plug lead resistance - too much of a good thing?

[Kccv23highliftcam]

Great thread revival!!

Now here's a thing.

My mk2 fiesta [nee cvh] workshop manual quotes 30k ohm as a maximum for each lead.

BUT the engine runs kent high lift cam with mechanical timing to match, twin 40's on a ported and skimmed head

So keeping the electronic ignition triggering side as "standard"

1. What coil-->sports or standard can you run sports coil with the electronic unit [dwell adjust I believe] ??
2. Plugs-->ngk R probably 6 heat rating..
2. What choice of lead based on

a. resistance (see 2 above)
or
b. diameter/quality..

If anyone can point to online [or other] information I'd be most grateful, lots of people want to sell you xyz without any real "knowledge"...

Edited by Kccv23highliftcam on Sunday 29th July 11:43

[Mr2Mike]

Unless you are running insane levels of compression the standard coil and HT leads will be fine. You should be concentrating more on optimising the mechanical advance, since it's unlikely the standard advance curve is remotely correct. Better still throw it in the bin and fitted a mapped system.

[Kccv23highliftcam]

Mr2Mike said:

Unless you are running insane levels of compression the standard coil and HT leads will be fine. You should be concentrating more on optimising the mechanical advance, since it's unlikely the standard advance curve is remotely correct. Better still throw it in the bin and fitted a mapped system.

Both fitted and spare dizzy's have had their springs worked.

Cheers.

[timbacct]

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)

What a fabulously informative piece. It has cleared up years of misconception on my part (and I trained as a Mechanical Engineer!) and helped me fix the problem with my 1951 Ferguson TED 20 tractor. Not quite the target of this forum but just as applicable. Thank you!