question on ign. coil resistor

[John S.]

The 1962 call's for the black dot 1931385 .o3 ohms resistor. Will the blue dot 1957154 1.8 ohms work ok on the 1962? Thanks john

[mike mccagh]

Re: question on ign. coil resistor

the blue dot will fire your points and the engine will run but it won't be at 100% of its .03 ohm resistor's presence.you probably won't realize the loss of power during routine operation. mike

[Duke W.]

Re: question on ign. coil resistor

The 385 is 0.3 ohms, not .03 ohms. The P&A catalog is wrong! Measure it with a digital voltmeter at room temp if you want proof.

I doubt if installing a 1.8 will result is any change in operation. The 0.3 ohm resistor was used to provide more ignition energy, but increased the chance of burning points, which I guarantee from my own personal experience is the case.

As a running change in '63 its use was limited to SHP/FI engines, which continued through '64.

For 1965 all Corvette engines with point ignitions, including SHP/FI, got a 1.8 ohm ballast and a 202 coil, and the 385 was never again used in production.

Duke

[Michael H.]

Re: question on ign. coil resistor

I'm a bit electronically challenged so pardon my lack of knowledge on this but I always thought the coil and balast resistor were supposed to be a matched set. I know that's the way it is for 63 and 64. The 087 coil was to be used with one part number resistor and the HP 091 had different number resistor. (can't remember which is which now and I don't feel like looking it up) I thought they weren't supposed to be interchangeable. This is the way the parts book breaks it down. Maybe 62 was different than 63-64?

[Duke W.]

Re: question on ign. coil resistor

I think the 107 coils used on solid axle (FI?) cars were also mated to the 0.3 ohm ballast. Primary resistance varies among coils, but not by much.

Maybe now is a good time to take a survey asking those with various service manuals to look up and post primary/secondary resistance for the various OE coils. Only the 091 is listed in the '63 shop manual because the change to the 087 coil and 1.8 ohm ballast on base and L-75 was a running change.

091 coil primary/secondary resistance is 1.02-1.13/8000-10,500 ohms.

Primary circuit steady state current is just the sum of ballast and coil resistance divided by system voltage. Of course, at high engine speed the points are not closed long enough to achieve steady state current(partly because of the "inertia" of coil inductance), which is when ignition energy starts to drop. Also, the ballast resistance will increase with increasing temperature, which decreases steady state current to protect the coil from overheating.

The 087 coil resistance data should be in the '64 Corvette Shop Manual Supplement, 107 in ST-12, and the 202 in the '65 supplement and later CSMs.

Maybe Jack can describe the equivalent mechanical mass-spring-damper circuit off the top of his head. I think it's resister-damper, inductor-mass, and capacitor-spring, and I think the equivalent layout is like a suspension - a mass at the end of a parallel spring and damper, and both an automotive suspension and inductive ignition are characterized by the same basic differential equation.

Duke

[Jack H.]

Re: question on ign. coil resistor

Duke's got the analog 'nailed'.... The only thing missing in his analysis of the Kettering ignition system is the contact resistance of the ignition points (FAR from 0-ohms, measure a couple and you'll see the point resistance is significant compared to surround components like the ballast resistor).

Big picture is points are normally closed allowing primary side current to flow through the coil and store energy in the form of magnetic flux. When the points open, battery supplied current stops flowing and the magnetic fields begin to collapse around the coil's primary circuit.

This induces current to continue flowing as the magnetic field collapses giving you electrical voltage on BOTH the primary and secondary side of the coil with the secondary side's voltage stepped up by the turns ratio. Without the condensor (capacitor) in the distributor adjacent to the points, you'd get a single, short burst of ignition energy...spark without dwell.

But, the capacitor 'rings' (storing induced current generated by the collapsing magnetic fields of the coil). Essentially, the coil and condensor play a game of 'catch' with the stored energy moving it from magnetic form in the coil to stored charge form in the condensor and that causes the spark waveform to persist or 'dwell'....

[Michael H.]

Re: question on ign. coil resistor

Thanks Duke, Jack. I think I'll just try to be content knowing that the 091 coil requires a different balast resistor than the 087 coil because you lost me way back in the beginning of your post. I don't understand electronics at all and probably never will.

This is exactly what I like about this board. The incredible amount of knowledge here is like walking into the largest library in the world and finding the exact info that you need. It takes a large number of people who specialize in different things to cover all the topics, and we have it all!

Michael

[Duke W.]

Re: question on ign. coil resistor

Although the coil and ballast can be considered to be "matched" there is not a unique solution. If you have a 091 coil 0.3 ohm ballast that has a habit of burning points, substitute the 1.8 ohm ballast. This was the '63 TSB "fix" for burned points. Actually, it recommended substituting a 1.8 ohm ballast during the winter months, since a colder ballast will have less resistance and allow greater primary current. Burned points with the 0.3 ohm ballast was more common during the cold winter months.

Using the 1.8 ohm ballast will cut primary current, and, therefore, spark energy, but I don't think the loss will affect performance, especially if you have proper heat range spark plugs and a reasonably correct mixture. Leaded fuels from the sixties fouled plugs much faster than modern unleaded fuels.

One unequivocal truth from ignition system research is that once you have sufficient spark energy to consistently light the fire, more ignition energy has no effect, however, the greater (about double) energy of the TI and HEI provide considerably more margin for spark plug deterioration such as fouling and gap growth, so a TI or HEI will fire a plug in poor condition that the single point won't.

Duke

[Duke W.]

Re: question on ign. coil resistor

Interesting point, Jack. I have to admit that I've never measured point resistance other than observing the "point resistance" scale on my dwell meter.

I would imagine that new points would measure very near zero with a senstive digital meter, but a few tenths when worn. Also, I can imagine that the "dynamic" resistance (engine running) may be different due to the high temperature at the contact surface and localized melting.

Do you have any representative point resistance data?

Duke

[mike mccagh]

Re: question on ign. coil resistor

not an electronics expert, but duke and jack's dialog might explain why the points gap is much more critical on the old 6 volt dodge and chevys i'm used to fooling with. the correct gap on these distributors is much more sensitive than the gap on 55 thru 67 non-ti vettes i fool with. mike

[Duke W.]

Re: question on ign. coil resistor

Critical to what... point burning?

Duke

[Mike M.]

Re: question on ign. coil resistor

critical to easy starting and running. mike

[Clem Z.]

Re: question on ign. coil resistor

the point gap changes the dwell time which will allow more time for the coil to charge up. the best thing for a point ignition is a "dwell externder" this allows the coil to start charging as soon as the points open not waiting for the points to close. it use a SCR as a switch. i have a sketch if someone wants to build one. i ran them on race cars back in the 60s before electronic ignitions became available and they work because when it would die the engine would not rev to max RPMs because of voltage loss to the coil. it worked much better than dual point distributors