I made the comment in the last spark plug thread that when I replaced the extended tip plugs NGK ZFR5N with colder, non extended tip plugs, ZFR6F-11, it was clear that the extended tip plugs ran better. I think that was a gross over simplification of things, and would like to explore the difference,and what that might mean to any one tuning up a 4 stroke, gasoline engine.

For starters, I have always tuned listening to the exhaust, with the goal of quieter, and smoother. There is nothing scientific in this. Over the years, I just seemed to be able to corelate a positive relationship between quieter/smoother with better performance, and better economy. So the comment that the extended tip plugs made the engine run better was based on the sound of the exhaust.

The colder, non extended tip plugs run fine, but at an idle, the exhaust pitch is deeper, and much more full of little burbly, back fire kind of noises. It has a gentle, but very clearly audible: p-pop......pop.........pop.pop...........pop.........p-pop........pop.pop...., sort of rythm to it. Keep in mind that I am running open exhaust, through the turbo, and you might not be able to hear this with mufflers on. My assumption is that this popping is unburned fuel from inbomplete combustion, at least at an idle. But like I said, the entire tone is deeper as well. I should point out that I am a piano tuner by trade, and can't help listening to things in this way.

So what does this mean, really? Am I way off track here? Is there something else that needs to be tuned now? Or is this sound really indicating something better, and a sign of impending greatness. Anybody out there listening to their exhaust to tune? What do you listen for?

The preignition was reduced somewhat by the colder plug. I feel like I am still headed in the right direction. I can get into it further before the detonation gauge starts to light up. So, I guess changing plugs was the right thing to do. I am just wondering what I lost, or if I lost anything.

Sam

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