Spark Plug Cross Referencing

I decided to check a few new plugs I have in stock for my "D". All of the Autolite 3077s measure ~1 ohm using an old analog VOM which passes some current thru the circuit under test.
Imagine my surprise when I checked two brand new Champion W-20s and found that one measured ~3 ohms as expected and the other one measured in the low Megohms!
I didn't set up a test circuit to determine if the questionable W-20 would arch internally at high (magneto) voltage.

Tip - I rotate plugs between the left and right cylinders frequently. This does a nice job of burning off the deposits that accumulate on the plug in the "lazy" cylinder.

Regards to all!
Jon

The older Deeres like other old tractors ESPECIALY when using a Magneto Ignition, were NOT designed for either resistor plugs or resistor wires. Those elements can reduce radio interference but are an added and unwanted load on the low energy ignitions and waste energy in the form of heat and I squared R losses versus discharging all the stored energy across the plugs gap where its needed the most. In addition the Mags have to work harder and can run at higher temperatures when one uses resistor wires or plugs !!!!! !!!

If we used the tractors as they were designed at full RPM and under a full load and at temperatures over 180 degrees, theres a chance the recommended heat range of plugs may yield that nice warm gray color. HOWEVER at light loads and low temperatures and especially at idling we need hotter plugs to avoid premature plug fouling, which is why myself and the other gents recommend use of the hotter plugs.

Hotter NON resistor plugs, keeping the temperature over 180 degrees, and not running the carbs anymore richer then necesary can help reduce premature plug fouling which is a problem ESPECIALLY when using low energy Magneto ignitions.

Best wishes yall, great discussion going.

John T

Magnetos are a gem of engineering, but of engineering done years before there was a need for resistor plugs -- these really to "quiet" the environment for hi-fi sounds systems among other things. They also were developed within the frame of battery-based ignition systems, and now electronic ignition -- surely nothing like you would find on your pre-1947 two-cylinder John Deere!

So when you use a resistor plug in plug as an element in a system designed for a non-resistor plug, you place an added unknown into the total circuit. The output impedance is no longer a match to the impedance of the magneto, and when a mis-match occurs, some of the energy which was to have been transferred is reflected back into the magneto. Reflected energy can overheat the coil, and enough of such abuse will eventually begin to show up in degraded performance.

I write tonight for all to hear -- men like Duane Larson and Glen Schueler have devoted an untold amount of hours trying to bring to us the vey best recommendations in spark plugs and magnetos. These are specialists! Listen!

And Eli -- I hate to burst your bubble, but the sparkplug crossreference you refered me to is wrong. It shows Autolite 388 as a direct replacement for Champion D23. You can go directly to Autolite and it will tell you that their 388 is a RESISTOR plug. 'Sorry! Listen to the experts. (PatB)

A tractor with an engine which is not in the best of shape may gain from a hot non-resistor plug, copper core wires, and if using a distributor system (or a gear-reduction mag) having the center electrode of the plug at negative voltage with respect to ground will at least have the plug/ignition system optimized.

And an ignition system like that can't hurt on any tractor.

In general Champion offers a wider heat range of plugs than AC and Autolite.

18MM non resistor spark plugs:

Champion D23 = AC C88L or Autolite 378
Champion D21 = AC C88 or Autolite 377
Champion D18Y extended tip = Autolite 3116
Champion D16 = AC C87 or Autolite 376 or Denso L14U
For gasoline stay with the hotter plugs and not the D16. Try Brillmans or some of the other vendors or NAPA local if you can't find them. Use the NON resistor type plugs listed above with a magneto and metallic core wire also. Autolite 388 and 386 are resistor type and NOT recommended for mag use although suppliers will tell you they cross to a D21 do NOT use them on a magneto! Info showing the 386 & 388 are suppressor (resistor) plugs can be found in downloadable charts at autolite.com

Non resistor 7/8-18 spark plugs:

Champion W18 = AC C77L or Autolite 3077 or Champion W89D

Champion W14 = Autolite 3076 & 3075

Can you explain to us what is the problem with resister plugs and a magneto? And what is a resister plug, anyway?

Kurt

For a mag John T covered the tech part of it very well above and I mentioned the parts life warranty issues.

Non resistor plugs may have a small sealed gap inside also but not as much and plug core construction will differ having a solid metallic core or laminated metallic cores with little resistance. Aircraft, military and industrial magneto ignitions have used non resistor "shielded" spark plugs and wires for years to control RFI, Radio Freqency Interference also. Some of those shielded type spark plugs are take apart type allowing for a replaceable resistor core to be installed for some pulse type ignitions and solid cores for magnetos as well. Older cars had lots of metal surrounding the engines and that would suppress some of the RFI but later lighter made vehicles and higher voltage pulse ignitions and composite body constructions don't suppress the RFI well.

Early resistor plugs used a carbon type core inside the plug between the metal rod cores to suppress the ignition noise with radios. RFI, Radio Frequency Interference, the tac,tac,tac you hear on radio speakers around a non resistor type ignition source is what the resistor or RFI suppressor type spark plug was made to control. Also later on RFI control was built into spark plug wire cables and some distributor rotors as well. Later resistor type plugs use a sealed controlled gap or gaps inside and different type metal cores and even composite materials to control the RFI. As many different designs of RFI suppressor plugs out there as brands. Other layered metal cores were used in the electrode part of the spark plug to also give longer electrode life to the plug. Most modern resistor plugs can not have the resistance measured properly with a common low voltage ohmmeter and it takes a high voltage pulse (1Kv) to first be applied to eliminate contact resistance and false high readings.

How "bout the rotor in a Wico magneto? Doesn"t this have a carbon contact that would act like the resistor carbon in a plug? Any problems here?

Kurt

PS Hope the fires stay out of your neck of Texas.

The fires were 150+ miles from here E NE of Amarillo but we are still dry for the potential of the same disaster.