Resistor Type Spark Plugs and Original Magnetos

[Nulli Secundus]

My 1938 500 Rudge race bike has an original Dirt Track BTH magneto (80+ years old).

I am using a NGK B9EV 14mm long reach plug with a spacer when a short reach one, without spacer, would be better. I now only have one spare plug.

They are now becoming harder to find and via eBay I found B9HVX, which I thought would be suitable replacement and better in terms of reach. Although the item was described as B9HVX and photographed as such, the supplier sent BR9HIX. They were not cheap and I was not happy because although the specification is similar the R in the part number signifies it is a Resitor plug. I returned them and got a refund.

However, after discussing this with Rapcom he thinks the resistor is less of a problem with an iridium electrode.

Is this correct? Can I use an expensive iridium resistor plug with my 80+ year old magneto and not have issues?

 

[Magnetoman]

he thinks the resistor is less of a problem with an iridium electrode.

The short answer is, a resistor won't be less of a problem in a resistor iridium plug. The long answer is:

When the voltage from a coil climbs to ~5-7 kV the electric field across a plug is high enough to ionize the air in the gap and the plug sparks. When that happens the resistance across the gap becomes very small so the inductance L of the coil discharges its ~10 mJ of energy in a time L/R which is a microsecond or so. With a resistor plug the discharge time is still L/R, except now there is 5-10 kOhms of resistance in the circuit so the spark takes much longer to discharge. Plus some of the energy is wasted in creating heat in the resistor.

The same things happen inside the combustion chamber with an iridium electrode, except the thinner electrode in that type of plug will initiate the spark at a somewhat lower voltage because the electric field near its tip is larger (because the dia. is smaller). However, if everything is working fine in your combustion chamber now, the lower voltage at the onset of discharge won't do you any good (or any harm). What matters is the total energy of the spark, which would be the same for a "normal" or for a non-resistor iridium plug. But, irrespective of the composition or diameter of the electrode, the energy in the spark will be less for a resistor plug because of the losses in the resistor.

If your magneto doesn't have a problem with a resistor "normal" plug, it won't have a problem with a resistor iridium plug. But, if it does have a problem with one, it will have a problem with the other.

Despite resistor plugs having worse performance than normal ones, because of the energy lost in the resistor, they're used because the longer L/R time takes the radio frequency emissions from the spark down from the range ~1 MHz (i.e. 1/microsec.), where they interfere with radio and TV signals, to ~1 kHz where they don't matter. When resistor plugs were introduced to allow radios in cars to function, their battery/coil ignition systems could be easily updated by slightly redesigning the coils to provide a bit extra energy if needed. However, such a redesign isn't as easy on a decades-old magneto.

 

[Nulli Secundus]

Thank you so much for taking the time to answer my question and in a way I could follow. I start my race bike on starting rollers and think the magneto is a good one, but do not know who overhauled it. I think I will try a resistor iridium plug.

Thanks again

David

 

[BigEd]

My 1938 500 Rudge race bike has an original Dirt Track BTH magneto (80+ years old).

I am using a NGK B9EV 14mm long reach plug with a spacer when a short reach one, without spacer, would be better. I now only have one spare plug.

They are now becoming harder to find and via eBay I found B9HVX, which I thought would be suitable replacement and better in terms of reach. Although the item was described as B9HVX and photographed as such, the supplier sent BR9HIX. They were not cheap and I was not happy because although the specification is similar the R in the part number signifies it is a Resitor plug. I returned them and got a refund.

However, after discussing this with Rapcom he thinks the resistor is less of a problem with an iridium electrode.

Is this correct? Can I use an expensive iridium resistor plug with my 80+ year old magneto and not have issues?

Anything that can interfere with spark production should be avoided. My vintage Sunbeam 500 single was raced by my brother on and off for around 20 years. We never used a resistor plug or resistor plug cap and there was never any complaint about radio interference at any of the race circuits we used.
We only ever used NGK plugs, usually a B8ES or B9ES. Try the links below. The first is for short reach B9 and the second for long reach B9.
B9HS
B9ES

 

[Nulli Secundus]

Thanks Eddy,

I guess these have a standard electrode, but they are nice and cheap

Thanks again

David

 

[BigEd]

Thanks Eddy,

I guess these have a standard electrode, but they are nice and cheap

Thanks again

David

I have run Iridium electrode plugs in some of my other bikes. Iridium is very corrosion resistant so a finer electrode can be used which should help spark propagation. The corrosion resistance should also mean that they should not need gap adjustment for longer periods.
I can honestly say I have not noticed any starting or performance differences using Iridium plugs. (Except perhaps in my wallet.)

 

[Bill Cannon]

Slightly off topic sorry.
Given that most transmissions are now digital why do we need resistor plugs at all?

Bill

 

[passenger0_0]

My transmission is still manual. ?

 

[roy the mechanic]

The short answer about resistor plugs is they are an integral part of the modern computer controlled fuel system, not intended for classics. I reckon they cost more because they last for 70-80 thousand miles, so over the vehichles lifespan the revenue to the makers is not reduced.