Spark gap and compression/detonation

[Moparfiend]

All things being equal how does spark gap affect detonation?

 

[moparedtn]

So many variables there, since each engine is unique and all...
Just going on what I've read over the years, running bigger gaps than stock can effect
timing (advancing it), so there's that possibility of producing/aggravating spark knock.

Same deal with heat ranges - hotter plugs aggravate pinging conditions as well when
you've gone too far in that direction (of course, there will be other evidence of the plugs
being too hot between discoloration and even blistering).

I've wondered the same thing though - since a lot of us run some manner of electronic
ignitions anymore, usually with some hot coil, should we increase spark plug gaps?
I remember the instructions that came with my Direct Connection electronic ignition kit
specifically saying NOT to increase plug gaps, so there's that I suppose.

 

[vance.dykes]

think of it like a top fuel engine more fuel more spark. a mopar ing, system can only produce 20 to 40000 kv and 30 to 35 is all the gap you can run any more and the spark will jump to the side of plug witch is a miss fire. run a c.d.i ing system and now you can run a 40 to 60 gap more spark with more fuel = MOPARER spark knock is from carbon build up and to much spark advance and low octane fuel.

 

[David Stillie]

No affect.

 

[dvw]

It doesn't
Doug

 

[451Mopar]

Really, the spark plug gap should not make any difference.
If it is an older / stock engine, make sure the valve stem seals are good. Oil in the combustion chamber can cause pinging (like really low octane fuel.)
Lowering air intake temperature also helps. using an intake gasket that blocks the heat crossover is a good place to start when reducing air intake temps (AIT)

 

[Don Frelier]

Predetonation is just that: spontaneous combustion prior to the proper ignition point in the cycle.
The plug hasn't fired yet.
The piston is still coming up and bam!

 

[vance.dykes]

Really, the spark plug gap should not make any difference.
If it is an older / stock engine, make sure the valve stem seals are good. Oil in the combustion chamber can cause pinging (like really low octane fuel.)
Lowering air intake temperature also helps. using an intake gasket that blocks the heat crossover is a good place to start when reducing air intake temps (AIT)

there specification for gap and in testing for spark to much gap and the system can be damaged . that is in the manual for testing high energy system. some ignitions systems wont jump a gap to large it would jump to the shortest i.e ez'ist ground .

 

[RJRENTON]

Predetonation is just that: spontaneous combustion prior to the proper ignition point in the cycle.
The plug hasn't fired yet.
The piston is still coming up and bam!

IMO...
YES...that supposition is correct. There are basically two types of spontaneous combustion: (1) pre-ignition is where the fuel charge is ignited by glowing carbon deposits. (2) Detonation is caused by the fuel charge igniting by the compression cycle AND the flame front ignited by the spark plug. When the two flame fronts collide, the resulting collision causes the noise and shock loads the piston, sometimes to the point of catastrophic component failure. Detonation is usually caused by a low octane fuel charge, too much spark advance (or too great of a rate of advance), too hot of an engine, and a lean fuel mixture, too high compression ratio, or some combination of all the above. Most every one says they hear the valves "rattling"....not true..at the instant of combustion, both valves are closed.
GM adopted the HEI ignition system in 1975 with the advent of the catalytic converter. To reduce rmissions, leaner mixtures were used, which required larger spark plug gaps to ignite the fuel charge. The standard 0.035" gap became 0.045" to 0.060" and Oldsmobile used 0.080". The HEI system was up tp the task. A few years later Exhaust Gas Recirculation was added to furthur reduce NOx emissions and the use of wide gap plugs continued.
There are various types of spark plug gap configurations, from projected nose plugs, to Champion's "J" gap to side gap plugs used in race car engines and blown dragster engines using nitro methane methanol fuels. When applied to the engine and operating conditions correctly the spark plugs have no difficulty igniting the fuel charge.
Champion Spark Plug Company had published very definitive information about the science of automotive ignition systems, what occurs, for just sbout every internal combustion engine made including race cars to chain saws to industrial applications. NGK has similar information available today.
To simply state one system is better than any other system is incorrect....it needs to be correctly applied.
BOB RENTON

 

[vance.dykes]

IMO...
YES...that supposition is correct. There are basically two types of spontaneous combustion: (1) pre-ignition is where the fuel charge is ignited by glowing carbon deposits. (2) Detonation is caused by the fuel charge igniting by the compression cycle AND the flame front ignited by the spark plug. When the two flame fronts collide, the resulting collision causes the noise and shock loads the piston, sometimes to the point of catastrophic component failure. Detonation is usually caused by a low octane fuel charge, too much spark advance (or too great of a rate of advance), too hot of an engine, and a lean fuel mixture, too high compression ratio, or some combination of all the above. Most every one says they hear the valves "rattling"....not true..at the instant of combustion, both valves are closed.
GM adopted the HEI ignition system in 1975 with the advent of the catalytic converter. To reduce rmissions, leaner mixtures were used, which required larger spark plug gaps to ignite the fuel charge. The standard 0.035" gap became 0.045" to 0.060" and Oldsmobile used 0.080". The HEI system was up tp the task. A few years later Exhaust Gas Recirculation was added to furthur reduce NOx emissions and the use of wide gap plugs continued.
There are various types of spark plug gap configurations, from projected nose plugs, to Champion's "J" gap to side gap plugs used in race car engines and blown dragster engines using nitro methane methanol fuels. When applied to the engine and operating conditions correctly the spark plugs have no difficulty igniting the fuel charge.
Champion Spark Plug Company had published very definitive information about the science of automotive ignition systems, what occurs, for just sbout every internal combustion engine made including race cars to chain saws to industrial applications. NGK has similar information available today.
To simply state one system is better than any other system is incorrect....it needs to be correctly applied.
BOB RENTON

you are right about GM ignition systems they could produce 50 -60 kv but we are talking about mopar ignition systems.

 

[Moparfiend]

Thanks for the replies guys. All things being equal was to try and examine just the effect of spark gap and detonation.

We know that a wider gap will be able to burn a leaner mixture. Does that mean it ignites sooner than a narrower gaped plug?

Bob Renton in your comments you mentioned the two flame fronts colliding causing the vibration and subsequent issues with detonation. If the gap changes the timing of that flame front then the timing of the collision changes. I am wondering about that timing event change here.

 

[RJRENTON]

Thanks for the replies guys. All things being equal was to try and examine just the effect of spark gap and detonation.

We know that a wider gap will be able to burn a leaner mixture. Does that mean it ignites sooner than a narrower gaped plug? IMO....NO when the spark occurs is the determining factor not the gap width.

Bob Renton in your comments you mentioned the two flame fronts colliding causing the vibration and subsequent issues with detonation. If the gap changes the timing of that flame front then the timing of the collision changes. I am wondering about that timing event change here.

You cannot assume timing variation relates to gap width. Once the burn starts, it goes to completion. IF the fuel charge cannot supress the increase in pressure change due to progressing flame front, the mixture will auto ignite, like a diesel engine, and detonation will occur. Ionization will occur around the center electrode, prior to the actual spark occurrence, which allows the actual spark to jump the gap to the ground electrode. All these events occur within milli-seconds. The voltage level, not the gap dimension, is the greatest influence. Wider gaps allow for greater exposure to the fuel charge; the higher voltage allows the spark to occur with longer discharge time. This is due to the coil's inductive reactance in the GM's HEI system. Mopar's original orange box ignition system was simply a points system replacement. The distributor just switched the coil current. The coil used did not produce the same energy level (voltage) as the HEI coil did. Chrysler did not use wide gap plugs, at least innitially. Detonation is related more to fuel's octane level and spark advance characteristics not spark gap.
BOB RENTON

 

[Moparfiend]

You cannot assume timing variation relates to gap width. Once the burn starts, it goes to completion. IF the fuel charge cannot supress the increase in pressure change due to progressing flame front, the mixture will auto ignite, like a diesel engine, and detonation will occur. Ionization will occur around the center electrode, prior to the actual spark occurrence, which allows the actual spark to jump the gap to the ground electrode. All these events occur within milli-seconds. The voltage level, not the gap dimension, is the greatest influence. Wider gaps allow for greater exposure to the fuel charge; the higher voltage allows the spark to occur with longer discharge time. This is due to the coil's inductive reactance in the GM's HEI system. Mopar's original orange box ignition system was simply a points system replacement. The distributor just stitched the coil current. The coil used did not produce the same energy level (voltage) as the HEI coil did. Chrysler did not use wide gap plugs, at least innitially. Detonation is related more to fuel's octane level and spark advance characteristics not spark gap.
BOB RENTON

Gotcha no time relation with respect to gap. So how does a colder plug prevent detonation all things being equal again?

 

[RJRENTON]

Gotcha no time relation with respect to gap. So how does a colder plug prevent detonation all things being equal again?

Not really....the heat range of the spark plug relates to the plugs ability to transfer the heat generated in the combustion chamber at the center electrode to the cooling system via the plug's insulator and shell. A hot plug has a longer center insulator, allowing the heat generated to travel farther, b4 bring absorbed by the cooling system. Conversely, a colder plug has a shorter insulator which absorbs heat faster allowing the center electrode to operate cooler.
IF a higher heat range (hotter plug) is used in an engine designed to operate with a cooler plug (low heat range), two things MAY occur. The center electrode would melt or the piston top may be damaged or possibly both. A hot plug may inniate pre-ignition or more likely detonation. While a cold running plug will NOT prevent detonation. A cold plug will, in all likelihood, become fouled resulting in a mis-fire resulting in a lack of power.
Not all spark plug manufacturers use the same heat range scale or criteria....Champion differs from NGK from Bosch, from AC, from Autolite or E3. You must do your own do dilligance for what ever brand you select to see where your choice of plug falls within the manufacturers heat range scale for your operating condition and engine. Under certain conditions (in a 1/4 mile contest with open headders) you will be hard pressed to actually hear the detonation, should it be occuring.
BOB RENTON

 

[Budnicks]

 

[Moparfiend]

Not really....the heat range of the spark plug relates to the plugs ability to transfer the heat generated in the combustion chamber at the center electrode to the cooling system via the plug's insulator and shell. A hot plug has a longer center insulator, allowing the heat generated to travel farther, b4 bring absorbed by the cooling system. Conversely, a colder plug has a shorter insulator which absorbs heat faster allowing the center electrode to operate cooler.
IF a higher heat range (hotter plug) is used in an engine designed to operate with a cooler plug (low heat range), two things MAY occur. The center electrode would melt or the piston top may be damaged or possibly both. A hot plug may inniate pre-ignition or more likely detonation. While a cold running plug will NOT prevent detonation. A cold plug will, in all likelihood, become fouled resulting in a mis-fire resulting in a lack of power.
Not all spark plug manufacturers use the same heat range scale or criteria....Champion differs from NGK from Bosch, from AC, from Autolite or E3. You must do your own do dilligance for what ever brand you select to see where your choice of plug falls within the manufacturers heat range scale for your operating condition and engine. Under certain conditions (in a 1/4 mile contest with open headders) you will be hard pressed to actually hear the detonation, should it be occuring.
BOB RENTON

So a colder plug doesn’t necessarily prevent detonation or preignition but too hot of a plug will cause one or the other. Makes sense.Thanks!

 

[451Mopar]

there specification for gap and in testing for spark to much gap and the system can be damaged . that is in the manual for testing high energy system. some ignitions systems wont jump a gap to large it would jump to the shortest i.e ez'ist ground .

My bad, when I wrote, "Really, the spark plug gap should not make any difference.", I was referring to detonation.
The coil discharges all its' energy in a very short time, so the difference in ignition timing between a 25-30 thou gap, and 40-45 gap might be fractions of a degree in timing.

More info: https://help.summitracing.com/app/answers/detail/a_id/5007/~/engine-detonation
https://help.summitracing.com/app/a...fbTRNMjhSd3RFJTdFU0lFejVNN0x6M0VMQWclMjElMjE=

 

[dvw]

In all my years of racing, non scientific results. Making the gap bigger never made the car any quicker. Making the gap to big at boost has occasionally caused misfire. Our boosted engines run .020", N/A .030". No extended nose plugs. Proper heat range. Proper octane fuel. Correct ignition timing. These are the key. The plug just has to start the fire. Nothing more.
Doug

 

[383man]

Thanks for the replies guys. All things being equal was to try and examine just the effect of spark gap and detonation.

We know that a wider gap will be able to burn a leaner mixture. Does that mean it ignites sooner than a narrower gaped plug?

Bob Renton in your comments you mentioned the two flame fronts colliding causing the vibration and subsequent issues with detonation. If the gap changes the timing of that flame front then the timing of the collision changes. I am wondering about that timing event change here.

A wider plug gap will take more voltage to jump the gap and it will be faster then with less plug gap. That's why they went to resister plugs to fight radio static many years ago. The lower voltage takes longer to fire and the trailing voltage would cause some radio static years back. The resister plug caused the voltage to be higher to jump the gap and it burns faster with less trailing voltage. It don't ignite sooner but the wider gap needs more voltage to jump the gap then less gap. The coil may be able to put out 60K but if the resistance of the secondary circuit (plug gap , rotor air gap and the plug and coil wires) will determine how much voltage is needed to jump the plug gap and overcome the secondary resistance. It may only take 8k at an idle to fire the plug so even if the ign can put out 60k it will only use what it needs to fire the plug so it would put out just over 8k since it will fire the plug with that much. If it had the wrong plug gap that was to wide then it will take more voltage to jump the gap and it could shoot it up to say 18k at idle and a 60k system will still easily fire the pug but a 20k system will be pushing it. Ron