F Y I

[Joe C.]

While the following data may not be exact for a specific engine, it does clearly show the trends that all engines follow. Further, remember that leaner mixtures require higher firing voltages than richer mixtures.

The calculated values are for air only and do not include fuel ratio effects. Nor do they include plug electrode materials, tip geometry, and accumulated deposits. Given a static compression ratio of 11.4 and a trapped (dynamic) compression ratio of 9.0, inlet temperature of 100F and inlet pressure of 1 Bar:

If the plug fires at -45 deg BTC the cylinder Temp will be 399F , Press will be 66psi
Required firing voltages at various gaps: .050" = 12.1kV, .080" = 18.1kV, .100" =22.1kV

If the plug fires at -35 deg BTC, the cylinder Temp will be 496F, Press will be 97psi
Required firing voltages at various gaps: .050" = 15.3kV, .080" = 23.2kV, .100" =28.3kV

If the plug fires at -25 deg BTC, the cylinder Temp will be 609F, Press will be 144psi
Required firing voltages at various gaps: .050" = 19.6kV, .080" = 29.8kV, .100" =36.5kV

As can be seen, larger gaps require higher firing voltages. But retarded ignition timing also requires higher firing voltages.
The worst scenario is a high compression engine, with large plug gaps and retarded ignition.

[Michael W.]

Re: Fyi

Thanks for posting that Joe, that's quite a relief.

Thankfully, my favourite Corvettes come from the era where high compression engines, large plug gaps and retarded ignition were not available. The retards were busy doing other things I think.

[Dan D.]

Re: F Y I

The "retards" were not busy doing other things. They were busy 'retarding' your timing.

Interesting post. Out of curiosity Joe, how did you obtain this data? -Dan-

[Scott M.]

Re: F Y I

Thanks Joe, interesting data.

[Clem Z.]

Re: F Y I

does not the ignition system build up more energy before it jumps the wider gap ???

[Joe C.]

Re: F Y I

The "retards" were not busy doing other things. They were busy 'retarding' your timing.

Interesting post. Out of curiosity Joe, how did you obtain this data? -Dan-

It is not my data. I started a thread re: spark plug gaps, and the effects of short-gapping plugs on another automotive forum, and this is derived from one of the more recent additions to the thread.

The data is presented strictly for comparison's sake. The quantitative results have not been verified by me.

Part of the discussion wandered into talk about the old Engine Analyzer tools, and oscilloscopes. It would be interesting if anyone equipped with either of these could carry out some experiments using conventional pre-emissions spark plugs whose normal gaps are generally .035".

I have short-gapped my AC R45 plugs at .030". They were originally gapped at .035".

[Joe C.]

Re: F Y I

does not the ignition system build up more energy before it jumps the wider gap ???

The entire discussion of spark plug gap and its effects on voltage, amperage, spark duration, reserve voltage, and numerous other variables is intriguing. I'm tempted to look for an oscilloscope and/or "engine analyzer" tool, like this one

http://cgi.ebay.com/ARM-DSO-Nano-Pocket-Sized-Digital-Oscilloscope-/200470598507

for experimentation.

More voltage is needed to jump the wider gap, which results in a "hotter" spark. This sounds all well and good, but whether or not the spark occurs is restricted by the amount of reserve voltage that your secondary ignition system has at any particular time. A marginal system will easily go to zero reserve voltage under various conditions, such as: lean mixtures, higher cylinder pressure, later spark timing, wide spark plug gap, excessive rotor to cap electrode gap, high secondary wire resistance, etc., resulting in "no spark" instances. In many cases, a "hotter" spark is unnecessary and spark consistency is much more important in order to prevent misfiring. The key is to find the smallest gap needed, which will ignite the charge under all circumstances. The smaller gap, if found, will serve to increase your secondary ignition's voltage reserve, resulting in fewer "no spark" instances.

The higher voltage required to fire across a wider gap results in a shorter spark duration (fewer decay cycles as current bounces back and forth between the spark plug and the ignition capacitor [condenser]). Shorter spark duration means that ignition timing is more critical and less forgiving.

[William C.]

Re: F Y I

Using less than recommended gaps was pretty much standard for high performance work back in the 60's, but with the advent of lower static compression ratios and improved OEM ignition systems with more reserve capacity the world swung in the other direction. Where .035 was just about the "standard" the HEI capability allowed GM to use gaps in the .060 range with much better performance in igniting the lean mixtures being forced by the mid 70's emission standards. That trend continued until the mid 70's when cat. converters were introduced. By that time, the Car groups were seeing the end in sight for Carburetors, and still needed all the benefits of the ignition upgrades just to survive, and regs continued changing regularly afterward.

[Clem Z.]

Re: F Y I

The entire discussion of spark plug gap and its effects on voltage, amperage, spark duration, reserve voltage, and numerous other variables is intriguing. I'm tempted to look for an oscilloscope and/or "engine analyzer" tool, like this one

http://cgi.ebay.com/ARM-DSO-Nano-Pocket-Sized-Digital-Oscilloscope-/200470598507

for experimentation.

More voltage is needed to jump the wider gap, which results in a "hotter" spark. This sounds all well and good, but whether or not the spark occurs is restricted by the amount of reserve voltage that your secondary ignition system has at any particular time. A marginal system will easily go to zero reserve voltage under various conditions, such as: lean mixtures, higher cylinder pressure, later spark timing, wide spark plug gap, excessive rotor to cap electrode gap, high secondary wire resistance, etc., resulting in "no spark" instances. In many cases, a "hotter" spark is unnecessary and spark consistency is much more important in order to prevent misfiring. The key is to find the smallest gap needed, which will ignite the charge under all circumstances. The smaller gap, if found, will serve to increase your secondary ignition's voltage reserve, resulting in fewer "no spark" instances.

The higher voltage required to fire across a wider gap results in a shorter spark duration (fewer decay cycles as current bounces back and forth between the spark plug and the ignition capacitor [condenser]). Shorter spark duration means that ignition timing is more critical and less forgiving.

GM had a problem when they went to .080 gap plugs because the voltage required to jump that gap got too large for the plug wires and it would arc thru the wiring to ground before it got to the plugs.

[Joe C.]

Re: F Y I

GM had a problem when they went to .080 gap plugs because the voltage required to jump that gap got too large for the plug wires and it would arc thru the wiring to ground before it got to the plugs.

Pre-emissions engines run relatively rich mixtures, and those with the higher duration cams run the richest. They also run more spark advance than "lean-burn" or smog engines. Lower reserve voltages eliminate the need for super-duper atomic Armageddon coils. They also eliminate the need for heavily insulated secondary wiring. Another benefit, is that carbon deposits in the distributor cap as a result of routine ionization of the air within during engine operation do not result in tracking/short circuiting as it would with higher output coils.