Spark Plug Gap

[MrLuckey]

What about an engine determines the correct spark plug gap? What modifications might affect the optimal gap? Surely its not just the strength of the ignition system (voltage) is it?

 

[Jaybird]

I think with gap it is best to go by the plug mfg's recommendation.

 

[MrLuckey]

I would "think" that a person should go with whoever built/designed the motor (maybe only the electrical portion) but what I am really looking for is:

What is it about an engine that determines what the optimal spark plug gap should be?

I am sure it has to do with ignition voltage at the plug, spark plug choice, and other electrical components but I am wondering if anything about an engine besides electrical details have anything to do with the optimum gap?

If so what?

 

[MrLuckey]

Hmmm - surely this isn't the first post to stump Rich and Eric :p

 

[Rich Rohrich]

Eddie - I was waiting for someone else to jump in and contribute :)

To hopefully get the ball rolling let's forget for a moment about the energy actually available from the ignition system and concentrate on some of the following points that influence how willing a mixture will be to fire and how that might in turn influence what the engine might require in terms of a gap:

- mixture motion near the spark plug firing point at the time of ignition

- air fuel ratio but more importantly the homogeny of the mixture near the spark plug firing point at the time of ignition

- shape of the center and side electrodes degree of gap shrouding etc.

- position of the plug firing tip in the chamber and it's relationship to (a)the mixture motion and (b) the degree of homogeny of the mixture near the spark plug firing point at the time of ignition

- temperature of the mixture and the plug firing point

There's a few more things but lets start with those and see if it will prompt any discussion :thumb:

 

[MrLuckey]

Somehow I KNEW I'd find a post from you this morning Rich :)

This sounds like its going to be complicated (I figured as much). I guess what got me to thinking about this was pulling the plug on GSR's KTM the other day. The gap seemed pretty tight so I checked the manual and it recommended .060. The plug I took out was set at .030 and the bike seemed to run great except for an occasional high idle problem that I haven't even been able to look into yet.

As to the manual recommendation, the plug we are using is not the one listed and the KTM has been ported so it got me to wondering what to do with the gap on the new plug. In the past I have normally just set the gap on the new plug to match the one I removed as long as the bike seemed to be running good.

I'll wait patiently for some of the other smart fellas to get this discussion rolling :confused:

 

[WoodsRider]

Working almost exclusively with diesel engines I've forgotten a lot of what I learned about electrical ignition systems. So from memory:

The spark plug gap provides resistance in the ignition system. The coil needs to build up enough energy in order for electricity to jump this gap. The heat of this coil energy jumping the gap is what initiates the controlled burning of the air-fuel mixture inside the cylinder. So the spark plug gap is critical for timing and efficient burning of the air-fuel mixture.

Too small of a gap will not allow enough coil energy to build up. The ignition point will be advanced in the cycle causing problems such as higher than normal brake mean effective pressure (BMEP) in the cylinder, higher than normal cylinder temperatures, higher idle speed and difficulty starting.

Too large of a spark plug gap will cause too much coil energy to build up. The ignition point will be later in the cycle (retarded) causing problems such as misfiring, low BMEP, lower than normal operating temperatures, lower idle speed and incomplete combustion.

This is only a stab in the dark since it's been almost 15 years since I've read anything on electrical ignition systems.

 

[MrLuckey]

Surprising there hasn't been more discussion. Well I think I'll take Aimee's KTM to Turkey Bay this weekend and I already have a couple plugs with the gap set differently so I guess I do some testing. I set the gap on the new plug at .050 and I have 1 ready at .040 and .030 (.030 matches the one I took out, the manual recommended .060). I am interested in seeing if I can tell the difference in .010 increments!

Thanks for the info Bill - that all sounds logical to me!

I wouldn't say that her bike was hard to start but it did seem to take a handful of kicks at times. And she did have a problem with slightly high idle at times so it'll be interesting to see what opening up the gap a little will do - if anything.

 

[WoodsRider]

On the diesels I work with, changing the injector timing one or two degrees can produce up to a 5% change in peak-load horsepower. Of course I'm talking about engines rated at 3600 bhp. Since the injectors are mechanical this means that opening pressure has a similar effect as the sparkplug gap in a spark-ignition engine. However small changes in timing on a diesel have a greater effect on power output since the heat of compression is used to ignite the air-fuel mixture.

I'm sure Rich knows more about how ignition timing will have an affect on the swirl of the incoming charge and how to properly gap a plug for cylinder that have been ported or had the combustion chamber reshaped.

IMO a stock engine will require a stock gap, but a modified engine will require some modifications to the ignition system for peak performance.

 

[jmics19067]

Woodsrider,

What you explain makes some sense to me but I thought the points <in old systems> is what made the coil charge and discharge. IE points open coil charges up points close coil discharges its energy to the spark plug. Today with CDI's and other electronic ignitions they would have a magnetic trigger to do the job of points.

To over simplify and to the extent of my knowledge I thought spark plug gap was determined by being small enough that the voltage created by the ignition system is able to jump the gap yet large enough to start the combustion process.

I thought the only tuning possibilities of the plug gap was at low rpm the points<magnetic trigger> is opening and closing slower ,allowing the coil to energize more , hopefully sending more voltage, allowing a larger plug gap, creating a larger flame kernel. The other end would be at high rpm with the points opening and closing quicker , the coil not producing enough voltage , you could close the gap to make it easier for the voltage to jump. Also on vehicles that don't have a battery a smaller gap might help it start easier at cranking speeds since the ignition isn't producing much electricity at 5 rpm.

Am I completely off base with the beginning of understanding how these things work?

 

[WoodsRider]

Jim - Like I said, everything is from memory and not dealing with spark-ignition systems for over 12 years has clouded that memory.

Again, from memory (and I'm really pulling this out of my arse), the ignition coil is simply a step-up transformer with primary and secondary windings. The points (or CDI) provide a path to ground for the primary circuit. When the points open there is no longer a path to ground and the magnetic field inside the coil induces voltage in the secondary circuit sending that voltage to the sparkplug. Although I don't remember the exact figures, it would step up the 12-volts in the primary to 6,000 volts (?) in the secondary due to the number of windings inside the coil.

The spark plug gap provides resistance for this voltage. The greater the gap, the greater the resistance. Different amounts of energy will be required to jump plug gaps of various sizes. This energy (the spark) is the ignition source. As the spark varies in intensity, so does the entire ignition cycle. The formula E=I*R (voltage = amperage * resistance) comes into play. Remember it takes one amp to push one volt through one ohm of resistance. While a spark is visible jumping the gap, it is heat energy that actually initiates combustion.

 

[biglou]

I think it's more like 30-60k Volts, Woods. But, that too is from memory! ;)

 

[MrLuckey]

Well I have no idea what kind of voltage to expect from a dirtbike but it seems like Chevy's HEI ignition was in the 45k range while some of the import stuff was in the 60-70k range. I do remember a big blue arc at night from an import coil to my hand pretty vividly though :scream:

 

[WoodsRider]

Lou - I think you're right.

 

[Jaybird]

.060 is twice what I've ever gapped any plug.

 

[jmics19067]

The points (or CDI) provide a path to ground for the primary circuit. When the points open there is no longer a path to ground and the magnetic field inside the coil induces voltage in the secondary circuit sending that voltage to the sparkplug.

yes I agree 100% but what I was trying to say was that I believed that the points closing is what triggers the discharge not the gap which is what I thought you where saying. :think:

.060 is twice what I've ever gapped any plug.

a lot of G.M. products<I know nothing to do with dirt bikes but I thought you should know> with HEI ignitions are supposed to be gapped at .045 or .060 depending on what the sticker on the radiator shroud says.

the more voltage the coil can produce the larger the gap can be . I am only making this statement based on the fact that early gm products with points usually had a gap of .032 with coils rated about 15,000 to 30,000 volts. Where the HEI coils can produce 40,000 to 60,000 volts and some vehicles have a gap of .060 twice the voltage twice the gap? I am assuming that the combustion chambers are of the same type and design for those two different vehicles. Now I can conjecture that turbulance in the combustion chamber has a major influence on spark gap only because of experiences welding in a shop compared to welding in high winds. The electrode in an arc welder has to be held closer to the work to maintain a decent arc in high winds compared to still air and I am assuming the same properties of an arc apply to both instances.

I do believe what Woodsrider says about a larger gap delaying the timing slightly. I can not back it up with anything except that it just make sense to me. I am thinking that the energyfrom the coil has to wait a moment to "ball" up at the spark plug electrode to cross a larger gap compared to "leaking" across a smaller gap. I also believe what Rich has to say about whats happening inside the combustion chamber. If you think about a stale fuel that isnt at a perfect stoichemetric<?> ratio near the plug it isnt going to burn as easily. Anything else that plays into the role is way beyond my knowledge or understanding at this point.

one of the neatest things <although terrible to work on>I have ever seen in a combustion chamber was on a Honda Prelude engine from the early 80's I think. If you can imagine this poor description;
The carburator was a three barrel two were progressive and fed thru a typical manifold into a combustion chamber right above the piston. The third barrel of the carb was fed thru a different intake manifold into this litttle pocket right next to the spark plug. The casting of the head had a drilled wall between the two kinda like a screen. It was a four valve head two for the exhaust one for the typical intake and one for the pre combustion combustion chamber<?>.

 

[WoodsRider]

Pre-combustion chambers, spark cells, energy cells and turbulence chambers are very popular in diesel and industial gas and dual-fuel engine applications. In diesel applications it is known is indirect or pre-chamber injection. And is sometimes used in conjunction with a glow plug to assist during start-up. These chambers contain a limited amount of oxygen so combustion is not complete, but the heat generated in the pre-chamber is enough to ignite the oxygen in the primary combustion chamber. These designs are early attempts to reduce emissions and improve fuel efficiency. Current direct injection EFI systems are much better and don't require the intricate casting and machining of a pre-chamber.

 

[steve.emma]

There is a lot of technial information going around here, but i think some of this is kinda getting off the point a bit. Its true that incorrect gaps do have a effect on engine performance but not to the extent that some people would have you believe remember were talking about dirt bikes here, most of the time you're just trying to stay on not worrying about wether 10 thou change in plug gap makes any difference.
I have done a lot of work with automotive spark ignition systems and the factors that effect plug gap the most are compression ratio and fuel mixture, weaker mixtures and highrer compression ratios both make it harder for a spark to jump the gap between electrode and ground. The amount that different plug gaps has on ignition timming is so small that it could not even be measured unless on a dyno in a lab. The pickup coil position and properties of the cdi unit control ignition timming. As for plug gaps effect on H/T resistance this is also true but again to a very small extent, spark plug and H/T lead internal resistance play a much greater role in this equation. Remember a coil has to capable of sparking a plug at very high rpm under the most serve conditions, so therefore most of the time a coil is running nowhere near its maxium spark output. Spark plug gap like everything else is a matter of comprimise, going too large or too small a gap will have a detrimental effect on engine power, but the amount of power gained by changing plug gap (from recomend) is so small that you would not even notice it, and you can prove this by by trying it one step at a time and see.
My advice for determining plug gap would be to stick to the factory specs and worry about something else.

 

[ohvrider]

One important factor is compression. Have ya ever pulled a fouled plug and checked to see if it sparked in the wide open air and it did? Under compression the gap is much harder to jump. I know this from an old Champion spark plug tester/cleaner I used back in the 80's. It used shop air to pressurize the plug as you watched it spark through a glass. No matter how fouled a plug was [almost] it would spark without pressure. Crank the pressure up and it went around the gap and quit sparking. So if you have a modified engine with oversize piston and milled head you couldn't get away with a larger than stock gap. All cars manufactured during the hei era were low compression smoggers. Therefore they could get away with a .060 gap. The hotter spark actually ignited the lean fuel mixture better. Cheapo spark testers show 60,000 volts to be about a one inch gap. It might take that much or more to jump a .060 plug gap in a 10,000 rpm dirtbike. I say go with manufacturers specs on plug gap.

 

[MrLuckey]

Well I'll try to post more tomorrow but a very short summary.
Manual called for .060 (Never tried it)
The old plug was .030
Installed .050 (didn't run very well at low RPM, big time bogging)
Misplaced .040 plug
Went back to .030 (bog dimished - almost disappeared, ran a lot better)
A little jetting for where I was riding would probably have been in order but we were having way too much fun riding :)

The difference seemed to be very obvious at low RPM and not at all or just barely at high RPM's.

 

[MrLuckey]

Also just wanted to clarify a couple things.
1 - The bike runs like crap with the factory recommended setting.
2 - I'm not looking for HP gains and don't expect any. I am looking for a crisp clean running bike, I'd even give up 1-2 peak HP if thats what it took for a really responsive throttle twist. Just wanting to understand more theory and reasons for the large difference in factory spec and what actually seems to work on the bike.

 

[mingy]

In my automotive experiece it's the voltage of the ignition system that makes all the difference and since you are not changing that I'm sticking with Steve.emma and saying "worry about something else."

Back when I used to bracket race cars we had guys running ignition systems with so much voltage they used to just break off the ground loop and arc from the eletrode to the casting :flame:

If you can modify a two-stroke ignition system to do that I'd be interested......... but I wouldn't recommend riding it in the rain.

 

[MrLuckey]

Originally posted by mingy
I'm sticking with Steve.emma and saying "worry about something else

Thats well and good with me if YOU don't want to worry about it but the fact remains that the manual calls for .060 and it runs like crap even with an .050 gap. Much better with .030 so I think I'll keep digging and asking to try and understand all the nuances a little better.

 

[Doc]

The timing of the spark for inductive systems (points or the current automotive transistor stuff) is controlled by the points opening, where the spark timing on CDI systems is controlled by the timing of the discharge of the capacitor (about 300 volts) through the primary of the coil. Thus, inductive sparks occur when the magnetic field collapses, while a CDI spark occurs as the field is being built. Inductive sparks are longer in duration for a given energy, but CDI sparks have a very fast rise time (time between when charge starts going to the plug and when the plug fires). Short rise times make fouling less of a problem. This is because a spark plug acts like two circuits, a capacitor in series with the gap, and a resistor in parallel to the gap. If the charge does not build up enough charge fast enough on the capacitor to reach a voltage sufficient to jump the gap, the charge will bleed through the resistor (deposits on the core nose) and the plug will not fire. Small gaps help prevent fouling because a lower voltage is required to jump the gap. When an engine is spinning slowly, the charge coil may not be able to charge the capacitor in a CDI system to the normal voltage (again, about 300 V). So, if you ride trials and you want to avoid killing the bike when the RPMs drop a little low, try a smaller gap. Probably the most active factor in determining the required voltage to jump the gap is the stoichiometry in the gap. Lean mixtures require a LOT more voltage to ionize the gap.

Doc

 

[MrLuckey]

WooHoo - thats the kind of information I was looking for! Thanks Doc, I'll have to read that a couple more times but keep it coming. Lets talk about this 'stoichiometry' some more!

Anyone else? Did the aftermarket porting significantly change what was required as far as gap is concerned?