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Spark plug firing voltage


Ken_P
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Hey all - so, working on a 1937 Packard 120 Touring Sedan. Original car, no mods.

 

I'm troubleshooting a rich running condition (excessive black smoke, carbon in exhaust, etc.)


Not looking to re-troubleshoot the whole problem here, although I may later if needed.


What I am looking for: is there a simple way to measure spark plug firing voltage, or does this require an oscilloscope?


If the latter, is that something an auto shop can do? I typically do all of my own work, but I don't want to drop a ton of money on an oscilloscope. A car from 1937 with points and condenser doesn't really require an oscilloscope very often. 😃

 

Thanks in advance!

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It will make no difference to your rich running condition.

 

Measuring plug firing voltage does require an oscilloscope, but generally you can get by without one. What is important is not the firing voltage so much as how much "headroom" you have. The firing voltage will be fairly low, but when you put load on an engine, it will go up. As long as your ignition is capable of supplying more voltage than what is required, it will not miss.

 

Wider plug gaps require more voltage, and shot wires can require a whole bunch! If everything is working as it should, you  are setting the voltage when you gap the plugs.

 

Things to look at with no oscilloscope: 1) make sure your plugs are gapped right  2) ohm test your plug wires and make sure there isn't a bunch of corrosion where they contact the distributor cap. 3) make sure the carbon button inside the distributor cap is contacting the rotor, and that no pieces have fallen off the rotor. The rotor is a more highly stressed part than just about anything else by the way. If there is doubt, replace it.

 

By verifying this stuff is ok, you can usually sidestep the oscilloscope. Also replace the condenser if there is any doubt.

 

To see how much voltage is available from the ignition, you can use an adjustable spark gap. It is sometimes enlightening and also relatively cheap. Put it on one plug wire, start the engine, and crank it out until there is no more spark. Not sure about the Packard, but most systems can make 16 kilovolts or more. Sometimes a lot more.

 

Although a rich mixture will lower the voltage demand, you can't substitute one for the other. If the mixture is too rich, you will have to figure that out and fix it. No amount of ignition tuning will help. Good luck, and I hope you get it sorted out.

 

 

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Edited by Bloo (see edit history)
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Put the car on a five gas exhaust machine. The chances of having a secondary voltage issue is small. Black smoke is obviously unburned fuel. Could be lots of causes. Maybe multiple issues. Start with the basics. 

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If the engine is not "missing" then the spark plug firing voltage is good enough. Look elsewhere for black smoke issues. 

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Thanks for all the help guys.


I'm cracking up - turns out, you can't give a bunch of car guys a vague problem with one specific question and NOT get troubleshooting advice! 😂

 

I appreciate everything. I do most of my blogging over at PackardInfo.com, but maybe if I can't solve it soon, I'll setup a post over here.

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Well, basically if it's firing at all speeds and under all loads, there's sufficient voltage. What that actual number might be is academic. The manual probably has a specification but ignitions are kind of all-or-nothing. Either it fires or it doesn't. The thing with electricity, it will only use as much as it needs to make the spark jump the plug gap. You could have 4,000,000 volts, but if it only needs 20,000 volts to make a spark, well, that's all that's going to jump. So you can have too weak a spark and it won't fire, or won't fire under all conditions, but if it works, it's working. On our six volt cars, I'd guess it's in the range of 20-25,000 volts. In essence, it's a very small amount of electricity (current) being pushed VERY hard (voltage) across a gap (resistance). Remember Ohm's Law, which is V = IR (with I being current). Current is fixed, Resistance is more or less the spark plug gap (plus resistance in the rest of the system, which is why you want low resistance wires), and Voltage is whatever it needs to be as a result of those two factors.

 

Does that help?

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Tracking on all of that. I'm a mechanical engineer by trade, but still remember Ohm's law. :)

I'm going to forget firing voltage, install my newly re-surfaced manifolds, and then go through the basic TS list again. Consider the post closed!

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OR you could measure your plug voltage the simple way.

Have a buddy switch her on and hit the starter.

YOU grab a plug with one hand and it's lead with the other.

The voltage is directly proportional to how far the hit throws you.

Don't forget your tape measure.

So much for "engineers", 'cause  I R 1

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