Shielded spark plug wires necessary?
 

Now that my CIS system is "safely stored" in a filthy pile in a corner of my shed, and my PMO carb setup is working, do I still need shielded spark plug wires, or are a set of Clewett wires like this acceptable to run?

(I may or may not have pulled a wire out of the boot while attempting to remove it from the cap, and am now considering replacements despite continued functionality.)

Ditch the shielded wires and get the Clewett wires.

If your engine ignition system comes with CDI, means up to 40.000v passing the wires, then it makes sense why the engineers at Bosch/Porsche switched to shielded wires and you should stay with this. Non shielded ones could work fawlessly but if a spark plug will fail or similair where the voltage wont be lead to ground the way through the spark plug it was meant to be, then it could happen that the voltage peak will jump arbitary to the next possible ground and that could be anyware in the closer area.

Many gasoline powered cars have sparkplug wires (including older Porsches), and to my knowledge most don’t use shielded wires …

Yep Clewett wires

Do you have an official Porsche reference for this?

I've seen the reason stated as they were trying to fix the airbox blowup issue and thought it was caused by errant sparks.

But there are ton and tons of people running non-shielded wires on their air cooled Porsches, and I've never read of someone saying "I had a spark plug fail and I had non-shielded plug wires and something bad happened".

Not saying you're wrong, would love to read an official statement from Porsche.

Read my explanation above, we're talking about CDi based ignitions with approx. 40.000 Volts. And not 3.2 Carrera DME ignitions or other non CDI based ones.

No official statement of Porsche needed its a fundamental electric based issue.
And btw. after the first years of using CDI they DID change to shielded wires in case of CDI cars and not only cause of fun.

And as I mentioned, ... yes in 99,99% cases it runs fawlessly, BUT if once the voltage spark searches himself for the next arbitary possible nearest ground – no matter why – then its just a matter of luck.
I would never want to save that minimal amount of money in relation to the risk when buying non shielded ones.

I suppose with the age of these cars and their electronic components, it would be hard to pin down using non-shielded wires as the actual cause of a failure.

I did find this bit of info about Magnecore wires:

"WARNING about metal shielded and so-called "Built-in Capacitor" ignition wires:

Although using a grounded metal shielding over the entire length of each ignition wire will certainly provide RFI suppression, and this style of wire is still used on low-revving piston driven aircraft engines, it is common knowledge (from experience) amongst automotive electrical engineers that it's unwise to use ignition wires fitted with grounded metal shielding over ignition cable jackets on a high-revving automobile engine — as the problems caused by any style of ignition wires which need to be grounded have proven to be so great, that using them should be avoided at all costs!

This type of ignition wire forces the cable jacket to become an unsuitable dielectric for a crude capacitor (effect) between the conductor and the grounded braiding. While the wires function normally when first fitted, the cable jacket under the metal shielding soon breaks down as a dielectric, and progressively more and more spark energy is induced from the conductor (through the cable jacket) into the grounded shielding, causing the ignition coil to unnecessarily output more energy to fire both the spark plug gaps and the additional energy being lost in the grounded metal shielding. This situation leads to engine power loss, and eventually to ignition system overload failures as the insulating ability of the cable jacket (under the metal shielding) breaks down.

Ignition wires promoted as having "built-in capacitors" are no more than solid wire or spiral conductor wires over which grounded metal shielding is fitted to only part of the wires' insulating jacket. These wires have all the disadvantages of wires with grounded metal shielding over the entire length of each ignition wire — without being able to properly suppress either RFI or EMI!"

I'm not an EE though so take that for what it is.

?? Read above ... I dont say "it will end up in failures" but in case of a not properly 40.000 volts flow where the target wont be reached cause of target ground failure (dont know the proper english words) likem in case of broken wires, pluf failure etc. the high voltage flow "could" behave arbitary.

https://vimeo.com/547541145/9d67423e95
Just to show how such a voltage even goes through isolated parts.

As said, many people do not have problems, but once the high voltage jumps over to low voltage ignition parts its a clear risk.

Safe money wherever you want, but honestly ... investing a few bucks more in shielded wirings and live with a better secured environment like it has been left the factory IMHO its worth it. :-)

There are 3 separate issues wrt to the plug wires
1) cross firing which was why Porsche originally installed the sheathed wires. There was an issue w/ air box explosions, My C3 w/ sheathed factory wiring experienced this). They were trying different things to cure this issue, sheathed wire was one. It didn't work.

2) transmission of energy to the plug, oe wires were solid metallic core, back in those days most oe's were using a carbon impregnated fiber core which was much less efficient at transferring power to the plugs, a solid metallic conductor will be the best at this

3) RFI- solid core wire emits RF radiation, which is mitigated by the installation of resistance in the pug connectors, sheathed wire does the same by creating a Faraday cage around the wire, Magnacor wire is the same idea but the Faraday cage is under the insulating sheath. The resistance can be in the connector or the plug, I don't even know if they still sell resistor plugs

When I replaced the sheathed wire, unnecessarily as I was throwing parts at a problem, I replaced it w/ ss core silicon sheathed w/ stock resistor connectors. Worked fine.

Clewet looks to be a modern solution

In my experience , the shielded wires tend to leak more than any other wire . I have sprayed water
on the plug wires , in the dark , and watched the spark show happen too many times .
I have also put my hand on wires to check on the contact of the plug head and zapped myself
far too many times . This can happen with new Beru, or new URO shielded wires .
I have never had this happen with Clewett Wires . I have plenty of sets out there with 100,000
miles on them and I have no concern for zapping myself, if I choose to touch the wires .
As written above , there are many articles negating the need for shielded wires.


Just my thoughts
Ian

+ 1, seems shielding the wires to ground makes much sense.

The Clewett wires to don't always make a firm connection with the plug. Insure yours are....

If your using a 6 pin cdi or even an msd you should theoretically use the shielded wires. Your other option is a spiral core/carbon core wire but it’s not really an option for the Bosch CDI because the resistance in the wire is too great.

Daniel