Variable coil input voltage

[sagaboy]

Voltage to the coil can varies between 15v with no load and 12.6v with heavy load (or even as low as10v with a faulty alternator). I wonder if the coil voltage output to the spark plugs will be affected by all these voltage variation during a drive--in other words--the coil output could peak spark with 15,000 v or as low as 8,000 v and varying spark voltage in between depending on the electrical load.

Shouldn't the varying spark voltage output affect the performance of our stock bug?

[david58]

Voltage to the coil can varies between 15v with no load and 12.6v with heavy load (or even as low as10v with a faulty alternator). I wonder if the coil voltage output to the spark plugs will be affected by all these voltage variation during a drive--in other words--the coil output could peak spark with 15,000 v or as low as 8,000 v and varying spark voltage in between depending on the electrical load.

Shouldn't the varying spark voltage output affect the performance of our stock bug?

If you are seeing 12.6 volts at the battery with a volt meter with the vehicle running with a load on the system then the charging system isn't doing what it was intended to do or you have added too many accessories that are pulling to much amperage. So to answer your second question coils are designed for a workable voltage change. Workable meaning normal, say like 13.2 volts to 14.6 which is a small voltage change. In that voltage range I don't think you will see any change in KV on a oscilloscope.

[sagaboy]

David58bug said: 13.2 volts to 14.6 which is a small voltage change. In that voltage range I don't think you will see any change in KV on a oscilloscope."

You mean 10/20% voltage supply drop does not produce a corresponding output drop in proportion?

[david58]

David58bug said: 13.2 volts to 14.6 which is a small voltage change. In that voltage range I don't think you will see any change in KV on a oscilloscope."

You mean 10/20% voltage supply drop does not produce a corresponding output drop in proportion?

That is not what I said re read everything I said......... .

[Marc]

The voltage across the plugs will only be as high as their ionization potential, regardless of what the coil is capable of producing. In other words, if the system were designed to function acceptably with 10.5v to the coil (let's say the coil primary-to-secondary turns ratio is 1 to 4000, so the maximum output voltage would be over 40,000v), upping the primary voltage would cause a proportional increase in maximum output voltage. At 13v, it could be over 50,000v....but if it only takes 25,000v to make the plugs fire, 25,000v is all you'll ever see.

[crocteau]

... upping the primary voltage would cause a proportional increase in maximum output voltage. At 13v, it could be over 50,000v....but if it only takes 25,000v to make the plugs fire, 25,000v is all you'll ever see.

Got me wondering... assuming the points stay open long enough for the primary field to collapse fully, would the higher voltage push more electrons across the spark plug gap? Would that do anything to improve combustion?

Sagaboy, I came across this and realized it might go along with what you were asking:"Experimentally varying the system voltage applied to a 12v coil at the SW or +ve terminal shows a fairly linear reduction in HT pulse duration as the voltage reduces, but the HT voltage at the plug does not start reduce until the supply voltage has been reduced to something less than 6v. This is because the HT voltage measured at the plug is controlled by the plug gap - as soon as the HT voltage rises high enough to jump the gap it will do so, which stops the HT voltage rising any further. The voltage at an HT lead that is not connected to a plug with a 12v supply at the coil, is much higher, and reducing the supply voltage shows a fairly linear reduction in voltage as well as duration." (http://www.mgb-stuff.org.uk/ignitiontext.htm#coil)