Sparky Challenge!

guido

Well-known Member
Hello,

How wide can a spark plug be gapped? With to- day's ignition system for small engines? I'd say pretty wide! Anyone know for sure?

Guido.
 
I changed the ones in my wife's '02 PT cruiser for the first time last year. Needless to say they were shot. I checked them out of curiosity, and the widest one was just over .100. So I'd have to say that .100 would definitely work, and given that the car was still running OK, even with nearly twice the gap, I'd say one could be opened up even more and still fire if the rest of the system was in good shape.

That said, I've got an old style tester that has spring clip for a ground, and then two pointed screws that you can hook two plug wires to. I've never measured the gap between the hot side and the ground, but it's at least 3/16 to 1/4 and a spark will jump that distance with no problem.

The main problem with too much gap is that, believe it or not, it can throw the timing off due to the time it takes for a spark to jump that excessive distance.
 
Not real sure where this is going, other than how long can you run a worn out plug...

I would think each different model would have a different limit, based on how much voltage the system can generate.

A couple of problems though, magneto ignitions require a minimum RPM to build sufficient voltage. The wider the gap, the faster the starter would have to spin the engine. Hard starting will probably be the first sign the plug(s) are due for a change.

The other problem would be possible coil damage. The harder the spark has to look for a place to go, the sooner it will break through the insulation and short to the iron core.

As far as intentionally gaping a plug wider, there is really no advantage other than trying to run a lean mixture for better economy. A wide spark gap will somewhat fire a too lean mixture better, but not worth the risk with an air cooled, carburated engine with fixed timing.
 
Hello Steve@Advanced,

Routinely I have been using wider gaps on small engines. Some time back I found out that the plug lasts a lot longer. A wider gap seems to keep the carbon build up at bay. It has worked well for me
with no hill effects on the ignition,

Guido.
 
Hello NCWayne,

You say .100? Not here say, but practical experience................... It works for me! The 3/16 to 1/4 applies to a plug outside the engine. How about inside? The engineers say that the modern ignition system can generate up to 13K volts. The also say that it takes 10.000volts to fire a plug under compression on a cold engine. Less when the engine is hot,

Guido.
 
A mag has an impulse so the starter or hand crank can turn the engine over very slowly & still produce a hot spark.
 
Hello bill in in,

I have no clue! I don't have a kilovolts meter. The little Weed Eater Blower I was playing with has the plug gap now at.060. Runs good and fires cold on the first pull, Couple more on 1/2 choke, and to work we go!

Guido.
 
Not that it takes longer to actually jump a bigger gap. It takes longer to build enough energy to jump a bigger gap.
Willie
 
Newer cars are different about timing, they adjust themselves, as the spark plug gap widens it'll retard the timing somewhat to make up for it. My brother has a PT Cruiser, it's his work car, deliver newspapers and knock around car. It was getting difficult to start, it has about 160,000 miles on it. Usually he buys a cheap car, runs it a few years then dumps them. More often than not the vehicle change is driven by an accident or catastrophic engine or transmission failure (lost a Taurus with 250,000 miles on it when the air intake got below water, when the scrapper came to pick it up they drove it on the flatbed, he claims that's when he noticed the hole in the block with pieces of rod hanging out). Was about to do this with the PT but when he started looking he thought everything was a bit overpriced. Decided to replace the plugs and filters, as he's taking the wires off he notices they're MOPAR wires, gets the plugs out and they're pretty bad. He's now assuming it ran 160,000 on the original plugs and wires. He reports it starts and idles much better now with new wires and fresh plugs and is happy- figures he saved about $3500 over replacing the PT.
 
You can increase the gap until the spark finds some other path to ground.

Contrary to popular belief, the spark voltage of an ignition system is not fixed. It is directly related to spark gap: the system will generate whatever voltage is required to jump the gap, which is why you can hold a plug wire a quarter inch or more away from the block and still get an arc. The problem is that if there's a weaker dielectric than the spark gap in the system, the spark will jump the weak link instead of the plug gap. That might be the plug wire insulation, the distributor cap or the plug boot. It could even be the insulation within the coil itself.

Increasing plug gap doesn't increase spark energy. The available energy is fixed at the instant the points open, and the voltage will rise until there's a spark somewhere. The manufacturer's recommended plug gap is a compromise that considers many factors including ignition system reliability and life. There's really no reason to increase plug gap beyond what the manufacturer recommends.
 
> Not that it takes longer to actually jump a bigger gap. It takes longer to build enough energy to jump a bigger gap.

Close. It takes time for the VOLTAGE to rise high enough to jump the gap. The amount of energy available was fixed the instant the points opened. The voltage rise at the plug gap is nearly vertical, but not quite so because of capacitance in the plug wire.
 
All systems have a "fixed" max voltage potential. Some can only reach 20,000 volts and some as high as 50,000 volts. The higher potential - the more gap can be jumped unless an easier path to chassis-ground is found.
 
Since Guido wanted this limited to small gas engines, the references to autos don't really help. All I know is that a gap that's a little too wide is almost always better than one that's too small!
 
The typical plug gap in free air requires about 12,000 volts to jump the 0.035 inch spacing. If there is a distributor involved, the rotor to distributor contact gap requires about another 3,000 volts. So, the coil must develop at least 15,000 volts to produce an ignition spark in free air and more in a combustion chamber with compressed air/fuel mixture. Many flywheel or rotating-permanent magnet magnetos max out at 20,000 volts. Many modern battery-coil systems can make 50,000 volts.
My 1995 Subaru Impreza last year put on a good demonstration on how wide a gap it could NOT handle. Had over 150K miles on the original spark plugs. Ran fine until one cold morning it would not start. Found the plugs were eaten up so bad they had over a 1/4" gap. Hammered them closed to .032" and it's been fine ever since.
Standard spark check on many older permanent magnet magnetos is hook in a large spark plug (like a D15) with the ground electrode missing. Most will jump that large gap with a bright blue spark but some older mags from the 30s will not.
Early Ford Model T cars had flwheel magnetos that made 4-30 volts and a coil that stepped up that voltage 500-1000 times. So that is a voltage potential of 2000 volts to 30,000 volts.
An ignition coil is basically a "step-up" transformer. A typical modern car had a coil designed to have 9 volts at the primary (when cranking) that gets stepped up 4000-5000 times by the time it leaves the secondary windings.
With small engines with breakerless ignition? If they are still flywheel magneto - I suspect they run around the same max voltage as they did when they had breaker points.
 
Mornin Guido, you come up with some pretty good questions, so here's my (somewhat rusty so no warranty or GOTCHA, its been a longgggggg time lol) electrical engineers response with some theoretical information some lay persons may or may not be aware of. I'm NOT here to fight or argue ONLY to try and help, so feel free to take or leave this, I'm not saying its right or wrong SO THERE LOL take it up with Billy Bob and Bubba down at the beer joint..........


"How wide can a spark plug be gapped"

NOTE the answer basically depends on the medium (fuel and compression) in which the plug must fire and the coils ability to produce a high enough voltage in order that current arcs across the gap. AND ALSO if the rest of the ignition system (cap and wires and distributor etc) will withstand the voltage without HV breakdown and arcing (other then across the plug gap). As the gap is increased its going to require a higher voltage before current will arc across the gap.

In older cars and tractors with modest compression and stock ignitions gaps may be in the say 0.020 to 0.35 range, while in modern automotive ignitions they might run wider, say 0.030 to 0.050 or so. BUT THE MODERN SYSTEMS HAVE COILS WHICH CAN ACHIEVE THE NECESSARY (for a wider gap) HIGHER VOLTAGES AND HIGHER VOLTAGE RATED COMPONENTS (wires and caps and systems)

ANSWER

If you still want current to arc across the gap to ignite the compressed fuel/air mixture WELL DUH

It can be gapped only as wide such that the coil can ramp up to a high enough voltage to cause current to arc across the gap. A typical old tractor coil voltage value, subject to the medium (fuel and compression) and the typical old tractor plugs gap to arc current, may be in the range of 8,000 to 15,000 volts. The coil may have the capacity to develop say 20,000 to 30,000 volts.

Soooooo say you want to widen the gap, fine, and as long as the coil can ramp to a high enough voltage to arc current across the gap it will work, but if the gap becomes too wide that the coil cant rise to sufficient voltage THE PLUG WONT FIRE

BUT HERES SOME ENGINEERING NEWS FOR YALL (sorry, gets a little deep and theoretical)

1) The coil stores energy when its conducting (when points closed) and when the points break open there is only X amount of energy available that gets mostly expended when the plug fires. When the points open and the coils HV secondary voltage ramps up and current eventually arcs the plug gap, most of the coils stored energy is expended in the form of Volts X Amps X Time (think of it sort of like watt hours the utility charges for energy).

2) Soooooooooo due to the fact there's only a finite X amount of energy available and most gets expensed across the gap (heat losses in other parts of the system) when the plug fires and since that energy is Volts X Amps X Time (like Killowat Hours utility charges except wayyyyyyyyy less here)

IF YOU INCREASE THE VOLTAGE (because of a wider plug gap) then either the Current and/or the arc time duration MUST NECESSARILY DECREASE. At certain gaps (in the medium and in laymens terms) the spark may be say a good visible blue but if the gap is increased the spark may appear more thin and wimpy yellow THATS BECAUSE IF VOLTAGE RISES THE CURRENT (the visible colored observation) IS DECREASED.

MAGNETO IGNITION still pretty much the same its only that the coils energy isn't produced by a battery pumping current through the coil, but due to the fact a magnetic rotor passes by the coil
which induces voltage and current then flows until its points break open.

SOOOOOOOO if you still want the plug to fire, you can only widen the gap up to the limit of voltage the coil can produce such that current will arc jump the plugs gap to ignite the fuel air mixture. A wider gap requires a higher voltage and since there's only X finite amount of energy available to get expended across the plug gap (Volts X Amps X Time) if voltage is increased then current and/or arc time duration decreases.

I look at it like there's a certain sweet spot at a given compression and fuel and at a given amount of X energy available where if the gap is at the sweet spot distance, the arc (current and heat evidenced by sparks appearance) is best suitable to ignite the fuel air mixture. In my opinion that's the gap where the bright visible blue spark takes place (but in compression now unlike if you're looking n at a plug outside the engine remember) versus a thinner wimpier yellow spark occurs which isn't as good at igniting the fuel. NO WARRNTY HERE JUST MY OPINION.

PS when the points break open the coils voltage
isn't instantly at x volts, it ramps up AND ONLY HIGH ENOUGH UNTIL CURRENT ARCS THE PLUGS GAP regardless if you had a stock 20,000 volt rated coil or a 50,000 volt so called HIGH VOLTAGE coil


That's my story n Ima stickin to it, take or leave it at your own peril, no freakin warranty

John T
 
Hello John T,

As always you put out good info, although NOT GUARANTEED,I Have only been turning wrenches since 1961, so I am still learning. Got to go, such a long post I'll read it later. Spot on for points ignition!

Guido.
 
"It takes time for the VOLTAGE to rise high enough to jump the gap. The amount of energy available was fixed the instant the points opened. The voltage rise at the plug gap is nearly vertical,"

EXACTLY RIGHT fellow Sparky (below is for the posters info, you have the handle on it already)

When the points are closed and the coils primary is conducting current but then they suddenly open and the coils magnetic field collapses THERE IS A FIXED X AMOUNT OF ENERGY AVAILABLE which eventually gets expended (except for heat losses) across the plug in the form of Volts X Amps x Time. The coils HV secondary is NOT instantly at X volts, it begins to ramp up and ONLY until its high enough that current arc jumps the plug gap.

Love sparky chat

John T
 
Hello MarkB_MI,

Spot on Mark! Time to built up the voltage means later spark? It happens later on degrees of Crank rotation, therefore later timing, You are thinking on the same lines He!

Guido.
 
Best wishes Guido, I always enjoy chatting with you.. I DO WARRANTY THAT !!!!!!!!!!!!!

John T
 
"Contrary to popular belief, the spark voltage of an ignition system is not fixed"

EXACTLY

The voltage necessary to arc jump current across a plug gap is a function of the medium (fuel and compression) in which the spark occurs PLUS the plugs gap. ITS IS NOTTTTTTTTTTTT FIXED and changes and depends upon the above variables. The maximum achievable coil voltage HOWEVER is for the most part a "fixed" design, such as it can achieve 20,000 or 50,000 volts, but the actual voltage at which the spark takes place depends on the medium and plug gap. The system is designed based on typical compression (which changes over time) and typical (which can change) fuel and a certain recommended and designed for plug gap,,,,,,,,,,and for a maximum voltage the system can deliver as required BUT THOSE THINGS ARE NOTTTTTTT FIXED and change over time.

The voltage at which a plug fires is NOT fixed (although the max available voltage can be), it changes and depends on the above ever changing variables........

John T Long retired EE so no warranty, I try to help NOT fight, take it or leave it at your own peril, but I believe it to be correct.
 
I forgot one last thing to add, imagine that silly me lol

So, say you have a stock typical old tractor ignition and you change out the stock coil for a new bright n shiny so called """HIGH VOLTAGE COIL"""

Sorry to disappoint you and after all that money you spent, as long as nothing else changed YOUR PLUGS WILL STIL FIRE AT "NEARLY" THE SAME VOLTAGE AS BEFORE. The coil voltage only ramps up high enough to fire the plug and once there IT DOESNT KEEP RISING SO IT FIRES AT SAY 50,000 VOLTS!!!!! (I bet Bubba thought his plugs fire at 50,000 volts now that he spent all that money on the 50,000 volt High Voltage Coil (and no other changes) huh??)


NOTE: Those after market coils may well have improved magnetic and electrical materials and may be more efficient etc so they may waste less heat energy thereby delivering a tad more spark energy then the old stock coil BUT BASICALLY the plugs firing voltage is a function of the medium (fuel and compression) and plug gap. Its true they may have the CAPACITY to achieve a higher voltage (if necessary due to a wide plug gap) and may be more efficient.

You get the most bang for the buck from a high voltage high energy coil if you couple it to a matching elec switch and store and impart more spark energy across the wider plug gap versus an old points system and stock coil.......

Nuff said

John T
 
Great explanation, John.

You touch on an important point, which is there is no free lunch. Increasing plug gap increases spark voltage, but necessarily reduces spark current and/or duration. There is only so much energy to go around, and it is predetermined by the primary ignition circuit how much energy is available.
 
>All systems have a "fixed" max voltage potential.

Not really. John explained it pretty well. It is more correct to say that an ignition system has a "design" voltage. The components are designed to work at that voltage plus a comfortable margin.

Although it is natural to think of the ignition coil as a step-up transformer, that's not accurate. The voltage induced in the secondary winding is a function of the rate of change of the magnetic flux. In ac transformer, that rate of change is a nice sine wave, so the output voltage is predictable. But in an ignition system, the flux rate of change isn't remotely sinusoidal, so the secondary voltage curve is steep and not very predictable.
 

That would be true if it operated under the same condition all the time... Problem is it don't.... Bubba may need more reserve at times the problem is in most situations he don't...
 
Can it be said great minds think alike lol lol or perhaps something less favorable???

The Conservation of Energy and Laws of Physics indeed dictate THERES NO FREE LUNCH

John T
 
Hello John T,

I just came in for a breather and read you first post. SOOOOOOOO Widening the gap will delay the spark in relation to crankshaft rotation? Would that me that the timing is delayed? Heck yes. More time to BETTER burned the fuel mix!
Just tuned my micro B&S weed Wacker. Two tanks of gas so far, it will be three before the day is over. Ho yea, the plug is set @.035, and all is well. As you say fun chatting with you......

Guido.
 
Hey Guido man, Its indeed true that a wider gap means more time before the voltage rises high enough to arc current across the gap

BUT IM TOTALLY UNSURE OF THE TIMES WERE TALKIN ABOUT AND IF ITS SIGNIFICANT AT ALL OR MAKKES HARDLY ANY DIFFERENCE.

Its a pretty steep voltage rise but as far as the exact amount of time I don't know....but dont think its all that significant BUT THATS ONLY A GUESS

John T

John T
 
All this engineerin" talk is beyond me. But I can offer one more tip for the outboard motor guys. If you have the option, try a couple of spark plugs until you find one that has the open part of the gap pointed toward the intake side of the chamber. Having the spark better exposed to the fuel/air might make a difference.
 
(quoted from post at 11:50:08 06/21/14) All this engineerin" talk is beyond me. But I can offer one more tip for the outboard motor guys. If you have the option, try a couple of spark plugs until you find one that has the open part of the gap pointed toward the intake side of the chamber. Having the spark better exposed to the fuel/air might make a difference.

My IH "H" started running on three cylinders and after determining which one it was that was dead, I took the sparkplug wire and held it away from the plug and it would run ok. When put it back on it would miss. Kinda proves some sort of theory about letting the voltage build up. A new plug fixed the problem.
 
(quoted from post at 07:10:14 06/20/14) Since Guido wanted this limited to small gas engines, the references to autos don't really help. All I know is that a gap that's a little too wide is almost always better than one that's too small!

Yep and small engines have weaker ignition systems than modern vehicles do.

And if properly insulated the spark voltage does not have to jump anywhere.
 
Hello merlynr,

YEAP! A shorted plug will fire, lIKE YOU SAID, if let if fire with a gap from the lead to the plug. Best way to test for a bad plug! Can't short the ignition out that way.............


Guido.
 
Hello merlynr,

YEAP! A shorted plug will fire, lIKE YOU SAID, if let if fire with a gap from the lead to the plug. Best way to test for a bad plug! Can't short the ignition out that way.............


Guido.
 

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