The last analysis just exceeded my ability to comprehend. I was good up until then.
Basically, the guys have both simply pointed out that irridum plugs can create the arc with less voltage. Circuitsmith alluded to the higher field density at the center electrode due to the smaller diameter, and TT pointed out that in addition to that the irridium can more readily release the free electrons that begin the arc. The term “apparent dielectric insulation” might be open to discussion because it’s a relative term, but the truth is that they both corrected my clearly incorrect absolute statement. I accept their corrections and hang my head in shame.
What Plugs Are Specified In The Owner’s Manual Or On A Sticker Under The Hood ? My Guess Is AC Delco And They Very Well Could Be Iridium Plugs. They Work Great. Use What Is Specified.
This Isn’t Rocket Science. Why Experiment ?
The Owner’s Manual is your key to successful maintenance.
What model-year is this truck and what engine have you got ?
CSA
Food for thought and question. One spark plug site shows that normal voltage required to fire the plug during starting and running, etc. is in the 20-25k volts range with Iridiums being slightly less.
Every car I’ve seen on the oscillscope (no matter make, engine, or plug) shows firing voltages of 10k volts or less. Anytime a voltage spike is shown up in the 15k volts range (even slightly less and definitely more) this generally means a bad spark plug and/or failing wire, etc, etc.
So where do they come up with this 25k volts thing?
When you’re looking on the scope the engine is idling, so the gas pressure in the cylinder at the time of spark is relatively low.
With lower pressure it takes less voltage to initiate a spark.
If the intake manifold has 20 inches of vacuum at idle the absolute pressure is about 1/3 of atmospheric.
And the cylinder pressure is about 1/3 of what it would be at wide open throttle (with no super/turbocharger).
If you could use the scope on a drive with the engine loaded and the throttle open you’d see the spark voltage go up and down with the intake manifold pressure.
p.s.: as an experiment you could have a helper do a stall test with an auto trans to momentarily load the engine.
The spark plug site (Denso) showed the voltages don’t vary that much from idle to elevated RPMs.
The engines I’ve watched on the scope show a voltage drop when the RPMs increase.
“The engines I’ve watched on the scope show a voltage drop when the RPMs increase.”
With the engine loaded?
The gas pressure at the time of spark should make a difference.
The decrease with RPM probably has to do with the coil having less time to build up primary current.
uh… YES!!!
Here is a picture of 4 iridium spark plugs at 225,000+ miles. They still work. You be the Judge… though I got them replaced with another set of iridium that will last another 225,000 miles.
My non-turbo PT Cruiser came with standard plugs that need to be changed every 30,000 miles and the upper intake manifold has to be removed. When I hit 30,000 miles I went to the dealer and found out that the turbo motors use the same coil packs and wires but use iridium plugs that last for 100,000 miles. If both my car and I last til 130,000 miles I will let you know how the iridium plugs worked.
I think you all have just saved me some money. I had put iridium plugs in my truck 100,000 miles ago, and am now coming up to 200,000 miles and was getting ready to replace them. Owner’s manual calls for changing them every 100,000 miles. I will take them out and check them, and if they look OK, put them back in. The truck will likely not last much longer, due to rust in upstate NY, but I am waiting for a more fuel efficient pickup to come along. I need a truck for all the things I do with one. Last time I changed the plugs, I had no trouble taking them out, and put anti-seize on the threads, so I anticipate no problems checking them. Thanks for all your input on this. I shpuld be able to get another 50,000 out of the truck, but if something major goes, then it will go.