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Rosco_NZ
 
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Twin Spark Firing using single CDI Box

It seems to me it should be possible to produce twin HV spark output from a single primary / twin secondary winding coil such as MSD 8224. This coil appears to have suitable primary resistance at 0.35 ohms so as not to overload the CDI box driver output. Each secondary spark would go to the twin distributor HV input terminal and fire each pair of cylinder plugs.

However I am sure I am not the first to think of this, however all vintage 911 twin spark implementations seem to double up on CDI units and 2x single coils. I get there may be some redundancy benefits for racing, but that's not me / many of us. I understand ignition timing could be adjusted independently with 2x CDI boxes, however from my investigations, twin ignitions for performance are fired simultaneously. (And any engine forced to drop to single plug with only 26 deg. advance will be very sluggish)

I am thinking this can only be the vintage crowd staying traditional ... any other good reasons????

Old 01-18-2021, 06:57 PM
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I see what you are thinking. These look a lot like the coil packs used in waste spark crank fire systems - three coils for six plugs (or double that if twin plug). In a waste spark system, the impedance (or resistance?) in the two cylinders, one open to the atmosphere, the other with a compressed air/fuel mixture, would seem to be different, but if this is significant perhaps the firing cylinder's mixture is lower impedance or resistance, so gets the most spark, or at least enough.

You are going to fire two "loaded chamber" plugs at the same time.

How did GM run these? Why did they use a coil with two plug wire outlets? I had a nice Chevy van in the 90s, but hardly opened the hood. Four packs for a V8? Looks like these were only used on V6s and 4 cylinder engines (3 or 2 packs). But this might confirm or deny your idea.
Old 01-19-2021, 09:42 PM
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Quote:
Originally Posted by Rosco_NZ View Post
It seems to me it should be possible to produce twin HV spark output from a single primary / twin secondary winding coil such as MSD 8224. This coil appears to have suitable primary resistance at 0.35 ohms so as not to overload the CDI box driver output. Each secondary spark would go to the twin distributor HV input terminal and fire each pair of cylinder plugs.

However I am sure I am not the first to think of this, however all vintage 911 twin spark implementations seem to double up on CDI units and 2x single coils. I get there may be some redundancy benefits for racing, but that's not me / many of us. I understand ignition timing could be adjusted independently with 2x CDI boxes, however from my investigations, twin ignitions for performance are fired simultaneously. (And any engine forced to drop to single plug with only 26 deg. advance will be very sluggish)

I am thinking this can only be the vintage crowd staying traditional ... any other good reasons????
I'm not really sure what you're asking. If your question is "can a twin plug ignition function with a sign CD/" the answer is yes. We've been doing it for 20+ years.
From the MSD website.

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Old 01-20-2021, 03:51 AM
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Quote:
Originally Posted by Henry Schmidt View Post
I'm not really sure what you're asking. If your question is "can a twin plug ignition function with a sign CD/" the answer is yes. We've been doing it for 20+ years.
From the MSD website.

Yes, you can do that. But, using a single MSD 6AL unit with two coils instead of two MSD 6AL units results in compromised performance. A single MSD 6AL is not up to the task. You will get less horsepower.....
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Old 01-20-2021, 08:42 AM
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Originally Posted by winders View Post
Yes, you can do that. But, using a single MSD 6AL unit with two coils instead of two MSD 6AL units results in compromised performance. A single MSD 6AL is not up to the task. You will get less horsepower.....
I've been doing it for years. Are we leaving something on the table? not so you'd notice.
Dyno both ways with no discernible difference.
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Old 01-20-2021, 08:50 AM
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Quote:
Originally Posted by Henry Schmidt View Post
I've been doing it for years. Are we leaving something on the table? not so you'd notice.
Dyno both ways with no discernible difference.
No race engine builder I know would use a single MSD with 2 coils....if they wanted the best performance anyway.

MSD for Twin Plug

Even MSD says you get half the energy at the plugs if you do this.
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Last edited by winders; 01-20-2021 at 08:57 AM..
Old 01-20-2021, 08:53 AM
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When I was designing my new race engine's CoP ignition system, I had the option of running two 3 channel ignitors or four 3 channel ignitors. With two 3 channel ignitors, I could have setup two coils on each ignitor channel.

The engine would run fine, but the CoP ignition system manufacturer told me that I would halve the energy at each spark plug and the engine would make less power.

My engine has four 3 channel ignitors....

Just because you can do something does not mean you should.......
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Old 01-20-2021, 09:05 AM
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Quote:
Originally Posted by winders View Post
No race engine builder I know would use a single MSD with 2 coils....if they wanted the best performance anyway.

MSD for Twin Plug

Even MSD says you get half the energy at the plugs if you do this.
Racing has it's limitaions as well as street performance.
The question was "will it work" and my answer was "it will work well and it's hard to tell the difference".
Is there always something that will make more horse power? sure but at what cost?
We run a single on street engines because lower compression engine don't need as much energy to trigger the required spark and if you have two CDs and lose one, you could very well damage the engine if you continue driving. When you have one DC that fails, the engine stops and there is no damage.
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Old 01-20-2021, 10:02 AM
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Quote:
Originally Posted by Henry Schmidt View Post
Racing has it's limitaions as well as street performance.
The question was "will it work" and my answer was "it will work well and it's hard to tell the difference".
Is there always something that will make more horse power? sure but at what cost?
We run a single on street engines because lower compression engine don't need as much energy to trigger the required spark and if you have two CDs and lose one, you could very well damage the engine if you continue driving. When you have one DC that fails, the engine stops and there is no damage.
Yes, it does work. But it is not how it was intended to be done. Just because MSD documents the way to do it does not mean it is what should be done. You get only 50% of the energy at the plug you would using two MSD units.

I could say:

"If you have two coils and lose one, you could very well damage the engine if you continue driving."

I have seen coils fail MUCH more often than MSD boxes.
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Old 01-20-2021, 10:38 AM
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Quote:
Originally Posted by winders View Post
No race engine builder I know would use a single MSD with 2 coils....if they wanted the best performance anyway.

MSD for Twin Plug

Even MSD says you get half the energy at the plugs if you do this.
Not trying to argue but I do want some clarification.
When you say "half the energy" how much is that. Real numbers please.
When you say "MSD says", did they mention whether or not they thought "1/2 the energy" would fail to perform to their standards?

I've had this discussion over the last 25+ years and no one has ever given me the empirical data.
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Old 01-22-2021, 08:02 AM
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Quote:
Originally Posted by Henry Schmidt View Post
Not trying to argue but I do want some clarification.
When you say "half the energy" how much is that. Real numbers please.
When you say "MSD says", did they mention whether or not they thought "1/2 the energy" would fail to perform to their standards?

I've had this discussion over the last 25+ years and no one has ever given me the empirical data.
I have not measure the energy because there are specs on the web. The old MSD 6420 has less voltage and millijoules (480v and 115mj) than the MSD 6425 (530v and 135mJ) listed here:

https://www.holley.com/products/ignition/ignition_boxes/street/parts/6425

This text from MSD says that running the ignition with two coils in parallel results in about half the power per coil. See here:

MSD for Twin Plug

So, if you are using an MSD 6420, instead of each coil getting 115mj, they are getting about 58mj.

MSD obviously thinks that running 2 coils in parallel on one 6AL ignition module works because they provide directions on how to do it. It doesn't mean that is what they think is ideal or what they recommend. In fact, they recommend one ignition module per coil.

What does a stock Porsche ignition module put out to a coil? I have no idea and I don't really care. Because we have data that shows that running with just one ignition module makes less power. This data comes from all over the place too. Builders are always looking for ways to get more energy in their ignition systems. They don't do this because it makes no difference. MBruns has document an almost 7% HP loss when running one ignition versus two.

Why would you purposely supply half the energy to the spark plugs that you could otherwise get? For convenience?

As I said, in my experience MSD coils fail much more often than MSD 6AL ignition modules do and would be just as hard to detect even with a single ignition module. So that argument is all wet. If you are really worried about ignition failure and detection, build a little switch box that allows you turn each ignition module off while the car is running. If one of the coils or ignition modules failed, you could find out.
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Old 01-22-2021, 09:34 AM
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You really seem desperate to argue the point. The truth is, these are not rocket science. They do not offer optimum performance on any level.
By your logic, "Why bother with air cooled two valve when four valve water cooled are available?"

All wet? please, coils fail. If we could run a twin plug on one coil, that might be safer. We can't. I have never had a Hi-Vibration MSD coil fail.
Ignition boxes fail. I have three failed 6 ALs in my shop right now. I'm talking to MSD about possible causes and they are at a loss to explain.
You insist that two is better. I agree. That was never the question.
I get it but "better" is subjective and application specific.
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Old 01-22-2021, 10:46 AM
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So you are trying to argue. At least be honest about it......
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Old 01-22-2021, 10:58 AM
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Henry has done his homework here, has run back to back on the dyno and noticed no difference in HP. To me that's not arguing, it's just stating facts that you seem not interested in hearing. Once a fire is started, adding more spark will do no good. These CDI ignition arguments "my spark is bigger than your spark" seem so pointless.
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Old 01-22-2021, 01:37 PM
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Quote:
Originally Posted by reclino View Post
Henry has done his homework here, has run back to back on the dyno and noticed no difference in HP. To me that's not arguing, it's just stating facts that you seem not interested in hearing. Once a fire is started, adding more spark will do no good. These CDI ignition arguments "my spark is bigger than your spark" seem so pointless.
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Talk to any engine builder that builds high performance engines and they will not agree with Henry's "facts". Running 1 MSD ignition with 2 coils is fine if you don't care about maximum performance. You will not make the power you could make with 2 MSD ignitions and 2 coils. You may not be able to run as high a compression and the spark plugs gaps would need to be smaller. The combustion would not be as efficient as it otherwise could be.
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Last edited by winders; 01-22-2021 at 08:04 PM..
Old 01-22-2021, 05:04 PM
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Scott, did you design your race engine as you stated? I thought it was a spec build to fit a particular race class and a true guru, William Knight, played a large role in the design?

Don't get me wrong, Henry and I have had a few disagreements over the years on this board. But we have also wished each other Merry Christmas and I have bought parts from him over the years. I do trust his experience and have called on it a few times where we were not as experienced as he was regarding the issue at hand.

That said, I would be careful debating with him the finer points as, if I understand correctly, you have never built a 911 motor?

Cheers
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Old 01-22-2021, 07:27 PM
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I was asked my opinion on this... First of all Henry does good work and I am appreciative there are good people like him out there. Here are my results and answer to my findings.. First the ignition energy necessary to complete a proper spark is codependent on a few things, like plug gap, compression, and type of plug used.. Silver is the best conductor and lower energy systems benefit with these. If you have a street level motor like 9.5:1 compression and you gap plugs at like 35 thousandths I would say there would be very little power difference between a single ignition vs two ignitions. Now if you have a 14:1 compression motor you may find the compression will blow out the spark. On Georges motor I had 14.3:1 on a 3.2 and made 377 HP, If I used 1 ignition it would not run as it could not make the 65 thousandths jump I like on the high compression motor. I used a digital MSD 6al and blaster 2 coil.. The wider gap makes more hp, the power difference with upper plugs vs dual plugs was about 17 hp on the motor,. I tested upper and lower plugs alone for curiosity the lower plug bank actually made a few more hp than the top bank.. not sure why... Any way in my opinion if you are trying to extract al you can out of a motor you are better with two ignition boxes or a much more powerful unit than 150 mj system.. Memory on MSD 6 series is around 150 mj ,not positive on number. This is a situation in my opinion where both answers are correct. I hope my answer here was not too bad I am on a phone call while writing and I am not great at multi tasking..
Old 01-22-2021, 08:25 PM
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Great clarification, thanks all for views & input. I learned a lot here so far ..

I think some parameters around the discussion might help .. I agree more spark is better ... to a point. There must a point beyond which no fatter spark across a bigger gap produces no further improvement. I agree as CR & rpm increases, this point will go further out. But air cooled 911 engines do not run high CRís like modern engines, albeit generally fairly rich.

So (for coils connected in series as per MSD diagram) would I rather have 1 spark in a sub-optimal location, or 2 (with the same energy output as 1) better located. I know which way Iím leaning as I think ~130MJ/2 will be more than adequate but provide all the well documented twin plug benefits ..

I think Iíll start shopping for that 2nd coil ..

For the record Iím proposing using Daytona Sensors CD-1 box. Tech support cautioned about running 2x coils in parallel as ~0.14ohms primary impedance too low & may burn out the output stage. Has anyone successfully run CDI coils in parallel, rather then in series? Series operation would halve the (~40+kV) voltage but maintain the same current at each plug.

I read somewhere MSD QA is supposed to have declined since production went offshore. Capacitors can and do fail.
Old 01-23-2021, 12:32 AM
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Twin Spark Firing using single CDI Box

I stand corrected on CR, thanks William. Sounds to me like both answers are correct, depending on CR & engine tune. For high CR full race engine, more is better. Iím guessing my 10:1 build, not so much.

Last edited by Rosco_NZ; 01-23-2021 at 12:38 AM..
Old 01-23-2021, 12:36 AM
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Talk to any engine builder that builds high performance engines ....edit....
This kind of discussion is always a hoot. Challenge the messenger to diminish his/her credibility.
I wasn't aware that after building more than 400 air-cooled 911 engines that none off them were high performance.

A few of my more recent builds......of course by Winders standards they probably aren't "high performance".

Factory 3.0 RS upgraded at the customers request for installation in a old Brumos racer.



Look closely you might recognize this 906 engine.



This fun little engine is a short stroke 3.2 we build is the first of a series of three for a company building multiple versions of a streetable RSR



Here's a neat little short stroke 2.8 that made 306hp on street gas with 9.5:1 compression.



2.0 engine to replace the factory engine in a 904/6



Of course these are all "old school" Porsche that left all their performance on my engine room floor.

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Last edited by Henry Schmidt; 01-23-2021 at 04:32 AM..
Old 01-23-2021, 03:23 AM
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