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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I really don't have anything intelligent to add, just that here in Boston my neighbor introduced a friend of his who rebuilt a carb for my 340 as "The Tuna". So named for his tuning abilities.

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/6 '67 Barracuda convertible, electronic ignition, 4-OD, street cam, SBP KH discs, 3.55 SG 7.25" 1" t-bars. Bilstein.
340 '67 fastback, Doug Nash 5-speed.
1988 Toyota pickup work truck

...or "Pluff....pluffpluff.....pluff....pluff...". You're hearing partial misfires. I listen to the exhaust when tuning, too, and I try to eliminate (or at least minimise) these misfires. You might try to adjust the fuel/air mixture, though it's a little worrisome that it increased when you moved away from the long-nose plugs.

I really, really wish there were such a plug as ZFR6N, ZFR7N, etc.

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Too many people who were born on third base actually believe they've hit a triple.

Talking about spark plugs, what are you guys running. I'm currently running champion RN12YC. Should I go colder and longer with my close to 10:1 /6. If so, what p/n? I get a slight misfire at idle and rattling when I press on the gas over 65 mph. And what about the gap? I had them gapped at 45 before but a friend of mine suggested trying 35. I'm running an MSD 6 and blaster SS coil.

Fred

Dan, I think your description of the sound is what I would hear if I had mufflers on the car. What about chucking a longer reach plug in a lathe and turning the threads off? I could do that easily. Slant, Fred, give these ZFR5n's a try. I think you will like them. If you are runing a carb, I would try to go one step lower on your power valve. Maybe that is higher, but the one that will open at a little higher vacuum reading. What kind of carb are you running? Does it have a tunable power valve? Or, step up the metering rods if that is possible with your carb. The old Carter 2bbl BBD's had a great, and very tunable metering rod system. Very simple to adjust without any disassembly other than removing an inspection plate. The engine in this car was very prone to detonation at light throttle when it was carbureted.

Sam

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