Can i use a rotor 1K with an ignition CDI (bhkz)?

[canaille911]

Hello,

i have a 911 2.7 1977 equipped with a bhkz ignition.
I use a rotor with a resistance of 5K. But now, I can't find a rotor (without rpm security)with the 5k resistance. I have bought a rotor ref 1234332215 and i have received the ref 1234332271. I have found that this rotor has a 1K resistance.
Can i use this rotor without any problem on the ignition system?

Thank you for your help

[Jonny H]

You can use it but will shorten the life of the coil. The resistor is used to limit the current. I would suggest using a 4/5k king lead to compensate for the 1k rotor.

The situation is worse with epoxy filled coils (like the current silver Bosch) as the epoxy is easily spark eroded. Original oil filled coils will cope better.

And yes, this means that pretty much all twin plug distributor setups (where there is no resistor in the rotor) will eventually destroy coils.

[canaille911]

Thank you for your answer. Where can i find this part? Sorry but my english is poor.

[Fred Winterburn]

I disagree with Jonny. Extra resistance on the secondary side raises the peak voltage before spark breakdown IE, it increases the voltage overshoot. The increased voltage overshoot creates more voltage stress on secondary side insulation including the coil. In addition, the energy transfer from the coil to the spark is reduced and that energy is dissipated elsewhere as heat if not recovered by the CDI. I would suggest that if the resistor is robust and designed for CDI systems to go for it. Most CDIs will make short work of the resistors which is why rotors are made without resistors for CDI use. Fred

Originally Posted by Jonny H

You can use it but will shorten the life of the coil. The resistor is used to limit the current. I would suggest using a 4/5k king lead to compensate for the 1k rotor.

The situation is worse with epoxy filled coils (like the current silver Bosch) as the epoxy is easily spark eroded. Original oil filled coils will cope better.

And yes, this means that pretty much all twin plug distributor setups (where there is no resistor in the rotor) will eventually destroy coils.

[Jonny H]

Hi Fred. The way I see it is that resistance on the secondary circuit prolongs the spark duration. Since Power = Energy / Time, this has the effect of reducing the power and therefore the likelihood of flashover inside the coil (which is the failure mode).

[Fred Winterburn]

Jonny,
The opposite is true. And this applies to any ignition whether it be inductive or CDI. Resistance on the secondary side shortens the duration because the energy transfer efficiency to the spark is reduced. This is easy to test. With an inductive system, where all of the energy comes from fluxing the coil, secondary side resistance effectively bottles the energy within the coil after after the spark extinguishes, which it does early because of the resistance. Some CDIs will recover some of that energy, so not a total loss, but detrimental regardless. With inductive ignitions especially, the voltage overshoot is much higher with more secondary side resistance. For either system, current control should be done on the primary winding. The risk of flashover inside the coil is reduced by keeping secondary side resistance low so that the voltage overshoot is lower, and so that any oscillations from energy left over once the spark extinguishes have lower peak voltages. Sometimes it helps to explain thing with extremes. Imagine the voltage stress that occurs on the coil an other HT insulation when a plug wire is pulled? Secondary side resistance does the same thing to a lesser extent except that it is there constantly. Fred

Originally Posted by Jonny H

Hi Fred. The way I see it is that resistance on the secondary circuit prolongs the spark duration. Since Power = Energy / Time, this has the effect of reducing the power and therefore the likelihood of flashover inside the coil (which is the failure mode).

[Jonny H]

Hi Fred. I think we can agree that the 5k ohm resistors used in the rotors were to reduce EMI by suppressing fast transient voltages.

Removing /reducing the transient suppression allows these events back into the ignition system which more likely is harmful to their components.

[Fred Winterburn]

Historically removing the resistor in the rotor has never been a problem with the longevity of any CDI I know of, or reducing any secondary side resistance whether it be in the wires or spark plugs. What's more important is to control the current and voltage at the CDI output itself. I suppose some designs might be susceptible to voltage peaks reflected back into the CDI and some might be prone to misfiring with circuitry that isn't immune to RMI. I like to see extra secondary side resistance limited to no more than 10 kohms for CDI and Kettering or the spark energy becomes marginal for either system. The problem with the Bosch rotor resistor is that most CDIs will make short work of them. Same used to be true for Champion resistor spark plugs. I modify the rotor to remove the resistor, but Aircooled.net sells modified rotors primarily for VW use but they often cross reference to many other Bosch distributors including Porsche. Fred

Originally Posted by Jonny H

Hi Fred. I think we can agree that the 5k ohm resistors used in the rotors were to reduce EMI by suppressing fast transient voltages.

Removing /reducing the transient suppression allows these events back into the ignition system which more likely is harmful to their components.