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Resistor spark plug caps


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I think this question is possibly one for Clive to field but any one else please feel free to add your thoughts.

 

I have just finished reading an article on the effect of resistor ( suppressor) spark plug caps on the duration and strength of the spark generated at the plug.

 

The article suggests that by using internal resistor caps the duration of the spark flare is increased however, the strength of that spark is reduced.

Further more the plug is more susceptible to shorting out as a result of either moisture, oil or fuel contamination.

 

All very interesting theoretical stuff.

 

However it struck me that this may be a contributory factor with bad 6v starting, the coil low tension voltage is sucked away by the starter so the high tension out put is lowered and the resistor in the plug cap drops the output even further resulting in a long duration low power spark or possibly no spark at all if the plugs are damp or the compression of the engine is high.

 

I know all US 6 volt ignition systems had suppressed caps but I am left pondering if the use of non resistor caps may be beneficial by giving a shorter but more intense spark at the plug.

 

Not good I appreciate for radio or TV reception........... comments and thoughts invited............

 

Pete

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Pete there are two purposes for incorporating resistance in the secondary circuit.

 

To reduce erosion at the gap (& the distributor cap). This is said to be in the order of 1,000 - 4,000 ohms, but is not sufficient to reduce RFI. To suppress RFI 10,000 - 20,000 ohms is needed.

 

Driver Training (All Arms) Vol II "Contrary to popular belief held in some quarters, they [resistors] do not affect the performance of an engine in any way. In fact, in some engines, they improve the cold starting"

 

I did some experiments to measure the current flowing through the secondary circuit & found it to be very low. I think the introduction of 10,000 ohms only reduced the HT by 7 volts.

 

The thing to bear in mind with the voltage for the primary circuit, provided the battery is fully charged, that this voltage is just to build up the magnetic field so it can collapse & produce a back EMF to be stepped up by the coil secondary winding. The equivalent 12v coil the HT output would be of similar strength. Admittedly a load from the starter motor will reduce the energising voltage a bit but it is not necessary for the coil to be 100% saturated with magnetism to give a satisfactory output.

 

Have you still got the reference to that article?

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Here we go Clive http://www.ultralightnews.ca/articles/resistorcapsandplugs.htm,

 

I found the article while looking for some suitable plug caps for an on going project (not aeronautical I hasten to add much more hum drum and non military).

 

I wasn't completely convinced by the findings so that's really what started me thinking and prompted my question here, have a read and see what your conclusions are.

 

Pete

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Driver Training (All Arms) Vol II "Contrary to popular belief held in some quarters, they [resistors] do not affect the performance of an engine in any way. In fact, in some engines, they improve the cold starting"

 

Is this why the old metal shirt button attached in series with the HT lead is believed to be beneficial Clive ?

 

Pete

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Pete thank you for that it makes an interesting read. It is written in sensible English by a bloke who has thought things through & sought to investigate things for himself, which is excellent. He writes:

 

"Once the plug fires the resistance comes into being. The coil cannot get rid of its energy in the shortest possible time due to the resistance.

 

This reduces current flow in the spark and it takes quite a bit longer until the energy in the coil has expelled itself via resistor and spark gap. This results in a longer spark. However the spark is weaker due to energy loss in the resistor."

 

Yes that is the whole point!

 

To reduce the erosion at the plug gap, we all know that the higher the current of a welder the greater will be the loss of metal from welding rod. That of course is why the designers & manufacturers incorporate resistance in ignition systems.

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Is this why the old metal shirt button attached in series with the HT lead is believed to be beneficial Clive ?

 

Pete

 

Yes it does work Pete but not with a metal button as that would just be a short circuit. Here is my effort some years ago.

 

IGN 30a.jpg

 

This the basis of the spark enhancers that are basically two self tapping screws with a head to head gap.

 

I don't know what effect it would have to increase the gap for the rotor arm, would it be similar?

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I don't know what effect it would have to increase the gap for the rotor arm, would it be similar?

 

Interesting point Clive, which brings me to ask how the rotor/ terminal gap was determined in the first place, in other words there must be an optimum clearance that is/was calculated by the designers when producing distributors and that begs the question are all rotor/contact gaps the same when new ?

 

Pete

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Pete I had always assumed it was a minimum gap to allow for manufacturing tolerances plus a bit to allow for slight wear/minor misalignment of the rotor arm to avoid the risk of the arm ripping into the posts in the cap. Interesting to shave some metal off the rotor arm & see if that did the job of a "spark enhancer".

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Well, for the rotor gap I suppose that there is probably the issue of producing a wider range of rotor arms, and then what happens when eventually someone fits resistive leads or caps to your system fitted with a resistive distributor?

 

trevor

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Slightly off topic but on the same track, I have worked on several Honda OHC mower engines fitted with resistor spark plugs which would start and run perfectly for about 3 minutes then die. The culprit in every case was the resistor plug which was failing in some way due to the effects of heat and long term vibration.

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Slightly off topic but on the same track, I have worked on several Honda OHC mower engines fitted with resistor spark plugs which would start and run perfectly for about 3 minutes then die. The culprit in every case was the resistor plug which was failing in some way due to the effects of heat and long term vibration.

 

Not off topic at all, very interesting in fact so what was the solution? was it a good old J8C?.

 

One of the reasons for starting this thread was to prompt a discussion about a topic that causes by far the most problems for owners of older vehicles. The interaction and interdependence of HT ignition systems is perhaps the least understood of all systems on a vehicle.

 

Pete

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Interesting to shave some metal off the rotor arm & see if that did the job of a "spark enhancer".

 

That's where I was going with that thought Clive,........ which would mean as the rotor slowly erodes the spark flare duration at the plug should get longer but the power of the spark will decrease ?

 

Pete

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Well, for the rotor gap I suppose that there is probably the issue of producing a wider range of rotor arms, and then what happens when eventually someone fits resistive leads or caps to your system fitted with a resistive distributor?

 

trevor

 

What indeed !

 

Pete

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That's where I was going with that thought Clive,........ which would mean as the rotor slowly erodes the spark flare duration at the plug should get longer but the power of the spark will decrease ?

 

Pete

 

But Pete the rotor doesn't erode. It receives the metal lost from the pillar electrodes in the cap, so in a 4-cyl a quarter of the metal lost it is shared between the pillars. Each losing a quarter share of metal. If the HT was wired incorrectly to give a positive spark then all the loss would be from the rotor arm (not to mention the other issues with a positive spark)

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Slightly off topic but on the same track, I have worked on several Honda OHC mower engines fitted with resistor spark plugs which would start and run perfectly for about 3 minutes then die. The culprit in every case was the resistor plug which was failing in some way due to the effects of heat and long term vibration.

 

Hi

 

Interesting thread, question for radiomike, how did you determine that it was the plugs at fault? Was it process of elimation, you replaced the plugs with new and the engine would run without dieing?

 

Cheers Phil

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But Pete the rotor doesn't erode. It receives the metal lost from the pillar electrodes in the cap, so in a 4-cyl a quarter of the metal lost it is shared between the pillars. Each losing a quarter share of metal. If the HT was wired incorrectly to give a positive spark then all the loss would be from the rotor arm

 

Is this due to true electron flow in a conductor then Clive? ......Hmm your going to have convince me that it wouldn't just take four times as long (on a 4cyl engine)

 

Time for brew and a think :coffee:.

 

Right back on it,.....Or are you saying the whole system in terms of metal deposition/loss is in equilibrium putting aside things like failing condenser and mechanical ware in bushings ect.?

 

Point taken about reverse wired LT on the coil.

 

Pete

Edited by Pete Ashby
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Pete I hadn't actually thought about an equilibrium of metal transfer. But I suppose it must be so, when arc welding I assume that all the metal from the rod does end up in the weld.

 

Well most ends up in the weld, the rest on your walls, clothes, in your lungs.... :undecided:

 

I sometimes wonder whether there is a maximum sustainable spark flame length for any gap, because the elements of the spark seem to travel in an oval shaped field rather than a cylindrical one. If we had, for arguments sake, just one gap in our HT system between coil and cylinder head then would the maximum size of this gap be noticeably less than the maximum combined lengths of all the gaps we do or may have: rotor, spark plug and air gap resistor? Is the fatness of the spark a limit on its maximum length?

 

trevor

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Hi

 

Interesting thread, question for radiomike, how did you determine that it was the plugs at fault? Was it process of elimation, you replaced the plugs with new and the engine would run without dieing?

 

Cheers Phil

 

I used the online diagnostic tool Google before I even looked at the first one, it was a well known problem! What I did not mention was that once the plug had cooled they would run fine again for 3-4 minutes before cutting out.

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Iv'e been doing a bit of this :readbook: and it seems the same question exercises the the biking fraternity greatly with as much huffing and puffing as can be found where ever any enthusiasts gather.

Iv'e also had a poke around one or two spark plug manufactures web sites as well some interesting stuff and well worth reading their technical pages (remembering they are in the business to sell you their brand of plug ).

 

On balance from what I have gleaned:

 

The fit or not to fit question:

 

Modern engines with radios, EMU's, electronic wasted spark coils and other electronic gadgetry should have and indeed need restive plug caps and in some cases restive plugs to reduce RF interference to a minimum.

 

Older manual distributor and wire wound coil systems with in house radios benefit from restive plugs OR restive caps but not both as the view is that this may have an effect plug fouling and possible misfiring at low engine loads.

 

Older manual distributor and wire wound coil systems without any other electronic devises do not need any additional resistance in the circuit over and above that which is produced by the HT lead note; this should be plain copper not modern carbon fibers.

 

The effects:

 

There is an awful lot of band width used up on the effects of using restive plug caps on older systems, the main issues seem to be:

 

Potential for poor cold starting, electrode fouling, rough low load running, over heating electrodes and reduced fuel mileage (not that the last one is much of an issue for most of us, the difference between 5 miles to the gallon and 4 is not worth the bother for us :))

 

So what are we to make of all that ? taxing ( and possibly exceeding :blush:) my weak grasp of basic electrical theory and with the use of the stub of an old HB pencil I found at the back of a draw I conclude that:

 

24 volt there is probably negligible effect

 

12 volt if you don't have to use restive caps don't use them, if you have to use the caps then don't use restive plugs as well

 

6 volt there is the potential for a lower power spark if using restive caps which may or may not result in poor hot starting depending on a host of other mechanical factors relating to the health of your particular engine.

 

In conclusion if you run a 6 volt system you may be better off without them The neighbors may not like you but you just may stand a chance of not having to be pushed out of the arena :red::red:

 

Comments and discussion welcomed......... I'm off to lie down for a bit :confused:

 

Pete

Edited by Pete Ashby
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I used the online diagnostic tool Google before I even looked at the first one, it was a well known problem! What I did not mention was that once the plug had cooled they would run fine again for 3-4 minutes before cutting out.

 

As an aside, many years ago while I was still young and foolish, before I got older, I took my modified Honda C70 down the Alps to ride the trails there. I soon discovered that as the engine got really hot, as in doing 5-10 mph up a very steep road, it would start to cut out. Changing the plug for another old one would solve the problem temporarily, as would leaving the bike to cool. A new plug would solve the problem for maybe a day.

 

The real problem turned out to be the location of the condensor, mounted inside the generator on the plate that divided the generator zone from the crankshaft zone, and the extra hot oil lapping around on the other side of the plate was killing the condensor. So the Honda engine may have a problem, and it may indeed be the plug, but it could also be something else.

 

Interestingly, I removed the condensor and replaced it with an old Mini condensor I happened to have, which I mounted on the frame in the cool, cool air about a foot of wire away from the points. This ran without fault for several years, as long as I had the bike, which begs the question of how close to the points does the condensor need to be?

 

trevor

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