Spark plug wires and ignition coil (specs / resistance)

[MoPar~Man]

I don't know if it matters if we're talking about a big-block or small block with the following stuff, but this pertains to my 318.

We're talking here about points/condensor, not electronic ignition.

Question 1:

Is there a rule-of-thumb when it comes to plug wire resistance if we're talking about points vs electronic or hei? It it prefered or recommended to keep plug wire resistance to something like 500 ohms per ft for points, but higher for electronic?

My current wires (Canadian Tire brand "MotoMaster Premium Silicone Constant Voltage" 20+ years old) are measuring 900 to 1400 ohms, which is working out to 500 ohms per ft.

A couple of brand-name wire sets that come up when plugging in the appropriate make/model/year are:

NGK 53420
Denso 671-8123

I can't find resistance specs for them.

Question 2:

Coil resistance.

I have 5 spare coils (pulled from junkyard years ago). One difference between them is the location of the screw terminals. 9 and 3 o'clock vs 10 and 2 o'clock. What is on the 318 now is 10-2.

The 9/3 might be for slant-6? I can't see any numbers on those.

The 10/2 spares have PN 2444242 and 2495531.

In total I have 3 that are 10/2, one on the car (can't see a number on it) and 2 spares. All 3 measure primary resistance 1.6 ohms. BUT - the spares measure 11k ohms secondary, but the one on the car measures 24k. (The 10/3 coils have secondary resistance 8.5k - 10k).

So I'm asking if these coil resistances are well documented and well known, to the point of being able to say the 24k secondary reading I'm seeing is not right.

(oops - I should have filed this under electrical / ignition...)

 

[CBODY67]

500 ohms/ft is pretty low and great. Considering the spec for the carbon core wires which came OEM were 1000 ohms/ft. NO difference between resistance on points or electronic ignitions.

As to coil resistance specs, they are in the appropriate Factory Service Manual. www.mymopar.com (free download if you don't already have it).

CBODY67

 

[MoPar~Man]

Yes I have the '67 service manual, and yes, I see it has coil specs.

Interesting that it shows coil PN 2444242 for manual trans, 2444241 for auto. Primary coil resistance is .25 ohms less for 2444241 auto (1.48 vs 1.72) and secondary is 650 ohms less (9900 vs 10550). I wonder why the different coils here, and obviously I don't have the right one!

For the 383 and 440, the same two PN's seem to be interchangable, regardless of trans, regardless if clean-air-package or not.

But the much higher secondary resistance I'm seeing seems to be out of spec.

What is VERY interesting is the spec plug - N14Y. Not RN.

That coincides with their plug wire specs, which are high. The manual says to replace cables if they're more than 30k ohms.

If the manual accurately describes these cars as delivered, then they put non-resistor plugs in the cars, but put all the resistance in the wires.

I've searched for why have resistance plugs AND resistance wires, and I've seen others ask it in various places, but there's never been an answer. I get why having a resistance in the discharge circuit is a good thing (minimizing RFI and increasing spark duration) but I've never seen an explanation why you'd need to have high-resistance wires AND a resistor in the plug.

And something I've never seen mention or asked about - what is the clearance between the rotor and tower contacts? The spark has to jump that also. I know it's not adjustable, but I wonder if it's designed to be less, much less, same or more than the plug gap.

I've never seen anyone market a "platinum" or "iridium" tipped rotor ! Why not? The rotor is taking quite a beating compared to any single plug.

 

[CBODY67]

When the cars were built, back then, it was common to use non-resistor plugs with resistor wires. Probably had to do with fewer FM radios in cars back then, with the main FM-band radios being in law enforcement vehicles. Not having resistor wires and non-resistor spark plugs would greatly interfere with the police FM radios, from what I readd back then. As far as production costs, the non-resistor plugs were less expensive.

The resistance on the manual transmission cars probably had to do with their possible higher shift points? Whereas it took a deliberate effort to manually-shift an automatic (which few people did back then, unless they were racing).

Back in the 1970s, I spent an afternoon filing the ground electrodes on the Champion J-14Y plugs in our '66 Newport 383. Then gapping them all to .035", per spec. When I had it at the dealership for something else, I asked them to hook the car to their Sunn engine analyzer to check timing and such. With the new plugs, the traces peaked at 9-10KV on the scope's scale, at idle. I was pleased. ANY coil ought to be able to build that much voltage, as most of them were rated at 30KV, as I recall. The thing to remember is that the coil will only build enough voltage to fire the plug, no more. Which means that hot rod coil which CAN produce 50KV does not build that voltage unless it has to. Under full load at WOT with a 13:1 compression ratio, for example.

As to RFI, even with the resistor plugs, it still can be heard in an AM radio with the hood up. With the hood down, the hood shields the outside radio antenna from it enough to become un-noticed, generally. In later years, resistor plugs and resistor wires are the norm. No problem putting both on earlier cars. That extra resistance only makes the coil build more voltage to fire the plugs. Widening the gap to .040" or so, increases the requirement a bit more, but it still works on a point ignition, from my experiences.

When I went to the spiral-wound Monel conductor spark plug wires ages abo, it was not specifically due to the lower resistance, BUT because there was one piece of wire between each of the terminal ends, rather than a tube of flexible carbon which could increase in resistance with age. Have you ever felt those older wires after 80K miles in an engine compartment? They are stiff and will crack if moved very much, i.e., "fragile", and need replacing. When in place like that, they can work just fine until that center conductor degrades and causes a gap in there.

In one respect, you can go crazy trying to justify all of those numbers. When the reality is that as long as there is continuity in the plug wires, any stock coil will fire the plugs at idle and other low-load situations. Even up to about 5000rpm. From my observations, the coils are much more generic in performance than some allude to. Just my observation on a stock motor.

Every time I've tried to check resistance on a stock coil, with a digital meter, the ohm readings did not correspond with the service manual, yet the engines ran perfectly. Once, I ended up with a "low resistance OEM" coil from a mid-80s L69 Z/28. I put it in my '77 LT 305 with a Holley 4160 and slightly warmed-up cam. I could tell NO difference between the two, but Chevrolet apparently thought it needed to be on the L69 4bbls (which might have been used in some stock-class racing activities where the lower resistance might give them a slight edge in power, possibly. And all parts on those cars had to be in the Chevy parts book.

To me, a low-resistance coil might make another 5 horsepower on a dyno engine, but in the real "street driving world", only about 85% of that 5 horsepower will make it to the ground. Only way to verify that is in a timed drag race. BTAIM

Sorry for the length. Short version. Specs can be nice, but if the engine now runs decently with some ignition items not meeting specs, which is more important? Getting the plugs to burn clean can be from a weak coil, not from an "out of spec" one by a few ohms.

Take care,
CBODY78

 

[MoPar~Man]

I'm measuring 24k ohms on the secondary on the coil I have on the car. The old/spare coils I have measure 11k, and the manual says the spec is 11k.

There's more than a few ohms different between 24k and 11k.

I still don't know why the engine idles like crap and the plugs are black.

 

[Gerald Morris]

But the much higher secondary resistance I'm seeing seems to be out of spec.

What is VERY interesting is the spec plug - N14Y. Not RN.

That coincides with their plug wire specs, which are high. The manual says to replace cables if they're more than 30k ohms.

If the manual accurately describes these cars as delivered, then they put non-resistor plugs in the cars, but put all the resistance in the wires.

I've searched for why have resistance plugs AND resistance wires, and I've seen others ask it in various places, but there's never been an answer. I get why having a resistance in the discharge circuit is a good thing (minimizing RFI and increasing spark duration) but I've never seen an explanation why you'd need to have high-resistance wires AND a resistor in the plug.

And something I've never seen mention or asked about - what is the clearance between the rotor and tower contacts? The spark has to jump that also. I know it's not adjustable, but I wonder if it's designed to be less, much less, same or more than the plug gap.

I've never seen anyone market a "platinum" or "iridium" tipped rotor ! Why not? The rotor is taking quite a beating compared to any single plug.

The higher secondary resistance* is IN spec for the following reasons:

1) Voltage increases by 3 orders of magnitude, namely, from 10 x 1.2 to ~3 x 10x10x10x10, to wit around 30,000 volts. Many modenr coils produce 45,000 volts, but I gave a conservative estimate to demonstrate that things are a little more efficient now. They had plenty high voltage coils even 60 yrs ago too.

To attain this thousandfold increase in voltage, one requires a thousandfold increase in the number of turns of wire to make the secondary. Even copper wire, especially the ultrafine sort used to make ignition coils, has a palpable resistance. Thus, the primary resistance, consisting of 1/1000 as many turns of wire, will measure down in the single digits. My Taylor coil has the rather common .7 ohms or 700 milli-ohm resistance, which I put in series with a resistor which upon heating up, measures about 1.2 ohms. This makes for a primary coil voltage around 7V when the engine is warmed up, assuring I won't be burning up my breaker points or the coil itself anytime soon! The secondary (spark plug) voltage from my coil comes in a nice 45k volts.

2.) Kilovolt spark potential is necessary to insure combustion! Gapping my plugs as per the FSM, I make sure my plugs are at .035", or 0.889 mm. Since in dry air, 11.6 kV is required to cross 1 mm, we need a minimum of 10,312 V, or 10.3 kV just to make the gap in a very lackluster fashion. So, 30 to 45 kV is quite modest. If one wishes to increase the length of the spark, widen the gap, AND RAISE THE VOLTAGE!

3. Higher voltage will cross greater distances, and higher conductive resistances (same thing actually) at a PRICE: REDUCED CURRENT AND POWER! So, make SURE there is enough CURRENT to the coil, both on the primary and secondary sides, to insure enough POWER at the spark plug! This can be attained by larger coil conductors, provided one has the power on the primary side to supply the needed current. WHY do you think those ACCEL Super coils are so damned BIG? Still, the voltage gain just with a heavier coil remains modest, until one increases the primary voltage and current enough to make a big difference.

This stuff doesn't quite scale up linearly, ALAS, as materials break down faster with more heat and discharge. That's your dime or thousand dollar bill.

Anyway, the old OEM spec coils were meant for commuters, grocery trips, moms, grannies and such. The 500 ohm/ft carbon core plug wires are meant for 30kV + ignitions, and suppressing electric noise.

I run Bronze Age straight copper 7mm wire for ultra low resistance to my plugs, FWIW, and filter noise at the stereo components and the tach instead. Gertrude's 1965/66 383 runs very clean for this, passing Emissions this summer on the first, untuned trial with flying colors so to speak. I'm trying platinum plugs with my very primitive ignition, though I might add a simple capacitive discharge box to the primary circuit as I drive a LOT at low revs, in urban traffic, and this might further improve my setup a bit. I'll tell All here when I have data on this.....

*resistance can be a bit misleading when estimating performance from an inductor. Manuals give these figures out for the purpose of troubleshooting at a low budget garage. IMPEDANCE is the proper variable to consider when RUNNING such a device, though that becomes a function of frequency, (rpm) regarding internal combustion engines. Still, an ohmmeter will tell Joe Greasemonkey if the coil is still good enough to use....

 

[MoPar~Man]

> The higher secondary resistance* is IN spec for the following reasons:

The measured DC resistance of a coil is dependent on the resistance per ft of the coil wire and the amount of wire (number of turns) the winding has.

Something has gone physically wrong with a coil that (when new) should have been 11k ohms but is now double that. If this came from the factory with 24k coil, but other coils come 11k, that would imply an impossibly large manufacturing tolerance. Or this is a completely different coil.

I have 3 coils, they seem physically identical, I could weigh them to add another dimension to this comparison.

Coil 1 is marked 2444242, CH-412, 249553, use with an external resistor. On the top in raised letters "Made in Canada". 1.55 ohms primary, 11.14k ohms secondary.

Coil 2 is marked 2444242, 2495531, use with an external resistor, Mopar, made in USA. No marking on the top. 1.72 ohms primary, 10.9k ohms secondary.

3'rd coil is the one I've had on the car since early 90's. Either I painted over all the markings, or for some reason it has none. In raised letters on the top it says Made in USA. 1.6 ohms primary, 24k ohms secondary.

My plan is to perform a few more compression / leakdown tests on 1 suspect cylinder, then put everything back to the way it was (coil, cap, wires) and run the engine and see if rough idling persists. Then I will change the coil to either #1 or #2 and re-test.

Today I have new cap and wires but will hold those back until I've tested an alternate coil.

 

[Gerald Morris]

> The higher secondary resistance* is IN spec for the following reasons:

The measured DC resistance of a coil is dependent on the resistance per ft of the coil wire and the amount of wire (number of turns) the winding has.

Something has gone physically wrong with a coil that (when new) should have been 11k ohms but is now double that. If this came from the factory with 24k coil, but other coils come 11k, that would imply an impossibly large manufacturing tolerance. Or this is a completely different coil.

I have 3 coils, they seem physically identical, I could weigh them to add another dimension to this comparison.

Coil 1 is marked 2444242, CH-412, 249553, use with an external resistor. On the top in raised letters "Made in Canada". 1.55 ohms primary, 11.14k ohms secondary.

Coil 2 is marked 2444242, 2495531, use with an external resistor, Mopar, made in USA. No marking on the top. 1.72 ohms primary, 10.9k ohms secondary.

3'rd coil is the one I've had on the car since early 90's. Either I painted over all the markings, or for some reason it has none. In raised letters on the top it says Made in USA. 1.6 ohms primary, 24k ohms secondary.

My plan is to perform a few more compression / leakdown tests on 1 suspect cylinder, then put everything back to the way it was (coil, cap, wires) and run the engine and see if rough idling persists. Then I will change the coil to either #1 or #2 and re-test.

Today I have new cap and wires but will hold those back until I've tested an alternate coil.

Those 3 coils all are well in spec. NOW, be SURE to get a proper ballast resistor for these. The .5 ohm ones are specified. This will give you a nice voltage division with the coil, with it running at about 9 V out of 14 when all is warmed up.

 

[Gerald Morris]

3'rd coil is the one I've had on the car since early 90's. Either I painted over all the markings, or for some reason it has none. In raised letters on the top it says Made in USA. 1.6 ohms primary, 24k ohms secondary.

My plan is to perform a few more compression / leakdown tests on 1 suspect cylinder, then put everything back to the way it was (coil, cap, wires) and run the engine and see if rough idling persists. Then I will change the coil to either #1 or #2 and re-test.

Today I have new cap and wires but will hold those back until I've tested an alternate coil.

Ditch #3. It's toasted.