While not Porsche specific, I can offer what I've seen first hand on the dyno with respect to ignition systems. (I was a nascar crew chief for over two decades and fortunately didn't pay for dyno time, other than race fuel).

If it's lightin' the fire, it ain't gonna make more power with a stronger ignition system. There is no more magic as long as the mixture is fired. But if you open up the plug gap, it will make more power, until you open the gap up just a wee bit too far and the engine won't start. Power increase was about 10 hp with increased gap (about 1-1.5%). And if your ignition system is strong enough, 0.100" gap worked really good, other than tending to crossfire in the distributor and blowing holes in the rotor. Be careful that the piston dome doesn't close up the gap on your plug though.

It was common knowledge that we used 2 complete and separate ignition systems but only one distributor and one plug per cylinder. MSD boxes would quit working more than we were comfortable with, so the other system was a backup to the first, with an "A-B" switch controlled by the driver.

With respect to twin plugs and disabling one side, it doesn't surprise me that it lost power, not because two are better than one, but that the ignition timing wasn't then optimized for single plug. Much like doing a "mag check" on piston engine aircraft. For a more correct test, test optimal spark timing under each scenario. I doubt that the power output would vary as much if tested that way.
FWIW, YMMV

Okay, you make contradictory statements in that paragraph. If it is just about "lightin' the fire", plug gaps wouldn't matter any more than energy at the plug. So, it's either just about "lightin' the fire" or it also matters how you "lightin' the fire".

I thought it was explained pretty well, but I'll explain it a little better. Without changing the spark plug gap, it will not make any more power. Any power increase comes from increasing the spark plug gap. But it will LOSE power if the ignition system is weak enough that the engine is misfiring.

Dannobee.. I agree with you. I am going to try to put it more direct for the person that asked the question as I was not direct enough in my answer.
An ignition system has the energy to complete a spark up to a certain distance. A bigger spark makes more power.. If you double the demand on an ignition system you reduce its energy to complete the spark by 50%. So both answers are correct.. An ignition can work with two plugs if the gap is not too large in most cases. If you add a second ignition source doubling the energy and you do not change anything else the result is probably very minimal or maybe nothing. I indirectly was trying to state that and did a poor job. This is my last attempt at this subject and I did it for clarity to the person who asked the question.. Depending how you look at it Henry was right and Winders was right. I am right in stating it should not have spun this far out of control to help a lesser experienced person gain resolution to their question. Lets not talk about duty cycle in the ignition system because that would throw more confusion to newer people.

Yes, of course, you have to tune to take advantage of twin plugs. You have to tune to take advantage of available energy in the ignition system. You have to tune to take advantage of the fuel you are using. Nothing happens magically in a engine. Any changes made have to be tuned for....including better and more air flow into the engine and exhaust out. What can be tuned? Cams, cam timing, ignition timing, spark plug gaps, spark plug types, fuel type and octane, intake type and size, header type and size, etc.

You can't take an engine, twin plug it, and make no other changes and expect it to be much, if any, better. You have to tune the engine to take advantage of twin plugging. The same is true for the ignition system. If you run one ignition box with two coils you have to tune for that. If you run two ignition boxes with two coils you have to tune for that. You would probably make small gains with out running plugs gapped to take advantage of the increased ignition energy, but not what you would gain by using plugs with the appropriate gap for the increased energy. Putting 106 octane race gas in car tuned to run 91 octane buys you nothing but some pre-ignition protection. You have to tune the engine to take full advantage of higher octane fuels. If you put an air filter on the engine that flows more air, you may gain power, lose power, or have no change. You need to tune for that too.....

The engine is a system. The system has to be optimized (tuned) for any changes made. This concept was a given for me.

I love these technical discussions where some of the real knowledgeable engine builders give us the benefit of their years of experience.

I have a non technical solution that I would offer:

If I build a twin plug engine and spend that kind of money, I am going to use dual ignition boxes because there is no downside other than cost of a second MSD box. Just checked and Summit sells the MSD box for $285. I may be missing something that would raise cost somewhat, but at this level of investment in an engine build, it is probably cheap by comparison.

You didn't miss anything. You are correct.

Well he may have missed something. I love these "it's only a few bucks more" arguments.

Please understand that this exercise if offered with humor not malicious intent


First, why buy the cheap MSD boxes. The higher performance boxes are only a few dollars more? The cheaper digital boxes seem to have a pretty high failure rate. For that matter, why not buy the Bosch CDs as used with early twin plug racing Porsches.....Of course those were special 8 pin CDs but hey.....why not. Lonny @ Permatune makes a great replica.

Then there's the installation ...Where are you going to install the second unit?
We like it out of the engine compartment away from RF signals and heat. Usually under the drivers seat.
I assume because your $285 is an "all in" the extra CD will install itself.

Now, in this "it's only a few bucks more" build is the engine going to sustain high RPMs?. Are we using stock rods....if so, ARP bolts are just a few bucks more.....oh wait...we're doing this "right" so why not Carillos...or even better Pankl titanium?

The only way to make "real HP" in a small engine is to spin it...right?
Careful, for that you really need a special crank...I have my own made to my specs but because you have a few more bucks, why not design your own?

Is this engine going to be built on a tired old case? Surely a new case is better and only "a few bucks more". I know because I have some....want to order one now? They won't last long.

As you can see "it's only a few bucks more" along with "while you're in there" are terms that need serious consideration every step of the way.
Cheers SmileWavy

I really appreciate all the input here, I love the way Porsche people are happy to have string and healthy debate to take our mutual interest forward. It does become an academic only discussion at times, but from my POV this great, feeding our requiring minds, as long as no one takes it too personally. Here’s a few observations ..
- It seems twin plug with reduced spark energy preferable to single plug with a lot.
- Modern CDI ignitions probably deliver more spark than is required for a standard engine.
- Therefore it may well be beneficial to really open up the plug gap in a single plug installation. Anyone actually tried .100 or anything near it in an old 911?
- Modern CDI boxes probably produce way more spark energy (150MJ) than Bosch CD boxes from 50 years ago, sufficient to ‘light the fire’ in a 10:1 street car.
- The solution needs to be appropriate for the application .. high CR, full race, best of everything. Lower CR street car, solution appropriate for state of tune.
- A single CD box firing twin plugs in a street car boils down to a reliability issue, as it works the box much harder.

Interesting read while stuck at Denver airport for 1 hour layover.
In the water cooled world you can never have enough water pump. In any engine you can never have enough ignition energy. Whether you use it or not.

In these hemi engines, there are absolute benefits to using CDI ignition. If you are looking for a less expensive setup or have some emission requirements to met then inductive is a good choice. But you should expect less energy at the plug tip.

I have not read here anything about how the fuel mixture is ignited under compression. The plug starts the cycle but it’s the heat that is generated in the chamber that ignited the fuel mass. The more energy dissipated the greater the explosion.

The leaner the mixture the more energy you need as well.

The Ignition function is always taken for granted, buts it’s the most important function and the last in the making of rotational torque.

Splitting the energy stored in the CDI must have an effect on the energy at the plug. If it has not been seen, I think other factors were involved.

CDI coils act as transformers which is why they need very lower inductance levels. Most modern CDI units produce anywhere from 100 - 500 mJ. Inductive ignition rely upon the windings within the coil to generate their energy. The Igniter is just a digital switch. The coils produces the energy at the plug tip. Most modern inductive coils produce anywhere from 35 to 100mJ. Typical they are around 45 - 55 as most modern car engines do not require high energy coils with small cylinder head chambers, and low RPM limits.

Neil, your correct, I tried to keep it basic for the average non tuner/engineer questions.. But the post got a little crazy I think. I am waiting for someone to point out you said explosion rather than Burn. I am not I was just shocked some time went by and no one jumped on it.. LOL
I thought about saying is we are going to split the capacitive discharge unit why run two coils, we should split the coil wire and feed both caps.. BUT I do not see that kind of stuff as good humor and it isn't productive and the goal needs to be help the people and not make drama. Most of the people on here, like Alton, Henry,Winders and you have their strengths and weaknesses compared to each other and all are pretty smart with good intensions. I encourage in the future we all put our best foot forward to help support each other and keep humor out of it.

I enjoy the technical discussion as much as the humor ! I have a lot more experience building Ducati engines where the air cooled variants also respond very well to a second plug.

Twin plugging my 3.2 is in my future plans and this thread has offered great advice.

Regarding the quality of the MSD (what I currently have on my 3.2) what are some superior choices others have used? M&W ?

Each of us have different opinions, I found crane digital boxes to be excellent years but they got worse. I have had 1 MSD go bad over about 200 in the past 30 years. The biggest issue with the MSD if often a coil issue in the magnetic distributors, quite often MSD's are wired in reverse polarity on the magnetic output from the factory distributer, they will run but it causes premature failure. The green feed wire from the star wheel often has the polarity reversed. The internal insulated wire should be the positive but often the outer braided wire is the positive. This has been the issue quite often, rather than fixing the distributer I have reversed the wires to get them correct on polarity more often than not. The MSD is far from the best but I can say it is rare for me to have a problem. It is possible others have have had terrible luck with them and does not like them. In my experience the least reliable ignition is the prematune. Many times I have gotten calls "my car is dead on the side of the road" and I replace the permatune a Bosch 6 pin and away they go. The Bosch is a good unit, on cars running carbs where the air fuel ratio is not spot on controlled by a fast ecu like a Motec 130 I prefer the MSD as I think is had more energy than the bosh and I can run a wider plug gap to help with a wide range of air fuel ratios and gain a liter power.

Just go to a race track and look at the air-cooled race cars. If they are not running the stock ignitions or CoP setups, odds are they are running some version of MSD. They just work and are not ridiculously expensive. My experience at the track has been coils are what go bad. It's best to get the sealed MSD coils as they seem to last longer. I have never had anyone come to me looking for a spare MSD ignition box...but have had people come looking for a spare coil on many occasions. I don't think I have ever seen a failed MSD ignition at the track....though I am sure they do fail sometimes.

Best coil for MSD is black Blaster 2 high vibration in case someone is looking. The 6Al digital works well thankfully they make them black now, I never liked the red.

Yes, the black coil is the sealed unit that can be mounted in any orientation. I think some people the red ones something other than vertical and that causes them to fail earlier than they should.

FWIW....I have been running a single MSD driving two Blaster coils in my twin plug RSR spec motor for the past 15 years on and off the track. Runs great.

Sure it does....it's tuned to run that way.

I also have a 3.0 twin plug, RSR style pistons, weber 40's, web120/104 cams PMS/Jarvis dizzy (MSD trigger), etc all running on a single older 6AL with two MSD high vibration coils. It's very powerful, I have a poop eating grin every time I drive it. I might do some DE or an AX but it's a street car, but it's my summer DD say at least 60miles every couple of days, so far it's very dependable.
Only real bad is the carbs are a bit thirsty and girls don't like the smell a carb performance car emits.

http://www.914world.com/bbs2/uploads...4034_thumb.jpg

Looks like nice motors, The MSD high vibration coil by memory is Gel and regular ones are liquid. The liquid ones do not live very well upside down. The High vibration ones like in photo above seem to not care how they are positioned. Speaking of the fumes from fuel, I think that can be adjusted out.