I recently took my car to have it dynoed only to have the results hampered by a recurrence of spark blowout issues. I've just about ironed out the problem at least in terms of establishing a blowout free boost limit without the use of ignition aids (Kenne Bell Boost-A-Spark). In the process of doing more reading on the subject I was drawn to the topic of spark plug resistance. Most of the discussion regarding what plug is best for boosted applications with ignition challenges is based on experience and hear say without much else to support the rational.

I have four sets of plugs as a result of addressing my ignition troubles and decided to take a multimeter to them. Here is what I found:

Two sets of Bosch Super Plus Copper Plugs #7980, that were preceeded by NGK TR6s all following the stock Delco plugs used in the 3900.

Installed after discovering I had a serious problem with spark blowout.
Plug resistances:
First set: 4.89, 5.36, 5.83, 6.29, 6.64, 7.26. Total resistance; 36.27 K ohms
Sec set: 4.43, 5.97, 6.50, 7.60, 7.62, 8.38. Total resistance; 40.50 K ohms
NGK TR6: 3.52, 3.56, 3.65, 3.88, 3.92, 4.08. Total resistance; 22.61 K ohms
OE Delco Plug: 5.58 K ohms only have one handy.

Pending,
AC Delco R42LTS used in the Turbo Grand Prix currently in the car gapped to .030 good to about 8 psi so far. I'll be removing them for a regap to .020 depending on the resistance. Otherwise I'll be reinstalling the NGK plugs which along with the Bosch plugs did sustain boost levels in excess of 10 psi however, because the problem was so unpredictable I can't say at the moment which was most consistent as I had more problems immediately upon installing the second set of Bosch plugs which has a higher total resistance and I can't recall all of the details surrounding the NGK plugs when they were installed since the MSD coils were driving them and later found not to do near as good a job as the used but good AC Delco coils that replaced them.

Long story short, check plug resistances where possible and or needed, so that you can better impedance match. Whether this will make a difference I don't know but when you consider the variance in resistance in each cylinder increases with the length of the plug wire, it might be wise to pair the highest resistance plugs with the lowest resistance (shortest) wires.

It won't make any difference. The resistance only factors in after the spark has jumped the gap, before that, no current is flowing so the resistance isn't seen. The biggest factor is the gap. Half the gap takes 1/4 the voltage to jump it, so a small decrease in gap greatly decreased spark blow out. The gap makes very little difference in the hp output of your engine. Larry

quote Originally posted by trotterlg: It won't make any difference. The resistance only factors in after the spark has jumped the gap, before that, no current is flowing so the resistance isn't seen. The biggest factor is the gap. Half the gap takes 1/4 the voltage to jump it, so a small decrease in gap greatly decreased spark blow out. The gap makes very little difference in the hp output of your engine. Larry

The problem I have with accepting what you're saying without question is that resistance, be it in the wires, the plug, or the plug gap, is indeed resistance. The plug resistance is in series with the gap resistance which varies for a number of reasons, but most significant here is air density due to compression levels. The voltage delivered to jump the gap is only as much as needed to do so. You don't get a 42 kv discharge from our systems to meet a 22 kv demand, that's why high output coils are often a waste of money without the complete aftermarket outfit designed to maximize their output.

If the resistance increases, so does the voltage requirement and if that requirement exceeds the ignition systems capacity to deliver, you generally get a misfire. The reduction in gap is an effort to restore proper and consistent ignition to an area that has exceeded the limits of the ignition system by reducing the resistance, so if gap reduction makes it possible for your motor to pull through to its rpm peak it does have an effect on hp output although indirectly as the upper rpm prior to power peak is where blowout is most disruptive. Whether 1-5 K ohms is significant enough to matter in my case I don't know but it's hard to ignore when there is a problem, given the inconsistencies in some of the plugs. If by chance I paired the two plugs with the highest resistance it stands to wonder.

Do you have any formulas to support what you're saying about the resistance of the plug itself not mattering, considering increasing voltage seems to be the common approach to addressing the problem barring gap reduction?

I believe you are speaking about the radio frequency interference RFI and non resistor vs resistor plugs where resistor plugs are documented to reduce spark energy in their effort to prevent ECM/radio interference.

[This message has been edited by Joseph Upson (edited 08-31-2013).]

No, you are wrong, the resistance is meaningless until the plug arcs, then the resistance comes into play with how much current flows. Once the arc is established it takes a smaller amount of voltage to sustain it because the arc is flowing through ionized air (and fuel). Bet if you measure the resistance of your spark plug wires you will find that they are much higher than the spark plugs are, plus the logner wires will have more resistance than the short ones do. This is because your "wires" aren't really wires at all but conductive plastic or fiber. Larry

[This message has been edited by trotterlg (edited 08-31-2013).]

quote Originally posted by trotterlg: No, you are wrong, the resistance is meaningless until the plug arcs, then the resistance comes into play with how much current flows. Once the arc is established it takes a smaller amount of voltage to sustain it because the arc is flowing through ionized air (and fuel). Bet if you measure the resistance of your spark plug wires you will find that they are much higher than the spark plugs are, plus the logner wires will have more resistance than the short ones do. This is because your "wires" aren't really wires at all but conductive plastic or fiber. Larry

Wrong about what? As for my plug wires, the resistances are all under 200 ohms ea or better yet less than 50 ohms per foot as stated by MSD, I measured them before install. I've already indicated the longer wires produce more resistance than the shorter wires aside from it being common physics knowledge.

Do you have formulas to substantiate what you are saying?

I understand ohms law; V = IR V= voltage, I = current, R = resistance.

So, I = V/R therefore reducing R = > I correct?


This is not a contest, I'm looking for information I can use to help solve a problem. I'm not saying you're wrong, I'm just not willing to ignore the general physics knowledge I have without appropriate formulas or documentation to justify it and at the moment I believe you are mixing radio suppression with spark energy resistance.

The formula shows that when no current flows no voltage is dropped across the resistance. This then shows that when the arc starts no voltage is dropped, durnig the arc voltage is dropped in the plug depending on the amount of current flow, but since the arc is already established the resistance has very little effect on it. My guess for you is that if you reduce your spark plug gap by 1/3rd to 1/2 your problems will go away and you will find no loss in power. Installing gapped down plugs is easy, give it a try and I bet you will be a happy person. When they say the spark is bown out it is really incorrect, the spark never starts because compressed air is a very good insulator. Larry

[This message has been edited by trotterlg (edited 08-31-2013).]

quote Originally posted by trotterlg: Installing gapped down plugs is easy, give it a try and I bet you will be a happy person. When they say the spark is bown out it is really incorrect, the spark never starts because compressed air is a very good insulator. Larry

Now you're confusing me. I stated as much from the beginning in regards to what I intend to do upon reaching the limits of the already gapped down plugs when I stated I would reduce the gap further to .020. It appears you haven't been reading every word of my long posts. I know spark blowout is a misnomer, but it's the most commonly used term and therefore better used when searching on the subject.

I appreciate your input, but I'm bent on proof where there is a conflict with what I do understand, for example, based on what you have said, I should be able to increase the plug resistances I have posted to 10,000 K ohms with all else the same and there should be no difference in performance, and I don't believe that by any stretch. The plug terminal to electrode (not the ground strap) resistance is in series and I can't ignore that or believe that it has no effect on spark energy at this point when all of GOOGLE research is telling me otherwise.

[This message has been edited by Joseph Upson (edited 08-31-2013).]

By your own formula you can see it only drops about 11% across the plug resistance. This is not much. Hope you find an answer you like, this is what I do for a living, so I am fairly sure the calculation is correct. A drop of 500 volts out of 45,000 isn't causing your problem, look some place else, like for a bad cap or rotor or coil, when the voltage required to jump the gap goes up because of the compressed charge the spark may be finding an easier path to ground some place else. I think you are just fixated on the plug resistance being the problem and I doubt it is. Larry

quote Originally posted by trotterlg: By your own formula you can see it only drops about 11% across the plug resistance. This is not much. Hope you find an answer you like, this is what I do for a living, so I am fairly sure the calculation is correct. A drop of 500 volts out of 45,000 isn't causing your problem, look some place else, like for a bad cap or rotor or coil, when the voltage required to jump the gap goes up because of the compressed charge the spark may be finding an easier path to ground some place else. I think you are just fixated on the plug resistance being the problem and I doubt it is. Larry

Didn't catch your other post, and if this is what you do for a living I respect that however, so far you have not produced anything beyond making statements to explain away the plug resistance possibly contributing to a problem in a system already near the limits where small percentages are relevant. 11% of 1oz vs 11% of 100 lbs is relative and significant if it's about to be dropped on your foot.

I didn't catch how you arrived at ~11% between the lowest and highest resistance plug and since this is in a DIS system the plug resistance would be combined for each of the 3 brand new ignition coils. If you know the science very well then what better person than someone of your background to help me understand scientifically what you are proposing. I can't take this subject to another forum with others having the same problem and say "It's this because Larry said so and he does this for a living.". If you ever get the time to post the formulas, or physics laws I'm far removed from to clarify, I'd appreciate it.

I recall during an undergrad physics course working with a formula to determine the voltage necessary to jump an air gap of a given distance taking into account atmospheric pressure and temperature that would be more helpful here, but unfortunately it was just ahead of when I took my dive into the motor world.

I just want to understand so help as best you can, otherwise I have to acknowledge the plugs used in race applications are usually resistor less for the very reason I believe, they deliver more of the spark energy than a resistor plug does.

OK here is the math: Your coil is about 3 ohms resistance, imput to it is 12 volts, so the step up to 45,000 volts is 3,750. Your coil will draw 4 amps on the primary, so with the step up of 3,750 you will be able to source .0011 amp at 45,000 volts. .0011 amps on the high voltage side in series with 5,000 ohms gives a voltage drop of 5.5 volts (yes, I was wrong, it is not 550 volts). So when the plug arcs, it will drop 5.5 volts across the plug internal resistance and the rest in the wire and the spark gap and the internal resistance of the coil secondary. This really doesn't happen because the arc resistance is very very low, so the voltage is really dropped across the other parts of the system, mainly the internal resistance of the coil I would guress. I would say this a an accurate reperensation of what happens in the instant the spark initiates. The big trick is to get enough voltage to the plug to fire the arc, once the arc is established it will take care of it's self. This is why I think you will find your system has a lower path of resistance to ground than your plug gap. Larry

quote Originally posted by trotterlg: OK here is the math: Your coil is about 3 ohms resistance, imput to it is 12 volts, so the step up to 45,000 volts is 3,750. Your coil will draw 4 amps on the primary, so with the step up of 3,750 you will be able to source .0011 amp at 45,000 volts. .0011 amps on the high voltage side in series with 5,000 ohms gives a voltage drop of 5.5 volts (yes, I was wrong, it is not 550 volts). So when the plug arcs, it will drop 5.5 volts across the plug internal resistance and the rest in the wire and the spark gap and the internal resistance of the coil secondary. This really doesn't happen because the arc resistance is very very low, so the voltage is really dropped across the other parts of the system, mainly the internal resistance of the coil I would guress. I would say this a an accurate reperensation of what happens in the instant the spark initiates. The big trick is to get enough voltage to the plug to fire the arc, once the arc is established it will take care of it's self. This is why I think you will find your system has a lower path of resistance to ground than your plug gap. Larry

I appreciate your patience and the above is something I can follow and apply. This problem has been such a burden that I've conducted several changes as well as made various purchases in an effort to combat it so I have a good idea about many of the working specs, like for instance my ignition coil resistances are ~14 ohms for the primary and right at 6 K ohms for the secondary, my voltage input at the coils ~13.7 volts due to a "Hot Wire" setup with a direct feed from the alternator through a relay activated by the old harness supplied voltage that measured in the 12 volt range.

I did read across a scenario involving leads and how adding a resistor to the circuit regulated the energy supply in a manner that increased efficiency and therefore brightness well above what was produced without it, however with ignition systems there are exceptions due to design and various other differences. Another confounding issue here is that the MSD and Accel coils are said to be manufactured with more windings to produce more voltage than OE coils yet when I installed a new MSD set with plugs my 10+ psi immediately dropped to 4 - 5 psi and when I reinstalled the old plugs I still did not easily recover the 10 psi boost mark.

So I have to explore all possibilities. I've hot wired the power to the module/coils, installed new AC Delco coils, 8.5 mm wires and now the plugs are all that are left along with plug gap which from experience .020 is about the lowest I can go before idle starts to feel like a misfire. I have sourced some suppressed and resistorless racing plugs that I plan to go check out tomorrow and decide whether or not to give them a try.

Thanks Larry, I really appreciate that.

The bottom line is getting the plug to do the initial arc is the trick. Once the plug fires the resistance across the arc (plug gap) is very low and the other componetnt in the system limit the energy in the spark. The resistance in the total system is the limiting factor in the spark energy, not just the voltage, you can have a very high voltage spark but if the energy (current times voltage) is very low it will not set off the charge. I really think you have some other problem, but you could also try some fine wire plugs, a spark more easily jumps a gap between two pointy electrodes than it does between two wide ones. Never know, let us know what your find in your system. Larry

quote Originally posted by trotterlg: The bottom line is getting the plug to do the initial arc is the trick. Once the plug fires the resistance across the arc (plug gap) is very low and the other componetnt in the system limit the energy in the spark. The resistance in the total system is the limiting factor in the spark energy, not just the voltage, you can have a very high voltage spark but if the energy (current times voltage) is very low it will not set off the charge. I really think you have some other problem, but you could also try some fine wire plugs, a spark more easily jumps a gap between two pointy electrodes than it does between two wide ones. Never know, let us know what your find in your system. Larry

I probably shouldn't refer to it as a problem but more of a challenge. I'm basically at the limits of the stock equipment and dealing with an issue I did not forsee. The compression is 11:1 and with boost on top of that it doesn't take long to find the limits of the stock equipment. I'm just trying to get as close as I can to the max boost pressure I want to run on the street without having to add another electronic device. I was having misfire problems for more than a year before I finally identified it, as boost was increased.

I hear what you're saying, but so far in dealing with my engine, theory has not always held true, for example so far to date the cheap Bosch Super Plus copper plugs with the wide electrode and ground strap have performed better than the narrow tip NGK TR6 irridiums to date although I'm going to give them another try to be sure. There are a lot of variables to account for like compression chamber design etc.

Mean while I checked the AR3924 resistor race plug resistance (didn't buy them) and it was over 8 K ohms so if I don't get the improvement I'm hoping for with the TR6 plugs gapped to .020 I'll try the non resistor race plugs AR3934 and see what happens. If that doesn't do the trick I'll have turn to the boost a spark.

[This message has been edited by Joseph Upson (edited 09-01-2013).]

I might be begging the obvious here, but Joseph have you tried indexing the plugs? Open side of the ground electrode pointed towards the exhaust valve?

Have you tried any other types of non conventional plugs? Spark plugs, even "premium" ones are a small investment and you might try plugs like the Bosch platinum +4 plugs (or any other plug that has four ground electrodes). Before you install them use a Dremel tool to cut off two of the four ground electrodes - two that are 180° apart from each other. Yes these plugs are better suited for four valve engines where the spark plug is top center of the cylinder and perpendicular to the top of the piston, but if regular plugs are not lighting off for you, give it a try. Can't be any worse that the results you're currently getting.

quote Originally posted by lateFormula: I might be begging the obvious here, but Joseph have you tried indexing the plugs? Open side of the ground electrode pointed towards the exhaust valve? Have you tried any other types of non conventional plugs? Spark plugs, even "premium" ones are a small investment and you might try plugs like the Bosch platinum +4 plugs (or any other plug that has four ground electrodes). Before you install them use a Dremel tool to cut off two of the four ground electrodes - two that are 180° apart from each other. Yes these plugs are better suited for four valve engines where the spark plug is top center of the cylinder and perpendicular to the top of the piston, but if regular plugs are not lighting off for you, give it a try. Can't be any worse that the results you're currently getting.

Thanks but one thing I've found consistently in reading up on the subject is that platinum plugs are an absolute no-no for boosted engines. I've also run across info suggesting that indexing the plugs is not worth the effort. The race plugs I looked at have the ground strap cut back to theoretically reduce impedance of the flame kernel but you have to be able to ignite the mixture to get that far and I don't see them being very helpful in that regard.

I'm on my fourth different plug type having picked up another set of Bosch coppers 1 heat range lower than what I was previously using with good results and gapped them to .020. I decided to give them a try instead of gapping the AC Delcos down from .030 because at sustained 8 psi I got 11 deg of knock retard on about 2 occasions which I don't ever recall happening before so a plug or two maybe getting unusually hot or it is possibly a rich source as although the avg AFR at the knock point was in the 11s the actual AFR preceeding and during the event was 10.4 and decreasing in small increments following the AFR trace in the log. Since it has run that rich many times before without knock I'm blaming the plugs.

Time will tell as proper sleuthing would be to gap the Delcos down first and see what happens so if the new plugs don't result in an improvement I'll be putting them back in at .020.

Funny this thread appears this morning as I think I have spark blowout (some misfiring at WOT- whatever you want to call it) and I was planning on switching to autolite 103 plugs today gapped to .040. I cannot feel it, but since I upped the boost to about 13lbs with a 2.8 pulley & E85, I see sharp irregular movements to from say 890mv down to say 730mv on the narrowband on my scans of WOT runs. At the same moment, the wideband is not picking up anything - which makes sense because the narrowband has much goes from roughly from 100 to 999 mv while the wideband goes from 1-5 volts. Questions for you.
1. What boost level are you at?
2. What fuel are you using.
3. How are you diagnosing that you have this problem?

Thanks & Good luck!


------------------
84 NB, 3800SC, E-85, VS Cam, 3.0 Pulley, 4T65E-HD, HP Tuners, AEM Wideband, Regal GS Gauges, S-10 Brake Booster. 1/4 mile -12.05 at 111.7mph

[This message has been edited by DimeMachine (edited 09-02-2013).]

quote Originally posted by DimeMachine: Funny this thread appears this morning as I think I have spark blowout (some misfiring at WOT- whatever you want to call it) and I was planning on switching to autolite 103 plugs today gapped to .040. I cannot feel it, but since I upped the boost to about 13lbs with a 2.8 pulley & E85, I see sharp irregular movements to from say 890mv down to say 730mv on the narrowband on my scans of WOT runs. At the same moment, the wideband is not picking up anything - which makes sense because the narrowband has much goes from roughly from 100 to 999 mv while the wideband goes from 1-5 volts. Questions for you. 1. What boost level are you at? 2. What fuel are you using. 3. How are you diagnosing that you have this problem? 4. Low voltage at the module doessn't help. Thanks & Good luck!

1. What boost level are you at?

Currently I have boost limited to 8.5 psi. The most I've seen on the gauge is a little North of 18 psi. The ignition breakup is rpm sensitive usually coming in by 4800 rpm in the form of a pop mostly but sometimes small intermittent pulsatile sensations that can be felt in the seat which I'm sure are misfires before the massive sparkout occurs.

2. What fuel are you using.

Currently 93 octane only, as I retune boost range to gasoline before turning the methanol back on. Lately when in use I've run about 100% methanol set to come on around 10 psi for the cooling effect on the intake manifold. I also have a preturbo nozzle which is part of the reason boost is holding at 8.5 psi with the current gate setting while the alcohol is off because it does increase compressor efficiency. The water content in the water/meth mix can contribute to spark blowout I've found simply because it is not flammable so reducing the ratio of water actually helps. At one point before I realized what it was the spark blowout was horrible coming in as low as 5 psi and coincidentally when I was running 100% water.

3. How are you diagnosing that you have this problem?

It was an un seen exhaust leak that helped direct my attention to it when trying to diagnose sudden consistent very low boost misfire prompted me to change to a different coil and module combo. One assembly resulted in the stereotypical pop at break up and the other assembly did not pop at all but instead went into a full misfire characteristic of what a grounding plug wire would cause. It was the pop by the way that I identified as blowout since prior to the coil/module swap it never popped, it just fell over and dumped fuel.

Once the loose manifold bolts were found and tightened, I switched to copper plugs 50/50 alc mix and the problem went away until I started running the car harder in the new boost range it achieved. Gapping the plugs down resolved that but because I had several things causing the blowout and I was only finding one at a time, the trouble kept resurfacing.

Causes I've found by experience:

Water ratio injected
Exhaust manifold leak
Spark plug gap (Smaller the better until idle starts to break up, for me that point is .015)
Spark plug type (Narrow tip irridiums are questionable although the NGK TR6 Irri are still questionable at this point)
Bad coil (Right after the right combo seemed to be in place and boost hitting 12-13 psi without trouble a blip appeared until boost was knocked down to 4 psi. An ohm check identified the bad coil).
Good aftermarket coils (With MSD coils it was worse immediately)
I found a 1 volt difference between the harness power supply and the output from the alternator so this can affect your available energy as voltage from the alternator decreases with heat build up.

I went through all of that so you can see how frustrating it can be to have the motor run good for a short while after a correction only to have it go back to the same trouble for a different reason.

The AC Delco plugs were holding 8.5 psi pretty well at the high .030 gap despite the knock retard incident which after a second review of the data trace showed it occurred as the throttle was being lifted gradually. Hopefully the new coppers will not require me to give them another shot at .020. The true test is getting the motor up to 5500 rpm with enough boost pressure on it to test ignite ability effectively. So far 4800 rpm is about as high as I can go without looking like I'm in a real big hurry.

[This message has been edited by Joseph Upson (edited 09-02-2013).]

Found the problem. Either the new AC Delco coils are counterfeit, or they were blemished kick backs purchased by a company that deals in bulk OE parts purchases and sold to me. I reinstalled the used factory coils with the cylinder ID numbers on them and so far the misfire issue is non existent up to 11 psi at this point, I'm still testing.

I inspected the new coils and found some cracks in the bottom of the casing on two of them along with a difference in the texture of the plastic compared to the old coils suggesting that they may have been sold to the aftermarket vendors for that reason as blemished instead of sent to the dealership stock provided they are originals. I also noticed a fine crack in the same region on the remaining new unused coil as well.

In a stock vehicle they will probably work fine for a long time but in an application that's going to tax them such as in my case they're nothing but trouble. By the time I took them off the car the most boost pressure I could run reliably without breakup was about 7psi.

I also have to start adding back the timing I removed due to spark knock in areas that were previously fine to make sure it wasn't the result of the coils breaking up.

From now on I think I'll take my chances with a used factory set from the salvage yard as I can't be sure at this point the dealer parts will be any better.

The used factory coil is on the right for comparison, note the difference in the plastic surface that makes the new coils look like they're encased in a brittle recycled plastic. The cracks start at the top of each yellow strip.



Here is a close up of the middle coil, the crack in the coil on the left in the same location is a little worse than this one.

[This message has been edited by Joseph Upson (edited 09-09-2013).]

At least now you have enough spark plugs to last the life of the engine. Amazing how easily it is to get fixated on a problem and the cause and not see the other things that may be causing it. Larry

quote Originally posted by trotterlg: Amazing how easily it is to get fixated on a problem and the cause and not see the other things that may be causing it. Larry

It's perfecty reasonable to expect brand new name brand parts to perform as such and not be at the top of the list for suspicion. New plugs followed the new coils as a process of elimination. That didn't fix it so I circled back to the coils and found both the direct (part) and indirect (manufacture specifics) problem.

At a paultry $10 expense for new lower heat range plugs prior to finally resolving the problem, my fixation was right where it should have been, with the ignition system mainly the plugs because in the past they have caused the exact same problem and narrowing the gap which you suggested and I had already done, is not always a cure all such as in this case and which is why I went back to the coils in the first place. Tnx for the input on plug resistance though.

This is for your boosted 3.9 liter engine correct? Rather than AC Delco coils, you should have gone with Denso OE replacement units. This stuff is all made in China today, but some manufacturers have higher quality standards than others.

quote Originally posted by lateFormula: some manufacturers have higher quality standards than others.

Tell me about it, until now I thought they were pretty much the same (OE that is). For stock use it's probably a non issue. I never expected to see what I found.

[This message has been edited by Joseph Upson (edited 09-09-2013).]