plug lead resistance - too much of a good thing?
Discussion
I changed the plug leads at the weekend, the replacements are the standard Range Rover leads which are labelled as suppression leads. I think this is normal and and I take it to mean they have a built in resistance to suppress radio interference. As normal with a TVR/Rover V8, it also has a set of plug extenders which I think have resistance built in. I've noticed that the engine is now a bit hesitant for the first minute or so after starting from cold. I wonder if this is caused by a weak spark - is it possible I've got the wrong leads?
They should be for a Classic Range Rover. I used the Lucas ones in the Griff and they have been fine.
AT about 25% of the cost of Magnecors I can afford to replace them a few more times. Not totally convinced about this they are better argument as most of the comparisons are duff grotty worn out leads against new ones. The tests I have seen that compared new with new indicated no real difference. Magnecors do last a bit better though.
AT about 25% of the cost of Magnecors I can afford to replace them a few more times. Not totally convinced about this they are better argument as most of the comparisons are duff grotty worn out leads against new ones. The tests I have seen that compared new with new indicated no real difference. Magnecors do last a bit better though.
Hold tight - here comes a major warble about sparks!
There are two phases two the ignition cycle, which impose different demands on the system.
When the spark is required the current supply to the ignition coil primary circuit is switched off. The resultant collapse of the magnetic field in the core of the coil is what induces the rapid rise in voltage in the secondary windings.
Current does not flow in the system until the arc across the plug electrodes has started (been "struck"). The resistance of the plug leads is only important after the spark has been initiated and current flows. So there are two distinct phases - initiation where voltage is important, and then the burn where the resistance of the circuit matters.
The voltage required to strike the arc is dependant on many factors, which basically boil down to how difficult it is to ionise whatever gas is between the electrodes of the spark plug.
Some factors that affect the secondary voltage required are the cyclinder pressure (increases with load, higher load requires higher voltage), spark plug gap, spark plug electrode size (thinner pointy electrode creates higher electrostatic field, so requires less voltage), fuel used (LPG is more difficult to ionise than petrol), etc. Hence this phase of the cycle just requires good insulation in the secondary side of the system!
Incidentally the above reasons are why many ignition secondary faults and misfires show up under load only. Hence the best place to find them is on a rolling road.
Once the arc is struck, the energy stored magnetically in the core of the coil starts to discharge as current flowing into the secondary system. At this point some resistance (i.e. leads, plugs, suppressors) is necessary to maintain a decent spark. If there is no resistance, then all the spark energy is dissipated in a very short time. This effectively makes it self-extinguishing. Where there is resistance the full voltage is still available to ionise the mixture and start the arc. The resistance ensures that the energy is not all dissipated at once, and thus a longer burn time will follow.
If the resistance of the secondary circuit is too high, then the current flow through the secondary system will not be sufficient to maintain the arc. The arc will then be extinguished, probably resulting in a misfire.
There are two schools of thought on the subject of ignition, since nobody I've found really knows how the process occurs. The Americans favour the short spark, and hence all leads etc., are minimum resistance. Everyone else goes for resistance of around 5000 Ohms, which gives a burn time approaching two milliseconds. Rover V8 systems commonly like 7,000-15,000 Ohms (coil lead plus one plug lead).
In these days of modern lean burn engines, secondary resistance is commonly higher - commonly 20,000-25,000 Ohms. This is coupled with more energy stored in the coil to give longer burn times, sometimes nearing three milliseconds.
So boggo RR leads are ok when new, but don't generally last all that long. Personally I would recommend changing them every 25-30K miles. As you know I am a great Magnecore nut though!!! (ACT stock the little beauties)
There are two phases two the ignition cycle, which impose different demands on the system.
When the spark is required the current supply to the ignition coil primary circuit is switched off. The resultant collapse of the magnetic field in the core of the coil is what induces the rapid rise in voltage in the secondary windings.
Current does not flow in the system until the arc across the plug electrodes has started (been "struck"). The resistance of the plug leads is only important after the spark has been initiated and current flows. So there are two distinct phases - initiation where voltage is important, and then the burn where the resistance of the circuit matters.
The voltage required to strike the arc is dependant on many factors, which basically boil down to how difficult it is to ionise whatever gas is between the electrodes of the spark plug.
Some factors that affect the secondary voltage required are the cyclinder pressure (increases with load, higher load requires higher voltage), spark plug gap, spark plug electrode size (thinner pointy electrode creates higher electrostatic field, so requires less voltage), fuel used (LPG is more difficult to ionise than petrol), etc. Hence this phase of the cycle just requires good insulation in the secondary side of the system!
Incidentally the above reasons are why many ignition secondary faults and misfires show up under load only. Hence the best place to find them is on a rolling road.
Once the arc is struck, the energy stored magnetically in the core of the coil starts to discharge as current flowing into the secondary system. At this point some resistance (i.e. leads, plugs, suppressors) is necessary to maintain a decent spark. If there is no resistance, then all the spark energy is dissipated in a very short time. This effectively makes it self-extinguishing. Where there is resistance the full voltage is still available to ionise the mixture and start the arc. The resistance ensures that the energy is not all dissipated at once, and thus a longer burn time will follow.
If the resistance of the secondary circuit is too high, then the current flow through the secondary system will not be sufficient to maintain the arc. The arc will then be extinguished, probably resulting in a misfire.
There are two schools of thought on the subject of ignition, since nobody I've found really knows how the process occurs. The Americans favour the short spark, and hence all leads etc., are minimum resistance. Everyone else goes for resistance of around 5000 Ohms, which gives a burn time approaching two milliseconds. Rover V8 systems commonly like 7,000-15,000 Ohms (coil lead plus one plug lead).
In these days of modern lean burn engines, secondary resistance is commonly higher - commonly 20,000-25,000 Ohms. This is coupled with more energy stored in the coil to give longer burn times, sometimes nearing three milliseconds.
So boggo RR leads are ok when new, but don't generally last all that long. Personally I would recommend changing them every 25-30K miles. As you know I am a great Magnecore nut though!!! (ACT stock the little beauties)
I've gone back to Magnicors which measure about 14K for number 8 lead. The LR ones measured 13 K for the same lead. And the coil secondary resistance was about 7K if memory serves. I guess this lot gives it a very long spark then? But the Bosch coil is allegedly high voltage (60 KV instead of 30 KV) which suggests a short intense spark. All very confusing! In any case it now idles OK and is fine under power but a bit troublesome under small throttle openings - more of a fuelling problem that spark I suspect.
Mark adams said:
Hold tight - here comes a major warble about sparks!
There are two phases two the ignition cycle, which impose different demands on the system.
When the spark is required the current supply to the ignition coil primary circuit is switched off. The resultant collapse of the magnetic field in the core of the coil is what induces the rapid rise in voltage in the secondary windings.
Current does not flow in the system until the arc across the plug electrodes has started (been "struck"). The resistance of the plug leads is only important after the spark has been initiated and current flows. So there are two distinct phases - initiation where voltage is important, and then the burn where the resistance of the circuit matters.
The voltage required to strike the arc is dependant on many factors, which basically boil down to how difficult it is to ionise whatever gas is between the electrodes of the spark plug.
Some factors that affect the secondary voltage required are the cyclinder pressure (increases with load, higher load requires higher voltage), spark plug gap, spark plug electrode size (thinner pointy electrode creates higher electrostatic field, so requires less voltage), fuel used (LPG is more difficult to ionise than petrol), etc. Hence this phase of the cycle just requires good insulation in the secondary side of the system!
Incidentally the above reasons are why many ignition secondary faults and misfires show up under load only. Hence the best place to find them is on a rolling road.
Once the arc is struck, the energy stored magnetically in the core of the coil starts to discharge as current flowing into the secondary system. At this point some resistance (i.e. leads, plugs, suppressors) is necessary to maintain a decent spark. If there is no resistance, then all the spark energy is dissipated in a very short time. This effectively makes it self-extinguishing. Where there is resistance the full voltage is still available to ionise the mixture and start the arc. The resistance ensures that the energy is not all dissipated at once, and thus a longer burn time will follow.
If the resistance of the secondary circuit is too high, then the current flow through the secondary system will not be sufficient to maintain the arc. The arc will then be extinguished, probably resulting in a misfire.
There are two schools of thought on the subject of ignition, since nobody I've found really knows how the process occurs. The Americans favour the short spark, and hence all leads etc., are minimum resistance. Everyone else goes for resistance of around 5000 Ohms, which gives a burn time approaching two milliseconds. Rover V8 systems commonly like 7,000-15,000 Ohms (coil lead plus one plug lead).
In these days of modern lean burn engines, secondary resistance is commonly higher - commonly 20,000-25,000 Ohms. This is coupled with more energy stored in the coil to give longer burn times, sometimes nearing three milliseconds.
So boggo RR leads are ok when new, but don't generally last all that long. Personally I would recommend changing them every 25-30K miles. As you know I am a great Magnecore nut though!!! (ACT stock the little beauties)
I know it's an old post but it's stuff like this that makes me glad I frequent PHThere are two phases two the ignition cycle, which impose different demands on the system.
When the spark is required the current supply to the ignition coil primary circuit is switched off. The resultant collapse of the magnetic field in the core of the coil is what induces the rapid rise in voltage in the secondary windings.
Current does not flow in the system until the arc across the plug electrodes has started (been "struck"). The resistance of the plug leads is only important after the spark has been initiated and current flows. So there are two distinct phases - initiation where voltage is important, and then the burn where the resistance of the circuit matters.
The voltage required to strike the arc is dependant on many factors, which basically boil down to how difficult it is to ionise whatever gas is between the electrodes of the spark plug.
Some factors that affect the secondary voltage required are the cyclinder pressure (increases with load, higher load requires higher voltage), spark plug gap, spark plug electrode size (thinner pointy electrode creates higher electrostatic field, so requires less voltage), fuel used (LPG is more difficult to ionise than petrol), etc. Hence this phase of the cycle just requires good insulation in the secondary side of the system!
Incidentally the above reasons are why many ignition secondary faults and misfires show up under load only. Hence the best place to find them is on a rolling road.
Once the arc is struck, the energy stored magnetically in the core of the coil starts to discharge as current flowing into the secondary system. At this point some resistance (i.e. leads, plugs, suppressors) is necessary to maintain a decent spark. If there is no resistance, then all the spark energy is dissipated in a very short time. This effectively makes it self-extinguishing. Where there is resistance the full voltage is still available to ionise the mixture and start the arc. The resistance ensures that the energy is not all dissipated at once, and thus a longer burn time will follow.
If the resistance of the secondary circuit is too high, then the current flow through the secondary system will not be sufficient to maintain the arc. The arc will then be extinguished, probably resulting in a misfire.
There are two schools of thought on the subject of ignition, since nobody I've found really knows how the process occurs. The Americans favour the short spark, and hence all leads etc., are minimum resistance. Everyone else goes for resistance of around 5000 Ohms, which gives a burn time approaching two milliseconds. Rover V8 systems commonly like 7,000-15,000 Ohms (coil lead plus one plug lead).
In these days of modern lean burn engines, secondary resistance is commonly higher - commonly 20,000-25,000 Ohms. This is coupled with more energy stored in the coil to give longer burn times, sometimes nearing three milliseconds.
So boggo RR leads are ok when new, but don't generally last all that long. Personally I would recommend changing them every 25-30K miles. As you know I am a great Magnecore nut though!!! (ACT stock the little beauties)
stevieturbo said:
Hats off to you...a 15 year old thread bump has to be right up there with one of the oldest !! lol
It seemed rude to have used the really good info to help me spec some new HT leads for the Lotus and not acknowledge the post was very helpfulPS I’ve seen older posts bumped from the archive
I too am very interested in the ignition system resistance. I am having missfires, I've got a set of leads I have taken off and measured the shortest at 4k and the longest at 11k Ohms.
I presume this difference will have an affect?
I've been suffering since I bought the car just over a year ago. I've replaced the coil to a bosch type, cap, arm, Leads, I've tried KV85 magnecor, DA4102 (britpart), the lucas set don't last long without extenders.
I have tried BPR6ES (resistive) plugs and I have tried most recent BP6ES (non-resistive?)
My latest setup as of today is TVR V8 Performance Double Silicone HT Leads 8mm Red purchased from powerspark with the BP6ES plugs which did make it alot smoother, the car then sat for an hour and displayed the same missfire/hesitant manifestation.
Any comments will be greatly aprreciated.
I presume this difference will have an affect?
I've been suffering since I bought the car just over a year ago. I've replaced the coil to a bosch type, cap, arm, Leads, I've tried KV85 magnecor, DA4102 (britpart), the lucas set don't last long without extenders.
I have tried BPR6ES (resistive) plugs and I have tried most recent BP6ES (non-resistive?)
My latest setup as of today is TVR V8 Performance Double Silicone HT Leads 8mm Red purchased from powerspark with the BP6ES plugs which did make it alot smoother, the car then sat for an hour and displayed the same missfire/hesitant manifestation.
Any comments will be greatly aprreciated.
So, am I right in assuming that all you have looked at is spark?
If so, why not look elsewhere?
The only time I personally have found a heat/time related misfire that was directly attributable was when I had an old bm on which the coil broke down under heat & would run terribly until left to cool down.
Re-sited it & it was better, changed it & it was sorted.
If the car is sat there idling for an hour before the problem becomes evident that is a lot of heat soak to get into something like a fuel line & cause evaporation of the fuel for instance.
If so, why not look elsewhere?
The only time I personally have found a heat/time related misfire that was directly attributable was when I had an old bm on which the coil broke down under heat & would run terribly until left to cool down.
Re-sited it & it was better, changed it & it was sorted.
If the car is sat there idling for an hour before the problem becomes evident that is a lot of heat soak to get into something like a fuel line & cause evaporation of the fuel for instance.
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