spark plug question

Can you use a CR9E instead of a CR8E ?


The heat range of the plug has to match what the engine does to it. It has to be hot enough to stay clean, and cold enough not to become damaged by heat. A CR9 will foul.


The heat range of the plug has to match what the engine does to it. It has to be hot enough to stay clean, and cold enough not to become damaged by heat. A CR9 will foul.

i concur.

go to the ngk site and they tell you what each denotation of the plug refers to.

you really want to use what is listed in the manual.

Just out of curiosity because I saw some Iridium CR9E's at my local auto parts store.

I remember reading in a manual (I think it was for my g-friend's ttr-125) that if you had jetting problem, to switch to the next temperature spark plug: too lean, go up one number and too rich, down one.

I was thinking that the higher quality Iridium plug would help burn the fuel more efficiently thus making less emissions. As for the jetting changes, maybe it could be taken care of via the power tuner...

Better performance through cleaner burning sounded good to me

But hey! as my name suggests I'm an artist, not an engineer and I tend have an idealistic view of the world!!!

Thanks for setting me straight!:thumbsup:

I remember reading in a manual (I think it was for my g-friend's ttr-125) that if you had jetting problem, to switch to the next temperature spark plug: too lean, go up one number and too rich, down one.

No, that's complete :thumbsup: I sincerely doubt it was in a Yamaha manual.

The heat range is of value to the spark plug only, as I said. Iridium plugs are advantageous because the Iridium electrode can be made a smaller diameter without eroding, and because it is a better conductor than steel. Electricity is solid particles in motion (albeit really tiny ones), and as such it has inertia, and as such it takes less energy to jump a gap from a sharp point than a fat, stumpy, cut-off rod. The bottom line is that the iridium plugs require less energy to create a solid spark, which makes them more likely to fire when partially fouled, cold starting, etc. Worth the money, IMO.

The "R" in the plug number is important too. It indicates a resistor plug designed to suppress radio frequency (RF) emissions. You probably don't have an AM radio on your YZ, but this is more than a courtesy for the guys in the pits trying to keep tabs on the ball game. It's a little rare, but RF from your own coil wire can occasionally interfere with the operation of the CDI on your bike.

The basic plug you want is a CR8E. The iridium version I like is the CR8EIX. It has a standard pattern ground electrode.

I am having an issue where the bike runs fine for 1 or 2 rides but then it will foul out. Is there any split tail plugs that are comparable to the CR8 that would be any easy fix instead of putting a leaner pilot jet in?

If you're bike is fouling CR8E's there's a problem, and a "split tail plug", or some other magic feather fix isn't going to cure it. Either you're too rich (which never surprises me), your spark is weak (in which case iridium plugs will give a slight improvement), or you're burning a lot of oil.

One thing a 426 or 400 does not like is being started and run for a minute, then shut off without ever warming the engine all the way up. The 450's seem more tolerant of it, but they have hotter spark, too.

your spark is weak (in which case iridium plugs will give a slight improvement),

Iridiums have more resistance than copper cores (about 1K Ohm more).... I would think if your spark was weak, you would want to choose a plug with the least resistance.... right?

The resistance at the integral resistor exceeds by several times over the resistance of either plug core. You should also understand that the center electrode of the plug is never made from copper, simply because it can't take the heat. "Copper core" plugs are copper only from the HT terminal to the carbon resistor. From that point, the electrode reverts to a high iron steel alloy.

Apart from that, the resistance of the plug gap is vastly greater than the resistance within the plug itself, regardless of the configuration, and especially under full compression pressures.

The issue is in getting the spark across that gap, and a smaller gauge electrode is more effective at doing that than a larger one. That can't be done with steel, but it can with iridium, platinum, tungsten, and other metals. This article goes into more detail:

Yeah... I already knew most of that. I just read through a conversation about plugs on a performance boating forum that I visit. The general consensus is that copper cores have less resistance and for weak ignitions, this would be a benefit.

That may be the consensus, but it has no basis in actual fact. It simply ignores too many factors.

That may be the consensus, but it has no basis in actual fact. It simply ignores too many factors.

I agree... lots of factors for sure. But the bottom line (and my point) is if you take a DVOM and check the resistance between a copper core and an iridium plug, you'll find 1000 ohms more resistance in the iridium.

Now thats 1000 more ohms of resistance in an ignition system that can be eliminated by using a cheaper copper core plug.

What about side gapping standard CR8E plugs or running a CR8EK, which is basically a side gapped plug with two ground straps?

You are obviously comparing a non resistor plug to a resistor type, and then attributing the difference to the material. Three feet of .015" iridium wire wouldn't have 1K ohms resistance.

Now take the VOM and tell me what the reading is between the top terminal and the ground electrode. THAT is the path that has to be traveled. Your VOM is using 9 volts or less to traverse the 1000 ohms you are complaining about. In the open air, it takes about 12,000 volts to jump a gap of .025". Air is an insulator, and the denser it is, the better it works, so under compression, the voltage required is much higher than that. The trifling 1K ohm offered by the resistor pales in comparison to the load imposed by the plug gap, and is inconsequential in terms of spark strength.

Ironically, from your point of view, increasing the resistance in the plug strengthens the spark, rather than weakening it. As the magnetic field of the primary windings collapses on the secondary coil windings, the voltage in the coil secondary begins to rise. If you have ever done any engine diagnosis using an oscilloscope, you will know that the secondary voltage will rise only as much as is necessary to create a current flow in the circuit, i.e., a spark across the gap. A shorted plug shows up as a low secondary spike, whereas too large a gap shows up as high voltage. What widening the gap does, then is hold back the spark until the voltage has risen to a higher level, at which time a much fatter, hotter spark jumps the gap. This spark is more likely to ignite the fuel in marginal situations than a smaller weaker one is. The downside is that if the gap is too big, the spark may not always occur.

It is simple electrical physics that "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."

"The development of precious metal high temperature electrodes (using metals such as yttrium, iridium, platinum, tungsten, or palladium, ...) allows the use of a smaller centre wire, which has sharper edges but will not melt or corrode away."

Iridium, platinum, etc., spark plugs very simply fire across a larger gap more reliably with less energy, and do so for a longer period of time. Not a magic wonder cure all, just an improvement.

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