Ignition wires, does it really matter what wire I use?

ignition wires

Ignition wires are used on almost every gasoline powered engine. They transmit the spark energy from the distributor cap to the spark plug. On most factory applications the wires are fairly bland, usually black in color and tend to blend into the engine compartment. Some wires, like the ones used on the 1978-83 911SC and Turbo models are covered in a metal braiding.

What if I want to change up the look of my engine compartment, changing the spark plug wires is a pretty easy job and usually the wires are very easy to see in the engine bay. What if I want to see a Red or Blue wires because it matches the color of the car or maybe I just like the color Red? What about all the aftermarket wire sets that I see they say they will improve my performance? Looks like a win-win, I get a cool color and more performance, how can I lose?

Factory ignition Wire Sets

The spark plug wire sets used by the manufacturer are specifically designed to match the type of ignition system installed. In Porsche models the ignition system used was designed by Bosch and the wires are built to specifically work with that system. Like every system in your car there are a set of design parameters that specify what type of wire can and cannot be used.

The ignition wires that have been installed on your classic Porsche are a solid core design. This means that the internal conductor that connects the two ends of the wire is made from strands of copper. The secondary resistance that is required for correct system operation is built into the individual connectors, either at the spark plug or on the coil. This means that every spark plug will get the same amount of energy for the same amount of time, regardless the length of the wire.

The amount of resistance is also a key factor to the operation of the system. Too little resistance will result in a spark that moves to quickly and is erratic in its delivery. Too much resistance and spark energy will be reduced including spark plug burn time. Different resistance values from plug to plug will result in slight timing offsets, which means not every cylinder is operating in exactly the same way.

AfterMarket Replacement Ignition Wires

Most of the time the brightly colored aftermarket wire sets are made from resistance per inch silicone wire. These wires were originally designed and used on domestic ignition systems and instead of having a solid metal internal conductor they use a carbon core to transmit the energy from the cap to the plug.

Using these types of wires will change the ignition system parameters, which in turn will change the way the car runs. One of the first issues with resistance per inch wires is that the longer the wire the more resistance the it has. This means that cylinder number 1 on a Porsche 911 will have a very different resistance number than cylinder number 6. This is because number 1 wire is only 12” long and cylinder number 6 is 36” long, so basically three times the resistance.

This difference in resistance results in each spark plug getting a different amount of spark energy. It will also create a timing offset and changes in the amount of burn time each plug gets due to different resistance values.


One of the biggest issues with many of the aftermarket ignition wire sets is their dielectric strength, or in other words their resistance to electrical leakage. Most of the carbon core resistance per inch wires on the market today are designed for use in ignition systems that produce relatively low amounts of secondary voltage when compared to those used in Porsche ignition systems.

Generally, a good quality silicone ignition wire will be rated to be able to withstand or contain 35,000 volts. When used in a conventional ignition system this is fine as most secondary voltages are around 25,000 volts. However, when using this wire in a 1969-83 Porsche 911 or the 1975-94 Porsche 930 application the secondary voltages are in excess of 45,000 volts. This means that there will be a high chance that the wire will not be able to contain the energy and has the possibility of leaking or shorting to ground.

Another characteristic of carbon core wires is every time a spark signal is sent down the wire it erodes a little bit of the carbon away. This means that over time with normal use the internal conductor will wear away resulting in its eventual failure.

ignition wire comparsion


By using a factory designed spark plug wire set, we will get a part that is designed for the system. It will have the optimal resistance at each spark plug in the engine. Each spark plug will receive the same amount of energy at the same time. The ignition wire is designed to contain the energy and is not prone to leakage. Lastly the wire will last longer as it does not degrade with every signal transmitted down the wire and a resistor does fail it can be replaced without having to replace the entire set.

The aftermarket set is often not specifically designed for the system it is being used on. Most of the time will not have the correct factory specified resistance, which will result in different levels of spark energy at each plug. Also, it will most likely not be able to contain the amount of energy that is being delivered through the wire. Lastly will have a shorter life span and the entire set will require replacement much sooner than the factory original set.

Now there are plenty of aftermarket parts that are made well. These parts can meet or exceed the factory specifications. You just need to be aware of what your individual system requires and compare those specifications to the aftermarket wire set. Remember we are not saying that an aftermarket wire set is bad or is of poor quality. It is just that they are not designed for use in this instance.

Have you thought about your points and coils? As part of your ignition system, aftermarket choices can be made here too.


7 thoughts on “Ignition wires, does it really matter what wire I use?”

  1. Interesting article Kurt.
    I have a Porsche 2.7S CIS engine in my car and need to replace the currently Stainless Steel Covered Leads that are on it.
    What brand and variant would you recommend?
    Is it worth doing any other ignition mods at the same time, engine is standard other than being rebuilt and running backdated heat exchanges and a Dansk Pea Shooter.
    Thanks in advance.

    1. Zander,
      The 3 pin CDI ignition system requires the 911.609.061.00 wire set this is the 1000 ohm resistor plug connectors with solid core wires. I always just use Beru brand wires. The only other things to check are that the rotor has a 5000-ohm resistor in it and the spark plugs are non-resistor type plugs with the correct heat range plug. If everything else is working correctly then I would not recommend replacing any other parts.

    1. Hello WC, We have been happy with the Beru Brand since Bosch is not in production. Porsche part number 616-609-052-10, Beru Number ZEF 303.
      If you do not like the short connectors, you can get a long connector set 616.609.052.12

  2. Hi Kurt,

    Great article! I have been chasing a rough idle and slight exhaust afterfire on my 1974 911 w/ CIS. I’ve gone from tip to toe testing and replacing ignition and fuel components but cannot find a smoking gun. It stumbles and sputters if the AFR goes anything north of 13.0 with no load but has no apparent issues under load at part to full throttle.

    After reading this article (and your service bulletins) I ran out to the garage to check my new braided Beru spark plug wire set and found that my cap to plug leads are 4k ohms, the coil to cap king lead is 1k ohms, and the rotor is 5k ohms (10k ohm total). My coil is original and seems to be fine but I’m wondering if it is getting weak and the additional resistance from the wires is causing an issue. I’m running a ClassicRetrofit 3-PIN CDI+ box at the moment, but I have the same issue with the original Bosch 3-pin. I have a Picoscope 2204 on it’s way to really see what’s going on, but was curious if you have any thoughts or suggestions?

    1. Justin,
      a rough idle with an occasional backfire will usually indicate a rich fuel mixture. Unfortunately using just an AFR gauge and trying to set up the fuel system is going to be really tough. At the bare minimum, a two gas analyzer is what you should be using, with a 4 gas being ideal. by using a 4 gas analyzer you will be able to see the fuel content, oxygen left in the exhaust, hydrocarbons, or unburnt fuel in the exhaust and this will point you in the right direction to solve the issue.
      As far as your ignition system is concerned it certainly is helping the engine run efficiently. The ignition wires are taking away spark energy from the plug and are the wrong ones for the car. The classic retrofit CDI is also taking away spark energy from the car as it only operates on a 300-volt primary voltage versus the 450 volt Bosch system. This will result in secondary ignition voltages being in the 30KV range instead of the 45KV. The next question would also be about the spark plugs that you are using and if they are non-resistor plugs or resistor plugs. in which case you could be adding another 5000 ohms to each plug circuit further decreasing firing voltages.
      To solve the issue I would start with an engine leak down to verify its mechanical condition. Then if all checks out return the ignition system to factory specifications with the correct spark plugs, ignition wires, and the original CDI box. Set the ignition timing to 35 degrees BTDC@6000 RPM vacuum hose disconnected and then lock it down.
      Then I would look at the fuel and intake system, make sure that the air box is not cracked or otherwise damaged. Make sure that you do not have any intake system leaks from either gaskets or orings. Remove fuel injectors and flow test, checking for poor spray patterns and injectors that drip or leak, injectors should also flow the same amount of fuel +/- 5% or each other, replace as required. Check CIS fuel pressures and make sure that they match the factory specifications in every way. The 1973-74 cars have a large number of updates in the fuel system so make sure that all updates are completed and functioning correctly. Make sure to check the sensor plate is set up correctly and that the roller bearing under the fuel distributor on the fulcrum arm is not seized. This can cause low-end misfires and erratic mixtures under light throttle. finally set up the fuel mixtures using a 2 or 4 gas analyzer. Don’t try and set up the car using one of the O2 sensor systems, you wasting your time and money as none of them are good for performing accurate fuel settings on these cars.
      hope that helps

  3. Hi Kurt,

    Thank you for the all this information it’s really helpful. I will be purchasing the correct wires, coil, and a few other bits from you to get the electrical system back on par. I was aware of the CDI+ box only operating on 300v primary and was always a little concerned. I did some more testing last night found that the system was struggling to jump a ~ 8-10mm gap at the plug end. Coming off the coil with the 1k ohm lead was a little better but not much. I currently have NGK BP7ES non-resistor plugs. The distributor is in good shape and recently cleaned and lubed, and I put in new advance springs (thanks to Parts Klassik). Timing has been set as you suggested and dwell verified at 36 degrees.

    I checked my compression and leak down numbers recently. The engine is the original non-S 2.7 variant but allegedly has 8.5:1 pistons and a S camshaft from a 1974 S engine. The engine was rebuilt in 2003 and has about 6000 miles on it. Compression across all cylinders was ~ 125psi average with a min / max of 122psi and 130psi. These numbers seem low to me but 8.5 x 14.5psi = 123psi, so maybe they are ok? Leak down numbers ranged from 3.3% to 6.7% with a 100psi input.

    I’ve gone through the entire intake system and replaced all intake runner base gaskets, injector sleeves and o-rings, brand new air box with all the ancillary seals, and put in new injectors. I even went as far as pulling a vacuum at at each cylinder at TDC to confirm the base gasket, injector seals, and intake valve seats are leak free.

    I flow checked the injectors and found two cylinders flowed +15% at idle with the others being within 5-8%. I noticed the spray pattern was poor at idle position. The spray patterns were actually a little better with my original injectors. I also get a drip at each injector every 2 seconds with the engine off and fuel pump running. I can get it stop if back out the AFR adjuster screw about three turns ( or just pull the plate down) but then it obviously won’t start or run. I have checked all my control pressures (with gauges) and they are perfectly in spec. I have the original 004 fuel distributor with the original 001 WUR and the TPCV is still hooked up. In my testing / experiments I noticed that the car runs way better with the TCPV removed from the throttle body shaft but still hooked in. With the TPCV properly connected on the throttle shaft, my AFRs shoot to ~17 and I get the bucking and surging just off idle and at low loads. When it’s off the shaft the bucking / surging is 95% gone but it runs pretty rich.

    I decided to open the fuel distributor which turned out to be futile. I was able to put it back together easy enough, but still have the +15% on two injectors albeit on different ports now. I also confirmed I have the correct 6mm OD x 2mm ID nylon injector lines. In hindsight I should have just sent the fuel head out but I couldn’t help myself. I am going to pull it and send to Flowtech as soon as I work out my ignition.

    I am curious to know more about the fuel system updates to the 73-74 cars you mention. I’m not aware of any other than maybe the -129 WUR and moving the fuel pump to the front of the car. Also very good to know about using 4 gas analyzer…I’ll have to track one down. It’s getting hard to find shops that can troubleshoot these cars and not just offer a rebuild. I really appreciate you taking the time to help.

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