Spark Plug recommendation needed

[Jeffrey S.]

Duke,
The "N" and "XL" plugs are identical except for the threading; "N" is half threaded and "XL" is fully threaded. The extended tip is identified by an "S" suffix. So an extended tip would be "NS" or "XLS". Some have said that the extended tip raises the heat range 1/2 a range. Do you know anything regarding this?
Jeff

[Duke W.]

I've heard over the years that extended tips "broaden" the heat range, So instead of a 45 you could run you could run a 44S because the incoming fresh charge will "wash" deposits off the extended insulator before they harden.

I'm not aware of any real science behind this. It's all anecdotal or maybe just pure speculation.

Whether to use a plain or extended tip is like asking should I use 10W-30 or 15W-40 CK-4 oil.

I'd be more concerned with the slight increase in CR with extended tip plugs, especially if I suspected the OE CR was not reduced, at least slightly, at some point over the decades.

So If I had a low mileage L-72 with evidence that it had never been rebuilt, I would use the 45XL, or equivalent. If it had been rebuilt and I know the actual true CR is no more than 10:1 and 93 PON fuel was readily available I think the extended tip would be fine.

So there are two basic options

1. Close your eyes and flip a coin - heads for plain tip and tailsfor extended tip.

2. Experiment with both and choose which one you think is best, but unless the extended type has more propensity to detonate I don't think anyone would be able to tell the difference.

Duke

[Jeffrey S.]

According to my AC Delco "All Product" book published in March 1971, The 45XLS plugs were used in 1960-71 Dodge and Plymouth; 1965-70 Rambler; Kaiser Jeep 1971. R45XLS the sam as above and 1971 Pontiac, GMC truck 1971. Engines are not specified.
Jeff