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Ignition Wires - looks neat but should you do it

Hi

I just noticed that Henry runs his ignition wires aligned all the way across the motor, I noticed years ago that Porsche twisted theirs, when I commented to a friend about it he said (we are both electical engineers and car nuts), No they should be like that to minimise cross talk, which is right, has anyone else noticed this?

Neven
Old 10-18-2004, 05:34 PM
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Cross talk? Sounds like something you'd worry about on a sensitive audio circuit not a 40,000 volt spark plug wire. You'd have to get enough inductive coupling to fire an adjacent plug before you'd notice any effect.

-Andy
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Old 10-18-2004, 06:37 PM
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Portsche used to twist them, by the time the carreras came out in 84, the wires were in a fixed loom to keep them properly spaced.

I would go with keeping them spaced out.

AFJuvat
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Old 10-18-2004, 06:38 PM
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Andy

Its exactly because its a 40kV pulse that you do get a decent amount of energy inductively coupled. I was just thinking the firing order is 1-6-4-2-3-5 when 5 fires 6 is 240deg behind (2/3 way thru intake stroke) even a weak spark in 6 could cause a
backfire

Based on this 'theory' the plug wires for 5 & 6 should be twisted,
can anyone confirm this? (I've got a 73E)

Neven
Old 10-18-2004, 07:21 PM
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Oops

I just realised that 6 is only 120deg behind 4 so should 4 & 6 be twisted

Now I'm confused, AF can you confirm?

Neven
Old 10-18-2004, 07:26 PM
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Neven,

I'll admit first that I haven't done the calculations nor do I indend to spend the time to. You could calculate the amount of voltage you'd see on the second wire based on distance between them and the length they stay together. I believe that you would find that the voltage is too low by several orders of magnitude to cause a spark on the second cylinder.

I'm also not sure that twisting the wires will result in less inductive transfer between them. I think when you twist wires in a radio or audio circuit you do so because you want both wires to see the same inductive pickup from a third source. Thereby canceling out the interference. It's been over 20 years since I studied this stuff so I may be getting at least some of it wrong.

That being said if it makes you feel better to twist the wires I don't think it will hurt anything.

-Andy
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Old 10-19-2004, 03:59 PM
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Andy

I'm an Electrical Engineer by training (these days I write software) but I think youd be surprised how much voltage is induced in a parallel wire, the amount is dictated by the current in the wire (which you could calculate from the energy in a spark and the length of it), And the passive wire is very high impedance (open at both ends), twisting means the wire is in the concentric field in opposing orientations so any induced voltage is cancelled. In the end I twisted 5&6 and 4&6 ie 4&5 run parallel (they are 360 deg apart) , I'm checking and original car on friday so it will be interesting to see if they are
the same. As you say it may be a minor thing but interesting all the same

Neven BE E&E
Old 10-19-2004, 04:21 PM
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