12 Spark Plugs

I find the spark plug issue interesting. Originally Champion spark plugs were two piece and had a number from 0 to 38. For 1932, Champion was starting to supply it's new plug series made with "Sillimanite" insulators. They were still two piece plugs but they had a "C" in front of the number. The earliest reference I've seen for the 12 cylinder engine is in the September 1932 issue of Automobile Digest, ( it is reprinted in the Auburn 1931 - 1933 Service Manual,, available from the ACD Museum Store). It calls for a C7 plug. There were three "C" series plugs available, C1, C4, and C7. The first two are 7/8" thread and the last is metric. I believe Champion was trying to reduce the number of plug styles and replacing the old numbers with one of the three plugs in the C series.

I think the Model A Duesenberg may have come with Champion 9 or Champion 10 and later could have been changed to Champion 7. I believe, after 1932, Champion recommended C7. The replacement plug today would be D16. The club Duesenberg Technical Advisor would have better information.

This whole thing got even more confusing because the center cores on a two piece plug could be purchased and they used a letter with the plug number.

The 1932 eights came with Champion 2 ( as listed in the February 1932 issue of Automobile Digest. I have an old Champion Chart that says the 2 was replaced by C4. I believe the C4 may have been installed at the factory during 1932. I also have an old Champion Chart that says todays replacement for the 2 is W18 and the replacement for the C4 is W16Y. It is a small heat range difference between the two.

I sometimes wonder how accurate the old charts were. I think asking for advise on what works is the best bet. My 1932 eight works very well with Champion W18.

Justin Kerns wrote: Andy,

Cord L-29 was 7/8" Champion C7. The W18 in your 8-98 are 7/8". So how could the Model A, which is a much earlier design than those, use 18mm? Maybe metric was adopted earlier in racing engines? I don't know the answer but it's worth confirming. What was the OEM plug for Duesenberg Model A & J?
Justin

To my biggest surprise all ball bearings in the rearend and transmission of the Duesy A are metric and of a very common size, as the ball bearings in the Auburn 8-98 tranny.
So I could buy these bearings for few Euros in every bearing shop around town.
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Andy,

It's easiest to trace if you know what the original factory plug was. So for example:

Jack is correct - OEM plug for the Auburn 12 was Champion C7. The only difference between this plug and the C4 was that the C4 was 7/8" and the C7 was 18mm. C7 was subsequently replaced by D16 and this is a plug that is still available cheap ($2.30 each) today.

Some folks have changed from the D16 to the D21 which is simply a hotter burning plug with a wider temperature range than the D16. The general consensus seems to be that if you run D16 and they get fouled then switch to D21.

In my case our engine came with D16 that are fouled but I'm going to replace with new D16 and see how they do. I don't want to increase the temp yet without personal experience with the engine. If the new plugs also foul quickly then I will try D21.

In the case of your Model A if it has D16 plugs now then it must use 18mm plugs and you have the same options as I do. Although Model A is pretty early and I'm surprised the plugs aren't 7/8". Many late 1920's cars still used 7/8" plugs and some even into the early 1930's such as Ford Model A, etc.

Cord L-29 was 7/8" Champion C7. The W18 in your 8-98 are 7/8". So how could the Model A, which is a much earlier design than those, use 18mm? Maybe metric was adopted earlier in racing engines? I don't know the answer but it's worth confirming. What was the OEM plug for Duesenberg Model A &amp; J?

I'm still learning so I could be wrong and I invite all comments please!

Justin

I am running Champion W18 in my 8-98, even if these have a bad reputation. These are readily available and pretty cheap. Until now I didnt find any other plug for substitution available in Germany.

Whats the difference between W- and J- and C- Champion plugs?
In a Duesy A engine I have had Champion D-16 plugs???
I didn?t found a cross reference to other spark plugs for these engines.

Any recommandations are appreciated.
Thanks
Andy

This post is similar to my other one on head bolt torque, in the sense that old repair and tune up manuals contain much information that is useful to today's owners. Chilton's Flat Rate and Tune up Manual (my edition is 1937) says the Auburn 12 takes Champion C7 spark plugs; for a hotter plug, it says H2. Gap at 0.025.

You can find Chilton's and MoTors manuals at flea markets, Hershey and elsewhere. They cost next to nothing. I wouldn't have an old car without them.

I run Champion J9 ( hotter )in the s/c cars and J6 in the non s/c cars. I have never had a spark plug failure in 20 years. Those numbers in 5 rib plugs are easy to find. Now the pre 1947 two rib plugs for authenticity are another matter.

I understand that C-4 was the factory plug so that is what should be run for authenticity. However this car will not be judged and will be a driver so I'd rather run something newer, more readily available and cheaper!

You guys bring up an interesting point. Do those of you who drive your cars regularly run the OEM plugs even when they are expensive and difficult to find? Or do you switch sets for judging? Or just take the ding for newer plugs during judging?

Justin

K Clark wrote: I think Champion C 4 is original for the 12.
K Clark

I agree with Ken, if Champion C4 came in the car, that is what you use.

I think Champion C 4 is original for the 12.
K Clark

Next installment in my series of "V-12 questions for noobies" is about spark plugs. What do you all run and why?

Our car has Champion D16 in it and they are pretty fouled. I've read that D21 is a direct replacement that runs hotter and provides more complete combustion. That is tempting but if it isn't broken I don't want to fix it. It could just be this car burns oil or the plugs are really old.

I'm not sure if either Champion plug is the best option. I've also learned that some folks run AC C86 and Auto-Lite 386.

Any thoughts on these? Anyone know what the factory plug was?

Thanks!
Justin