911T ignition coil advice needed

[Forssling]

Hi All,
I have restored a -71 911T engine. I'm about to start it up for the first time very soon. It's been pointed out that the coil I have is not working for my setup.

• Beru 12 Volt Blue Coil ZS 172
(3.3 ohms, 40000 volts)
https://www.hot-spark.com/1-Beru-ZS-172.htm
• Original (restored) CDI
• 123-ignition distributor

1) Which coil will work for me? Does not have to be original setup. Flame thrower? What is the numbers (resistance etc) crucial for my setup?
2) Can I run the Beru coil above for the break in?
3) What kind of problems will this set up give? (Beru coil)

ps. I found recommendations for MSD 8202 coil. Any comments?

Thanks for any input,
Jörgen Forssling
Gothenburg
SWEDEN

[HaroldMHedge]

I believe Parts Klassik has a good replacement coil for your 71 911T CDI system.

[Forssling]

Originally Posted by HaroldMHedge

I believe Parts Klassik has a good replacement coil for your 71 911T CDI system.

Thanks for your input, and you're right. But I'm in Sweden so I will avoid to order from US if possible. In my research I have come up with these candidates available in Europe:

• JP Ignition transformer Bosch Style black Perma Tune USA 502 00
• Beru ZS 109
• MSD Porsche Blaster 2 8202
• Bosch 0221121001

Anyone have any preferences among these or any other candidate?

KR
Jörgen

[john walker's workshop]

I use the MSD 8222 epoxy filled coil with the bosch CD unit with no issues.

[Forssling]

Originally Posted by john walker's workshop

I use the MSD 8222 epoxy filled coil with the bosch CD unit with no issues.

Thanks! I’ll check this out.

//J

[Bob Ashlock]

I recommend the original Bosch 0 221 121 001. It was specifically designed to work with the Bosch CDI. I have compared all kinds of aftermarket coils and the only one that is a close match is the ************ replica. The majority of other coils have high inductance. You don't want that with a CDI unit. The low inductance coil (really a step-up transformer) is designed to pass the fast rise time discharge energy straight to the plugs. A high inductance coil, designed to store energy and then release it later, slows/spreads the discharge pulse and lowers its amplitude. I'm not saying aftermarket coils don't work... they will give a spark. But my observation side by side is that they give a weaker more orange/yellow spark compared to the Bosch coil that gives a bright blue/white spark with a good sounding "snap". Another factor that plays into this is the turns ratio. The Bosch coil is 1:10 step up. The pulse discharge from the CDI is 400V, so X 10 makes 40KV. A typical inductive coil has a much higher turns ratio. So you would think higher output voltage. But that doesn't always happen because of the pulse spreading/suppression effect caused by the inductance. My 2-cents worth. This is probably like starting a discussion about motor oil.

[mysocal911]

Originally Posted by Bob Ashlock

I recommend the original Bosch 0 221 121 001. It was specifically designed to work with the Bosch CDI. I have compared all kinds of aftermarket coils and the only one that is a close match is the ************ replica. The majority of other coils have high inductance. You don't want that with a CDI unit. The low inductance coil (really a step-up transformer) is designed to pass the fast rise time discharge energy straight to the plugs. A high inductance coil, designed to store energy and then release it later, slows/spreads the discharge pulse and lowers its amplitude.

Actually, the coil (8222) recommended by John functions very well on the 911.

Originally Posted by Bob Ashlock

Another factor that plays into this is the turns ratio. The Bosch coil is 1:10 step up. The pulse discharge from the CDI is 400V, so X 10 makes 40KV. A typical inductive coil has a much higher turns ratio.

Both coils have a turns ratio of about 100 and not 10. Remember, 10X 400V is 4000V which would be an inadequate spark voltage, i.e. both provide about 40KV.

[Bob Ashlock]

Thanks mysocal911 ........oooops, my little math error. Maybe that's the 'new math' ...

[Jonny H]

Originally Posted by Bob Ashlock

I recommend the original Bosch 0 221 121 001. It was specifically designed to work with the Bosch CDI. I have compared all kinds of aftermarket coils and the only one that is a close match is the ************ replica. The majority of other coils have high inductance. You don't want that with a CDI unit. The low inductance coil (really a step-up transformer) is designed to pass the fast rise time discharge energy straight to the plugs. A high inductance coil, designed to store energy and then release it later, slows/spreads the discharge pulse and lowers its amplitude. I'm not saying aftermarket coils don't work... they will give a spark. But my observation side by side is that they give a weaker more orange/yellow spark compared to the Bosch coil that gives a bright blue/white spark with a good sounding "snap". Another factor that plays into this is the turns ratio. The Bosch coil is 1:10 step up. The pulse discharge from the CDI is 400V, so X 10 makes 40KV. A typical inductive coil has a much higher turns ratio. So you would think higher output voltage. But that doesn't always happen because of the pulse spreading/suppression effect caused by the inductance. My 2-cents worth. This is probably like starting a discussion about motor oil.

^^^ What he said ( with the correction to the maths in the turns ratio. I believe the Bosch coil is actually 1:78)

Check our website next week. We've just complete 3 years of development of a true CDI coil. First production units should be here next week.

[Forssling]

Thanks All for your input! I really appreciate it. My concern about the Bosch coils has been about the quality issues that I've read about. I have now ordered a 8222 that was easily available for me here in Sweden.

My engine is a stock T, the only upgrade is the 123 distributor. If I run into problems with the 8222, I will check out the other options.

Take care,
Jörgen

[mysocal911]

Originally Posted by Jonny H

^^^ What he said ( with the correction to the maths in the turns ratio. I believe the Bosch coil is actually 1:78)

Check our website next week. We've just complete 3 years of development of a true CDI coil. First production units should be here next week.

If you are referring to using the 8222 coil, as a replacement for the original small black Bosch coil when not available,
it doesn't result in a less effective spark for the original Bosch CDIs. Some aftermarket CDIs with shorter spark pulses
than the Bosch CDIs may experience marginal sparks. As an example, the 8222 (MSD coil) was designed to
work with MSD CDIs (multiple sparks) without any problems.

So let's not label all CDIs with the shortcomings of some aftermarket CDIs which claim multiple sparks,
but whose second spark is too short, providing marginal benefit when not used with the original black Bosch coil.

[mike fitz]

Hi Jonny will your coil work on my aftermarket CDI if so put me down for one .....mike fitz

[Jonny H]

Originally Posted by mysocal911

If you are referring to using the 8222 coil, as a replacement for the original small black Bosch coil when not available,
it doesn't result in a less effective spark for the original Bosch CDIs. Some aftermarket CDIs with shorter spark pulses
than the Bosch CDIs may experience marginal sparks. As an example, the 8222 (MSD coil) was designed to
work with MSD CDIs (multiple sparks) without any problems.

So let's not label all CDIs with the shortcomings of some aftermarket CDIs which claim multiple sparks,
but whose second spark is too short, providing marginal benefit when not used with the original black Bosch coil.

I did not mention CDI boxes and multispark but since you have, it's the spark interval that's the issue with 'slow' coils, not the spark duration. If the time between multiple sparks is too long, the benefit of having them is negated.

The MSD coil is much slower to 'charge' and 'discharge' than the Bosch. It takes around 600us to 'ring out', compared to 170us for the Bosch coil. MSD units' multi-spark interval time is 1000us and cannot be changed which I believe is because of the coil performance.

Our CDI+ system has an adjustable spark interval, of which the default is 400us which takes advantage of the fast Bosch coil. This provides the speed necessary to be well before TDC to gain extra power. You *can* use our system with 'slow' coils, but you have to detune the CDI+ to run with a longer spark interval.

To answer the OP's question about the Beru coil, this is an inductive coil. Although it may run the engine, it will not be optimal for the CDI unit. The time taken to charge and fire the spark will be much longer than the correct coil, which will effectively 'distort' the advance curve proportional to RPM. The spark itself may be sporadic over a longer period of time, rather than a short concentrated burst.

[mysocal911]

Originally Posted by Jonny H

Our CDI+ system has an adjustable spark interval, of which the default is 400us which takes advantage of the fast Bosch coil. This provides the speed necessary to be well before TDC to gain extra power.

What is this hyperbole about "speed"?

Originally Posted by Jonny H

Although it may run the engine, it will not be optimal for the CDI unit. The time taken to charge and fire the spark will be much longer than the correct coil, which will effectively 'distort' the advance curve proportional to RPM.

And more hyperbole! It has never been mentioned, until posted by you, nor proven that using the 8228 or the blue coil with Bosch CDIs results in engine power loss.
Again, don't conflate the shortcomings of your CDI with the Bosch CDI.

Let's avoid having Pelican Parts Forum members wasting their money on changing their coils they've been using for years with no problems.