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???? :rolleyes:

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.

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.

http://forums.pelicanparts.com/uploa...1611146981.jpg

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.

No race engine builder I know would use a single MSD with 2 coils....if they wanted the best performance anyway.

http://forums.pelicanparts.com/porsche-911-technical-forum/231256-msd-twin-plug-4.html#post5786585

Even MSD says you get half the energy at the plugs if you do this.

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.......

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.

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:

http://forums.pelicanparts.com/5849689-post69.html

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.

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.

So you are trying to argue. At least be honest about it......

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.
David
Performance EngiNerding

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.

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

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..

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 CR’s 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 I’m leaning as I think ~130MJ/2 will be more than adequate but provide all the well documented twin plug benefits ..

I think I’ll start shopping for that 2nd coil ..

For the record I’m 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.

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. I’m guessing my 10:1 build, not so much.

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.

http://forums.pelicanparts.com/uploa...1611403186.jpg

Look closely you might recognize this 906 engine.

http://forums.pelicanparts.com/uploa...1611403186.jpg

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

http://forums.pelicanparts.com/uploa...1611403186.JPG

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

http://forums.pelicanparts.com/uploa...1611403186.JPG

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

http://forums.pelicanparts.com/uploa...1611403186.JPG

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

Sorry last night when I was posted I was on a conference call and I could have done a little better.. Here is the best way you can go in my opinion, I hope this helps. There are more factors than the few I posted like how much advance your motor needs, the maximum RPM, air fuel ratio accuracy, and I could think of a couple more but it is not necessary. The great thing about this forum is there are a lot of knowledgable people trying to help keep the air cooled world alive both professional and amateur and more often than not there very intelligent.. Rosco, Back to trying to help you with your decision. There are a few advantages to twin plug on a N/A motor one is total timing needed. The amount of advance here has much to do with the stroke for example on an 80.4 motor twin plug I don't seem to make additional power past 22 deg and on a 70.4 mm stroke I make power gains on twin plug up to 28 or even 28 degrees. I have run 82.4 stroke motors up to 28 degrees advance without any issues but zero gains in power therefor for safety and longevity I run the minimum necessary to achieve the highest TQ/HP result. With Single plug I have similar issues but like on a 70.4 stroke I need usually 36 degrees to get peak power. So there is not one right answer for all motors. The decision also becomes very important on piston shape and head shape if not stock.. I do not really like twin spark on a street turbo build. Many do not realize the main purpose of twin spark is to ignite the fuel on both sides of the dome causing a higher efficiency motor making more power with increased reliability and less pre detonation. Now cams play a huge part in these choices on timing but I will save that for another day. The larger the motor the smaller the dome on the piston at the same compression. For example I have a 4 liter head I am in the works on that is like around 50 cc's therefore at that displacement my piston is nearly flat for a 10.5:1 build and the pistons are about 40% lighter enabling much higher RPM's. without slinging itself apart. The higher the RPM the more stress. Therefore the size of the motor your doing has a lot to do with this. Back to subject as this could get too long. I would twin plug your motor and try the 1 MSD 6 AL digital and 2 blaster 2 high vibration coils. I have used a few of these remade Bosch CDI boxes with "more energy in a smaller box than MSD" with negative results.. I am still going around with MSD engineers on why cant I use there gold drag boxes and split the signal to two coils.. Then I have the same amount of current as t 6al boxes.. They seeming could not answer me.. But most of my builds I use weapon X coils and drivers. Back to subject again on Porsche standard distributers. I prefer the counter clockwise distributers with magnetic star wheel. Answer, I would twin plug your heads and try the MSD. and MSD coils and I have done this on budget builds and if the spark does not provide the spark needed you can add a second unit. If it never gets too cold where you are for others reading you need less energy than in cold climates. I know there are a few variables I have not listed but I think I covered the needed points to help with your decision. My email is KNIGHTRACE@MAC.COM I am glad to help anyone with a question. No one is always correct, no one knows everything and we should try to learn each day. Things like this discussion in my opinion are several sharp people injecting there viewpoints with the goal to help people like you and more often than not you end up with a team result often increasing the accuracy and outcome of your motor without costing you any money and often saving you money. One of my blessings was someone who helped me greatly over 25 years in my knowledge, his name is Dick Elverud. I was in my early 20's and didn't know much as I was an engineer not a mechanic and he put thousands of hours in teaching me things and I do my best to give back to new people in the Porsche game to pay it back to the community or the best that I can. I have also learned things from non professional people through the years that challenge us from time to time. My opinion is the outcome of these motors is based on about of effort put in. I kind of helped Mike D'Silva kick off his shop in Australia and he for a short time in the game is doing very well and I am impressed with his ability to learn most things needed and how quick he learns. He as many learn faster than I did in my opinion. Please reach out to me if you ever need my help I will do my best to help the Pelican community. I actually purchased my first 3.2 motor to build from Henry when he worked with Class Action in the early 90's. WHAT I wouldn't give to go back to that day with my knowledge today.. I have great respect for Henry because he is honest and does good work. I will not name names here as one shop is still in business in California but the reason I started my business building motors in the early 90's is because I was robbed by two criminal business with horrible outcome's in the early 90's. I said to myself I know almost nothing but I am honest and ANYONE could do better than these two shops. Therefore I have spent decades trying to help people not get robbed like myself. I guess that is how I got the name "Super Hero" on this forum. Believe me I did not pick that name and the only way I look at it it could be worse. I hope I helped you reach a final decision and your welcome to reach out to me if I can help. Thanks, William Knight

Not to write another book here, BUT smaller displacement motors often make more HP per liter but are actually less efficient than larger displacement engines in most cases at least in N/A conditions. I do not design motors based on HP goals. Like In Winders 3.6 my entire goal was the most cumulative energy over time to help it go faster around the track, I was not focused on peak HP. The more efficient the engine is the more energy it puts to the ground. If cars and driver are equal the better engineered motor will win every time.

That was a great post for the Porsche gizmos but what I want to know is what happened to that Browning Hi-Power 9mm you were talking about selling/trading a few years back?
I ended up with an FN 5-7 but I'm still interested in the Browning.....

http://forums.pelicanparts.com/uploa...1611408352.jpg

Well that took a turn

Sent from my Nokia 7.1 using Tapatalk

Great thread, thanks for the information.

Henry, all the other folks talking dyno proven horsepower in this thread are comparing twin plug to single plug except you. Could you post a dyno comparison of back to back twin plug single/dual MSD boxes ?

-Lorin

Horse power gain on twin plug vs single plug on a 3 liter or 3.2 usually is about 15 hp with a 120/104 cam.. Back to more important stuff. Henry I unfortunately sold the GP Browning for like $700 sadly. I remember you were going through a bad phase where you were selling all of your firearms.. I lost most of mine in a boating accident last year.. But I still have one or two.. Friend of mine who was Secret Service went to the 5.7 he said the secret service went to that as well. Myself I am still an old fashioned 10 mm person... I had some Liberty 60 grain ammo at 2200 fps. Think it is adequate. I love to kill steel, paper and even clay pigeons. Think I am going to shoot clays today....

I have done about 50 3 liters with big port heads, not 3.2 as there too big for even a 3.2 in my opinion, With an oversized intake valve with radian cut and 10.5:1 compression and 120/104 camshaft I average 272 hp at the rear wheels. cant remember TQ but I am SO OVER 3 liter motors I hope never to build another 3 liter.. I gave these numbers so Henry doesn't have to dig..
Secret recipe for someone who wants a great street motor.. 74.4 3.2 crank, big port 3.0 heads, 464/465 cam from webcam.. makes about 260 at wheels and will eat the 3.0 making 270 on a track or auto and put a smile on your face for the money spent.. William Knight

Thanks for the data, William.

My question was the horsepower difference between twin plug ignition using 2 MSD boxes VS twin plug ignition using 1 MSD box.

Only Henry has mentioned dyno testing this difference.

Most of the dyno numbers I have stored in my cranium are all that's left. Back them, dyno results were on paper. If they got transferred to the computer it was rare. Then, every time I upgrade my computer, I seem to lose data.
One of my favorite dyno results was from a late seventies 2.5 race development that showed zero horsepower gains from boat-tailing 911 main webs. I remember but don't have the sheets.

I know, weird.... Bruce Anderson claimed 7-10 horse power.

Ok sorry I didn't understand the question, I had JB racing dyno a 3.4 motor 10.5:1 compression 3.4 with 50 mm PMO's it made 341 HP with two ignitions and with one it lost 14 hp.. The reason I asked them to do it was a client named Mike Lindstrom brought a car in the shop that had 1 MSD and ran fine... I was like why am I using two.... The power may be worth $250 more. I learned that day you could use one box.. I don't have a dyno of it with 1 box but I have the dyno on the 3.4 with 120/104 cams. I am not a fan of those cams but they work. I suspect the difference on a 275 hp motor would be closer to 8 or 10 hp. but I am guessing.

I agree with Henry.... I don't even boat tail cases... However it is my belief the smaller the motor the more it helps. on 3 liter cases and larger your wasting your time. Winders 3.6 made 434 HP and 340 TQ. and the thought of boat tailing the case never crossed my mind. I wasn't even trying for peak HP, I didn't care as long as it was enough to satisfy the client. I only cared how wide the power was because he had a 87 mph 1st gear and 49 lb rear wheels. it made between 250 to to 270 tq from 3700 rpm to 7700 rpm. Have Dyno and race video. Not wanting to post here as I am not trying to compare to anything just backing up what Henry stated and prove you can make power without boat tail and hopefully save people some money. Pauls 3.4 was boat tailed before I got it and it looked like they used a chain saw. I like a radius vs the square on the cases we make but it is only because it add a little strength and doesn't hurt power.

I bet if you posed this premise to Dawe, Knight, Aase, Mirage, GAS, Lowrance, etc., they would agree with it:

"You don’t use a single MSD with 2 coils unless you need to do so due to costs reasons. …or you don’t have the space to put the second ignition module. It should not be standard practice."

Porsche always used 2 ignition modules on twin plug engines.

Twin Spark Firing using single CDI Box
 

Great info here, thanks all for sharing. If you’re a racer, I’m sure a 2nd MSD box is small beer. Being half Scottish, and living in road car world, I want to understand the cost / benefit.
For clarification I’m building a 7R mag case high compression 2.7RS replica engine running MFI.

Part of the discussion is to understand whether I need to provision mounts for a 2nd Daytona Sensors CDI box being car reconstruction is underway.

I’m getting the heads twin plug prepared as I hear benefits of reduced pumping losses, reduced heat generation and improved drive-ability. So that’s underway. Initially I’ll run single plug using a distributor and then lock the distributor and tune the curve in software. I’m somewhat concerned with rotor phasing running a standard distributor and up to 38 deg advance. I have access to 100 Octane fuel here down under. (Might be optimistic at 10:1) Any world experience here guys?

I’ll then install the JB Racing twin plug distributor and hook up the 2nd plug bank, initially with a single CDI box. I’m also making provision for a TPS driven off #6 TB shaft. I would have thought someone would have designed a bolt TPS & billet TPS mount .. I’ll design something in CAD and get it machined if it hasn’t been done before...?? Given all the MFI engines running around with MSD boxes, I would have thought someone would have taken the next obvious step to extend to 3D ignition. The reduced timing with twin plug would also much reduce risk of cross firing inside the distributor.

I’m anticipating some dyno tuning to optimise ignition at part throttle settings. Alfaholics sells an off the shelf system for the GTA replica and is full of praise for the difference 3D tuning makes, especially being able to tune out some rough running with big cams.

I will be interested to see the difference between each development as the engine gets improved.

In the meantime, I have an out of round tunnel to contend with, even after paying a well known Arizona shop to align bore to standard.

Just a couple of quick questions.
During your dyno test, did you try to tune for ignition changes? IE: mixture , plug gap, timing to see if a single CD would perform with different parameters?
What type of plug wire did you use? Resister, solid core, spiral core?
While I would agree that the higher the compression the greater the need for a higher energy spark, but did you try your test on a milder (95.5:1) engine?
What type of distributor did you use to trigger the CD?
What type of CD did you use?
Was your CD analog or digital? We found that the analog out performs the digital MSD hand down in this application.
I'm not challenging your finding but if your goal was the prove 2 is better than one, perhaps what you saw of confirmation bias.
Ultimately, for high compression racing situations, more is generally better but for spirited street performance, more is quite often, just more hype.

Oh my God.....

Why would you twin plug a Porsche engine? Because you need more spark energy for it to work properly. So why would you ever pick an ignition option that gives you less spark energy?????

Porsche, when they twin plugged 911-based engines, uses 2 ignitions...1 for each coil.

A few answers and possible suggestions

Mag cases offer real challenges. Sometimes you just have to bite the bullet and source a different case


http://forums.pelicanparts.com/uploa...1611440354.jpg

http://forums.pelicanparts.com/uploa...1611440354.jpg

http://forums.pelicanparts.com/uploa...1611440354.jpg

I'll let your guru answer that question.
At no time have you come close to suggesting that "less spark energy" is not enough for excellent engine performance.
We might hypothesize that excessive "spark energy" can be detrimental to the life of ignition parts.

I just took apart a set of "race" heads that had crazy stiff springs. At 39mm installed height, the measured seat pressure was over two hundred pounds.
I wonder if that genius thought "it's a race engine, more is better"?

An answer to Henrys question the best of my ability as I do not have much experience doing single ignition on twin plug with two coils. I did not do any testing to see if or how I could make it work by changing plug gap, plugs. Nothing.. It was on a dyno eating money. So it was more of a curiosity for me. It did run but I don't know how long the plugs or ignition would or wouldn't last. My background is Electrical engineering with a masters in electricity. BUT I have put no time in real testing as there was no reason. I strongly suspect a low compression N/A motor at like 9.5:1 with low resistance plugs and no large gap would work very fine and give no issues. I understand our only goal here is to help people with questions and in this instance., I probally could come up with at least 25 variables to effect the answer down to the cams. The test we did was a 10 minute experiment. I did not even know it would run until that day. The motor had 50 mm PMO,s and a JB racing twin plug dist. I know I help some people today that do not care if their motor makes 15 hp one way or the other, They want twin plug because it looks cool. On these questions with mechanical injection I know enough to give you Henrys phone number as that is not my domain. I have done maybe 10 MFI motors in my life but I am far from an expert. I hope the Pelican community is grateful for people Like Henry and myself and others who take time out of their day to answer questions. I am grateful for clients like Winders, his passion pushes people like myself to a higher level than I would have probably achieved without him. Hopefully Rosco has all the answers he needs and maybe this post will help many others in their builds. Rosco, make sure on the JB racing dist, only use one mag signal if you use two ignition boxes, there is a parallel splitter for that. Lots of people do not know that one mag signal is to be used as a spare. I do not want to get into a long discussion on this it just keeps the boxes in time with each other, so that you don't have a leading or trailing upper or lower bank. WK

And with that it seems like we're done. At least I am.
Cheers

LOL! You don't even get what William was saying there. He didn't architect the engine for peak power. If he did, he would have used different cams with different timing. Our goal with the engine was not peak power. It was a broad power band with usable power. I had a general range of power I was hoping to get based on what I expected my competition to have. But agreed with William that having faster lap times was more important than top speed. But, because of the choices William and I made for the engine, it made more peak power than I expected by a large margin yet had the broad power band we wanted for best overall performance.

With the CoP setup we used on the engine we were presented with two options. One, was running 2 3-channel ignitors splitting each channel between 2 plugs (coils). The other was using 4 3-channel ignitors with each plug having a dedicated channel. The manufacturer told us either would work. They said sharing the channel would halve the power to the plugs and only recommended doing so if the cost of the ignitors was an issue. As the engine would make more power and be more efficient when running a single channel per plug. This all sounds familiar, does it not?