Comparison of #3 vs #14 AC Rapidfire in a "C" 3800 with Delco Ignition

Shortly after I purchased my Reatta I replaced the spark plugs with what I thought were six AC Delco Rapidfire #3s. Turns out I was partly right.

Those who have read my previous article on the differences and interchangability of the stock (88-90) Magnavox ignition with the later Delco ignition may have noticed that the Delco has a slightly greater spark energy than the Magnavox and that the original plug for the 3800 had a .060 gap, the same as the RapidFire #14 while the AC Delco recommendation in 2000 was for a #3 with a .045 gap, perhaps in recognition that the Magnavox might not reliably fire the larger gap.I note that Delco is now recommending a #14 for just about all n/a 3800s

Since the #14 Rapidfire is recommended with the Delco ignition and several people have recommended it even for the Magnavox, I decided to try a set. The fact that I had noticed a very slight uneveness in the idle at a stop light was another factor.

Pulling the front bank first, I made an immediate discovery. I had not one but two different RapidFire #3s in the engine.



Left to right Rapidfire "late" #3, "early" #3, #14
Note the green band at the top of the "early" plug.


AC Plugs (super trivia: AC stands for Albert Champion who, upon selling his first factory to GMC, immediately went into competion by starting the Champion Spark Plug Company) for many years used a green band at the top of the plug as an identifier making them easily identifiable even in a V-8 engine. Of course when the plug boot became much longer as today, the band was covered. The most noticable difference is in the shape of the electrode - in the "early" plug it is broad and flat like old style plugs. The "late" one and the #14s have a very small diameter pin style electrode. The salient point is that perhaps this mix of two plug types (four were "late" and two were "early") may have be at partial cause of the miss at idle.

Be that as it may, the most difficult thing about the front bank as usual was getting the plug boots off. After that a 5/8" plug socket, 1" extension, and swivel head rachet made short work of the first three plugs.

As usual the rear bank involved a blood sacrifice (much easier than the Fiero though) but did have another surprise: The Bosch p/n 12014 O2 sensor I had installed is considerably taller than the Delco unit (it barely fits into my O2 sensor socket). Tall enough that I could not get the socket assembly onto the drivers side plug on the rear bank without removing the sensor. Just something to consider when purchasing a replacement sensor.


        Bosch O2 sensor and socket.                 Recess with O2 sensor removed & socket on plug.

Sharp bend in wire at top of sensor is from socket. Insulation is intact and seems to work properly. May look into a crows foot scocket before doing the next one. JPG of recess was taken from side but Bosch O2 sensor and socket extension almost touch when sensor is in place.

With a matching set of #14s installed and after allowing to warm up to closed loop idle, there was no trace of the earlier misfire and is now very smooth. The first tank of gas (filled to same point at same pump) which was 292.4 miles of which about 200 miles was at 70 mph posted but included two ten minute traffic jams and the rest mixed local driving and took 11.98 gallons of 87 PON or 24.4 mpg, about 1.5 mpg better than I would have expected

Note: there were no shields on the rear bank when I removed the #3s and did not replace them. I saw no sign of burning on the plug boots though one was very difficult to remove. For more on the different RapidFire spark plugs see the page on RapidFires.

Contents copyright 2004 by Padgett except where indicated.