Spark Plugs & AFR changes
 

So I have been doing this shit for like 1000 years (or so it seems) and I thought I would post this up as no one has ever bother to do so in the internet forum shit hole world that is non ebay porn related postings.

So here goes:
Who has ever noticed that their full load AFR changes, when you put in a different spark plug (say hotter)?
* No audible misfire, no knock.
* No loss of power.
* Nothing to alert you to say cooler plug not being fully functioning.

I did a test today where when swapping out one set of spark plugs of cooler heat range to a hotter type found an air fuel mixture that normally data logs at ~11.5 to 11.7 range leaner to 11.9 to 12.1 range. Absolutely nothing else changed (same boost, rpm, type of load test, exactly same power and performance measure) just swap out spark plugs, that's it.

Suppose it is not something people are too exposed to unless you have a car fully instrumented as I do or are not testing it every week as well, but wonder what other cunts have seen found over the years, when running legitimate back to back tests ;)

What about fuel? did you buy exactly the same gasoline? Even at the same station, they can get their fuel from a different supplier.

The more lean "mix" could only be incomplete combustion if all other factors are equal. The unburnt oxygen will make the O2 sensor read leaner than the mixture really is. As an oxygen sensor, it can't see the unburnt, fuel, just the available oxygen.

Absolutely nothing else changed, only a heat range in spark plugs. gave that result. Swapping plugs back went back to same base.

I have seen similar things in small scale methanol/nitro engines using glow plugs of different heat ranges.

So the hotter type plug does a worse job. Interesting... I will have to think about why this might be.

For clarification:
Were you running Water injection?
Was this completely under boost, or did it exhibit the same behavior under vacuum/idle?

I have seen mileage improve by changing heat ranges, but I thought I went to a hotter plug to improve the economy. I apologize - unlike your awesome data acquisition systems, I have to rely on my ever-more-sketchy memory.

I will think about what would cause this and post again if I can think of something.

On my system when I have checked it the hotter plug seems to be burning better (not misfiring as much).

In the small scale stuff you can run an engine much richer on a hotter plug v's a cold plug (cold plug needs to be set leaner) to stop misfiring....

Anyway its an interesting thing I noted, am repeating experiment with full CDI ignition system and see what that shows.

interesting. since fuel and air volume are close enough to the same, does that mean a richer AFR = more complete combustion, or is it the other way around?

ive noticed differences too, i'm currently playing with a 12A PP with a 48IDA, and i've tried a few different plugs in the "tuning the carb" phase.

in the idle/low speed range the rx8 plugs actually wanted more fuel than the autolites.

the W/b on a p port is kind of a slippery slope, its on there...

Yep, it's common. Surprised more people don't notice this.