How much voltage at the ignition points?

[Greg L.]

I'm getting ready to run my engine in the chassis for the first time in about 5....or is that 6 years? and as I wire up the ignition a thought crossed my mind...

I don't have any resistance between the battery and the ignition coil and was wondering if this is okay to run the engine like this for a half hour or so or wil I toast the points? If not then what can I do to drop the voltage to the coil and what should the voltage be?

Thanks.

[Joe R.]

Re: How much voltage at the ignition points?

It's my understanding that GM intended to have about 6 volts across the coil when the engine was running, and jack that up to 12 volts only during cranking to get a hotter spark.

I think it would be unwise to run the engine for 30 minutes with 12 volts suppled to the coil. Both the coil and the points would be under excessive stress during this period.

Chances are that everything would hold up just fine, but if you are doing engine break-in you don't want to risk any problems that would force you to shut down the engine prematurely. Even if everything holds up for 30 minutes, the extra heat dissipated in the coil might do some damage that would weaken it enough so that it fails later at an inconvenient time.

I suggest that you buy an appropriate reproduction ballast resistor and install it in your system. Keep in ming that the ballast resistor gets VERY HOT during normal use, so mount it accordingly.

[Greg L.]

Re: How much voltage at the ignition points?

That's what I was afraid of Joe.

I suppose then that I could just grab any any ballast resistor from an auto parts store? I seem to remember that the Dodges of the 70s had a resistor mounted in the engine compartment so maybe I could pick up one of those?

Just currious...did any Corvettes ever use an actual ballast resistor or did they always use the resistor wire like what my 69 has?

[Jack H.]

Re: How much voltage at the ignition points?

Corvette used a discrete ballast resistor through 1967, except for TI equipped cars where the function was embedded/distributed via the engine wiring harness.

Yes, you can run your engine for a short time at full voltage with the only side effect being the coil will run 'hot' and the current through the points will be higher. The old dirt racers from the 50's/60's used a 'poor boy' ignition trick by INTENTIONALLY running a 6V coil in their 12V cars to insure a hot spark at max RPM.

Few who did this ever reported coil melt downs while on the track. But, here you're talking about an aged, classic ignition coil, that may have see 'abuse' in its prior life, so all bets on a 'guarantee' are off.

Last, your original question quoted 'static' electrical parameters (12 Volts, Etc.). Understand, auto electric systems are dynamic which is why some use the term B+ to indicate the battery level.

Most batteries, deliver 2.2V per cell when fully charged meaning a 12V battery is NOT 12.0 VDC. It's voltage level varies based on charge, and internal condition.

Then, when you start the engine and the charging system kicks in, the voltage level climbs to re-charge the battery with B+ levels in the 14 VDC being common. Bench power supplies made for 'automotive' use typically output 13.8 VDC.

Over on the ignition primary circuit, voltage levels will change depending on whether the engine is off, points closed; off, points open or running. When running, the voltage level is a VERY complex issue because you're dealing with a DC circuit that's being actively switched (ON/OFF) with energy storage devices (coil/inductance; condensor/capacitance) introducing SIGNIFICANT transients and flyback voltage considerations.

Few with auto electric knowledge quote specific DC voltage levels for these reasons....they depend on how/when you're looking at the circuit and they're highly time dependent!

[Joe R.]

Re: How much voltage at the ignition points?

Hi Greg:

If you decide to add a ballast resistor, try to use one that has the correct resistance. During the C2 era, most ballast resistors were 1.8 ohms, but some were 0.3 ohms.

I don't know the resistance of the C3 resistance wire you are trying to emulate, but a nominal goal would be for the ballast resistor to have the same DC resistance as the primary winding of the coil. Then, under static conditions, the voltage across the coil would be about half the available battery voltage..

[Greg L.]

Re: How much voltage at the ignition points?

Well I know the proper way would have been to use a ballast resistor but it looks like I wouldn't really need it for what I'm doing here so I threw caution to the wind this weekend and just fired it up as is.

I ran it for about 15 minutes and it went pretty well except for a small exhaust leak between the heat riser and exhaust manifold that I have to fix. Oil pressure is a bit above 50 psi cold and about 43 hot. I primed the fuel system by removing the return hose to the tank at the tank and then blowing into the tank fitting to "pressurize" the tank until fuel came back out of the return line. That little trick even filled the fuel bowl so I just pumped it a few times and it fired right up!

Check out the contraption.

Oh that puddle of oil in the one pic is from the mechanical oil pressure gauge fitting that I mounted for the run.

[Mike M.]

Re: How much voltage at the ignition points?

"Necessity is the mother of invention".
Nice radiator support!
HaND

[Greg L.]

Re: How much voltage at the ignition points?

That's my old suppport and it had a bit of rust in it. I got lucky and found a near mint one on ebay a few years ago and deceided to keep it just in case I needed it for something...which I did. It was actually pretty solid with just the two lower bolts but just to be safe I installed the optional and very rare 1x3 wooden brace.

I thought someone might find the pics interesting because it shows just how little set-up you really need if you ever want to test run a fresh engine without the body.