there shold be a ground connection to the coils, or they could just use the 12 volts internally?

Wait a second, here...

Hold on a minute...

What's all this talk about replacing spark plugs about??

:blink: :dribble:

The positive rectifier creates a negative impact on the excremental vault, resulting in a hemorrhagic output. Wait a minute.... what was the question???

Ordered some today I live to waist money.

Zorlac said:

I would think that since the ground electrode has so much more mass than the iridium center electrode, erosion from metal deposition would be somewhat of a moot point.
I think the real advantage lies in the small diameter of the center electrode causing a higher concentration of electrons allowing them to arc over at a lower voltage and therefore more reliably.

The PO had iridium's in mine, and since I had already bought new ones I put them in, although after looking at the iridium's that came out, I'da never changed them, I still have them.

I'm treating my FJR like your SAAB, they're not coming back out.

Click to expand...

Ding, ding ding... give that man a prize!!

Yes, you are quite right, the smaller diameter, pointy center electrode will allow the arc to begin at a lower voltage (for a given gap) more reliably. But that would only apply if/when the center electrode is the negative (electron donor) side of the arc. When reversed, ie when the center electrode is positive referenced to the side, the shape of the side electrode would be the important one.

The goal in using the iridium is that they can make that nice pointy shape electrode and it won't erode quickly like it would if it were softer metal. But the pointy electrode only matters if (when) it is the negative one. Ideally they would make the side electrode a pointy one of iridium, too, then we would be covered in both directions.

I believe that the schematic is accurate. It is an easy way to design a waste spark ignition system and is used on other vehicles as well. My old '97 Ford Exploder with a 5.0L V8 has a similar ignition system in that half of the plugs are fired positively and half negatively. It's one reason Ford specified dual platinum plugs for it. The side electrode is also made of platinum, not just the center.

FWIW the original standard plugs that I took out at 16k miles were still very good looking. I'm sure we replace them all far too soon.

A lot of good points made here,

I prefer the iridiums (based on years of bike tuning experience) due to the longer plug life performance. If you may tend to

not change your plugs often (as Mamaha recommends), the iridiums are the best choice.

BTW, the factory plug change interval recommendations are surely based on the stealerships financial obligations.

WW

Spark plug info:

"Electrons are emitted where the electrical field strength is greatest; this is from wherever the radius of curvature of the surface is smallest, i.e. from a sharp point or edge rather than a flat surface. It would be easiest to pull electrons from a pointed electrode but a pointed electrode would erode after only a few seconds. Instead, the electrons emit from the sharp edges of the end of the electrode; as these edges erode, the spark becomes weaker and less reliable.

At one time it was common to remove the spark plugs, clean deposits off the ends either manually or with specialized sandblasting equipment and file the end of the electrode to restore the sharp edges, but this practice has become less frequent as spark plugs are now merely replaced, at much longer intervals. The development of precious metal high temperature electrodes (platinum, iridium, etc.) allows the use of a smaller center wire, which has sharper edges but will not melt or corrode away. The smaller electrode also absorbs less heat from the spark and initial flame energy."

God help us the engineers are at it again.... :dribble: The great electron debate of 2009 on the FJRForum.

They work, use'm and go. No worries.

Researching a little bit, I found this site, from which I quote:

In a waste spark system, the two spark plugs will share the same coil. So therefore, in this system both spark plugs are attached to the coil and they are fired simultaneously. While one cylinder is toward the end of its compression stroke, its piston pair is nearing the end of the exhaust stroke, and both plugs are then fired.
Even though the spark plugs are manufactured exactly the same, if they are re-installed after there has been some extended use, they should then go back into the cylinder from which they came from. The reason for this is because the electrons will align themselves favorably with one direction of current flow. While the one plug will receive positive potential (voltage) at the wire side, the piston pair will receive the negative potential. The negative potential is what causes the second plug to fire instead of the voltage just going straight to ground after firing the first one.

Click to expand...

So Fred W's point is correct, in that current flow is opposite in each plug on the coil. They each flow to ground, though, I don't follow that they complete the circuit through each other at ground. If I successfully learned to read (not a given by any means,) I see it as one plug has a high positive voltage and they other has a high negative voltage. Ground is 0 volts, so you still have the needed potential across the spark gap. The plugs are still parallel, not series. Think of it as a "tee," like so:



Without that ground, there is no circuit and no spark.

As for the impact of the direction of flow, the fine point of the iridium electrode is still the focal point of the spark, whether it "emits" electrons or "collects" them. I'd never considered the possible effect of moving a plug from one side of the coil to the other as stated in the article. My assumption is that the guy knows of what he speaks, but I saw no credentials. B) I do know that the iridiums last longer, and by enough to offset the price difference.

FJRBluesman said:

God help us the engineers are at it again.... :dribble: The great electron debate of 2009 on the FJRForum.
They work, use'm and go. No worries.

Click to expand...

This from he whose signature speaks of the Piston Return Spring.

Spark ONLY takes place when the magnetic field collapses. The field will collapse ONLY when the primary circuit is opened after current was flowing. This results in inducing a current into the wires and since there are many more turns on the secondary windings, a higher voltage results.

Bogey907 said:

I think all the guesses are wrong. A transformer has a voltage induced in the secondary when the magnetic field on the primary changes, meaning when the input current starts or stops. When the input turns on, the secondary will spark, and then when the input turns off it will spark with the opposite polarity. The on transition will happen at TDC on one cylinder, and the the off transition will happen at TDC on the other.

Click to expand...

What's a spark plug?