Simple but not so simple...?

Why are such high resistance numbers a good thing for spark plug wires?

 

JJJ,

Yes...and No!

In simple terms it's about striking a balance between a low resistance to maximise the transmission of the spark energy to the spark plug, whilst also suppressing the unwanted EMI and RFI that can bugger up everything from radio reception through to spiking your ecu.

So thats why you don't just go for the thickest lead you can find. There's a sweet spot for every car.

Paul

 

JJJ
Have you swapped in a new plug on that over-fuelling cylinder? if not, worth doing.

 

I have not but will. Yesterday, which I thought was going to be a fun and straightforward thing (ended up offering a rough spot or two), I installed a set of Denso 8mm HT wires. Althought I didn't record all the figures, I noticed that the new compared to the old offered MUCH higher resistances at every lead. I needed to measure in the 20K ohms scale! Big numbers and I didn't and don't understand why.

I will change the plug and plan to change the FI in that cylinder as well, as soon as I get it in my hand. This weekend at the latest.

 

Thanks Paul... But can resistances that need to be measured in the 20k ohms range be considered low resistance?

When I started looking up information about checking resistance in HT leads I was SHOCKED by the numbers and what was considered desirable... Just didn't seem right. Those kinds of numbers anywhere else ain't,,,, desirable. Ya know?

 

JJJ,

I don't know enough about our engines to know the optimal resistance. If one thinks about the ignition circuit to a plug in 2 phases, the first phase is to have low enough resistance to get enough current down to the plug. With distributor or coil-on-plug systems, the voltage is not super-critical, compared to say a magneto system which requires very strong voltage. Witness the very early IC engines which just had a bare solid wire to the plug.

The second phase is the burn phase as the spark flows across the electrodes. If it were unresisted, you would get too fierce / quick a burn cycle. If it is resisted, it causes a slower burn. In lean-burn engines, the resistance required tends be in the 20k ohms range, whereas older burn technology engines probably work better with about 6k ohms resistance.

Paul

 

Interesting... Still over my head for the most part, but interesting. So, resistance (a certain amount) slows the spark at the plug? Slows the current?

I guess I'm wondering where all the heat goes that usually accompanies resistance. At my battery cable on the XK8 once, corroded wire, I had a very LOW resistance which produced enough heat to start cooking the wire. I can't remember the resistance but it was in the single digits IIR.

I just drove the car for the first time since the change and I am noticing differences. One, I'm not smelling gas. Two, much quieter. 3 (I think) more power. And the idle has seemed to change for the better.

I replaced with DENSO wires who usually make good and right stuff. Hoping they have there resistances right!

I measured a few of the NEW HT leads and was finding numbers in the mid to hi 6k values - and around. Clearly corresponded to length. I saved the OLD wires and was reading as low as 1.5 1.9k. Now I want to test them all.

I have so so much to learn.

Don't get me started on transmission problems and figuring out what to do with issues with a transmission that sat flooded in H20 for xxxx weeks and months on my rescue XKR. OMG. Gotta,,,, love it?

 

Paul
Is that correct? I have no idea about such things; but I had always thought that when the spark jumped, it jumped at the same speed! Now maybe a longer spark duration - but is that a matter of the coil potential or the plug resistance?
Enlightenment please!

 

Yeah! MeeeeToooo!

 

I'm no expert! (As you all well know!) But the resistance in both the plug lead and the plug itself affects the spark burn time. I tried to google something that explained it better. Have a look here:

Spark Burn Time (crypton.co.za)


The simple way that I view plug leads is that:

- they need a good core to carry the voltage
- there's an optimal resistance required as part of the spark ignition design characteristics of the engine
- they need to have sufficient insulation to avoid arcing or unwanted grounding
- they need to be able to suppress EMI / RFI (even if it's just to maintain decent radio reception!)

My dear departed Dad, God bless him, used to say that, unless modern technology had improved things, you needed a damn good reason not to fit what the manufacturer had originally designed.

Having said that, my original LPG-converted Range Rover ran much better on some nice new Magnecor leads!

Paul