Spark Plug Gap for Upgraded Coils

[DienstXIV]

Hi Everyone, I have a 1975 911S and I just installed a Bosch Red Coil with 1.8ohm resistance and about 45,000 volts. I have the stock CDI box and a newly installed 123 Ignition distributor. I have been told that if you upgrade your coil you can increase the spark plug gap to have a more complete burn and in theory leading to better hp and MPGs. Is this true?

Can anyone recommend what I should have my spark plugs gaped to? Right now I have them gaped to the recommended gap in my spec book which is 0.7mm or 0.028inches.

[Oregon 911]

Hey what’s the difference between the “Bosch red coil” and the regular Porsche coil that came with your car? I noticed in pelican parts they show two coils the Bosch coil with the silver body and the Porsche coil that looks exactly the same that’s got the silver body. Are those the coils that were made in Brazil that everybody talks about having bad luck with?
Yes I’d like to know if I change coil is there a reason for me to change my spark plug gap at all? I have a 1974 911 and I had some ignition problems so I sent my CDI box to Bob Ashlock and I sent my coil to him he checked out the original coil and told me there was nothing wrong with my CDI box but he did upgrade the capacitor in it now I’m getting a hotter spark. I’m still using the original Porsche coil that came in the car and in the future I plan to send the Bosch distributor to advance distributors and have them do a complete rebuild and provide a new curve for the dwell
Marion.

[reclino]

I am pretty sure the red coil is not compatible with CDI. The failure of your coil may take your CDI with it.
I thought all CDI Porsche distributor trigger the CDI with a variable reluctor, but it looks like 1975 still had points triggering it. Very interesting, but I think the need for careful dwell control is mitigated by the CDI The mechanical advance curve, can be tweaked, but dwell curve is confusing to me, not sure what that is.

[mysocal911]

Originally Posted by Oregon 911

he did upgrade the capacitor in it now I’m getting a hotter spark.

By that "upgrade", you may have a "hotter spark" at idle, but at high RPM you will have a spark with less energy, i.e. a weaker spark.

[911pcars]

Conduct a test.

Drive your vehicle at highway speeds for X miles with the 0.028" gap. Record your fuel consumption.

Install second set of plugs (or regap plugs) at e.g. 0.040", then repeat. Compare your mileage.

As for power comparison, you might have to perform A-B tests on a road dyno, same day, same test/operating conditions. Unless your particular engine specs create borderline ignition issues, I don't think the difference will be significant. Other test variables are up to you (e.g. ign. timing, advance curve, etc.). Testing will confirm.

Typical inductive ignition systems (points, coil) produce voltage only sufficient to fire the spark plug. No load (idle) voltage could be 4000V, max. load voltage around 14-20K volts. Your coil is capable (advertised) of producing more, but only when needed (high compression, lean A/F mixtures, etc.). In addition, 45K volts will require a beefy voltage path (wires, insulators, spark plugs, etc.). OTOH, coil-on-plug (COP) systems use dedicated coils to increase ignition efficiency instead of relying on a single mega-coil.

An interesting read, among many others on the subject:
https://www.tomorrowstechnician.com/diagnosing-secondary-ignition-problems/

MHO,
Sherwood

[mysocal911]

Originally Posted by 911pcars

Typical inductive ignition systems (points, coil) produce voltage only sufficient to fire the spark plug. No load (idle) voltage could be 4000V, max. load voltage around 14-20K volts.

What's your basis for making such a statement? Please provide reference data and fully explain your comments.

[John at J&S]

The spark gap acts as the voltage regulator.

Say your ignition system is capable of making 30kV, but at idle it only takes 4kV to make the spark. That's all you will see with your test equipment.

What happened to the other 26kV? That's just a reserve capability.

With ten pounds of boost it might take 20kV to make the spark, and that's all you would see with your equipment.

[mysocal911]

Originally Posted by John at J&S

The spark gap acts as the voltage regulator.

A better descriptor is transient suppressor.

Originally Posted by John at J&S

[B]
Say your ignition system is capable of making 30kV, but at idle it only takes 4kV to make the spark. That's all you will see with your test equipment.

What happened to the other 26kV? That's just a reserve capability.

With ten pounds of boost it might take 20kV to make the spark, and that's all you would see with your equipment.

Yes, we are aware of that! The question was for 911pcars, i.e. why is it just unique to "inductive discharge (points, coil)". Read his post.

It happens any time the applied voltage exceeds the breakdown voltage between two electrodes.

[John at J&S]

I didn't mention CDI or inductive. It should apply to both.

[mysocal911]

Originally Posted by John at J&S

I didn't mention CDI or inductive. It should apply to both.

I wasn't referring to you. It's 911pcars.

[911pcars]

Originally Posted by mysocal911

The question was for 911pcars, i.e. why is it just unique to "inductive discharge (points, coil)". Read his post.

I think I said that was the case with inductive ignition systems (points and coils). I don't know enough about modern digital ignition systems to explain how they manage maximum spark. I did speculate that the voltage needed isn't fixed but determined by the resistance to that spark forming due to compression ratio, A/F mixture as well as engine load, similar to the firing voltage for inductive points and coils.

I thought the attached link added more b.g. info. I sense there's another issue on your mind. Care to share?

Here's another video on various ignition coil systems, in order of efficiency:
https://youtu.be/L_hx8I6GzyU

[Fred Winterburn]

Except that it doesn't with CDI. I believe the overshoot on the secondary with CDI is due to leakage inductances which can't be gotten rid of entirely. Turn up the scope intensity and watch the firing line continue upwards at the same slope after the spark is struck (measure this on the secondary side). There are two voltage components after the plug fires. I can't duplicate this with an inductive system so it is not the instrumentation giving an erroneous reading. The continuing voltage spike is probably far less damaging with small plug gaps as there is far less current associated with it, but it does exist regardless of plug firing voltage (CDI only). Fred

Originally Posted by John at J&S

I didn't mention CDI or inductive. It should apply to both.

[safe]

Why not just use the stock CDI, coil and spark plug gap right out of the box? Works perfectly adequate. Has for the last 40 years and probably the next 40 too.

[John at J&S]

Fred, I envisioned something like that happening, but have no experience so I didn't mention it.

I thought it might be stray capacitance that would do it, but that would just make the spark stronger. For a few nanoseconds.

[Fred Winterburn]

I could have described it better too. I should have stated that the slope of the secondary voltage rise remains unchanged even after the plug fires. While the spark voltage is dropping after the plug fires, a voltage component continues to rise with the same slope as the voltage rise before the plug fired. The difference with CDI and inductive being that even as the gap is breaking down, the capacitor is still nearly at full voltage so that voltage is (somehow, but I'll leave that to a coil designer) multiplied and appears on the secondary despite the majority voltage component declining with spark current leading voltage. Once again, CDI only. Fred

Originally Posted by John at J&S

Fred, I envisioned something like that happening, but have no experience so I didn't mention it.

I thought it might be stray capacitance that would do it, but that would just make the spark stronger. For a few nanoseconds.

[mysocal911]

Originally Posted by safe

Why not just use the stock CDI, coil and spark plug gap right out of the box? Works perfectly adequate. Has for the last 40 years and probably the next 40 too.

That sums it up!