MSD for Twin Plug

[tom1394racing]

I am getting ready to install my MSD6AL on my '73 RSR clone. The motor is a '78 SC with twin plugs mods including the dizzy conversion by Jerry Woods. I've read and been told that I can drive two coils with a single MSD unit. I have two of the MSD Blaster coils and a single MSD6AL unit.

Does anyone have any wiring diagrams, photos or a description on how to wire the single MSD to dive two coils?

[jpnovak]

I have never heard of driving two coils with a single ignition. Seems like it would overload the ignition box.

You can use a single trigger to signal two boxes, each running a coil.

Oh yeah, post some pics of the modified distributor. Please include some internal pics and adapter plates if you can.

[randywebb]

yes you can. it works just fine. very simple - just branch into the wire.

yes do post pics of the JW mods

[Steve@Rennsport]

Originally posted by jpnovak
[B]I have never heard of driving two coils with a single ignition. Seems like it would overload the ignition box.

You can use a single trigger to signal two boxes, each running a coil.

Jamie is right and MSD will confirm that as well.

One coil for each MSD box.

[ErVikingo]

Before changing to Electromotive, my setup used 2 MSD boxes as indicated by MSD.

[DW SD]

Please do post the pics - very interesting thread.

Thanks!

Doug

[kenikh]

Never mind, post deleted. I was reading it wrong.

[randywebb]

So Steve -- what will happen to an msd box that runs 2 coils at once?

FWIW, I think you know who recommended that to me...

my car seems to run ok with it...

[tom1394racing]

Well...I am confused. Differing opinions from the experts (Jaime, Randy and Steve) on whether you can drive twin coils with a single MSD unit. Scott and Jim at Auto Associates both say that only one MSD unit is needed for the twin plug application.

Attached is a photo I snapped at the Vintage Races at Lime Rock this past fall. I think it's Prescott Kelley's 911ST. It clearly shows a single MSD driving two coils.



I would guess that the only potential issue with a single MSD might be a drop in driving power (primary current) to the coils. Anyone else have any experience with this?

Randy...Would you mind posting a description of your wiring. Is it simply a matter of splicing the second set of wire into the MSD unit?g i

[Steve@Rennsport]

Randy:

Two coils place much larger current load on the MSD unit and it runs too hot. Further, neither coil gets the full 450-480 VDC primary voltage so ignition performance is compromised.

This keeps you from running the wide plug gaps that give more HP and a better idle, too .

Bottom line, it becomes a durability issue.

[tom1394racing]

Steve

That makes sense to me.

Thanks

[randywebb]

Thx, Steve.

"experts (Jaime, Randy and Steve)"
- I am going to defer to Steve (and Jamie).

Yes - that's what I did. Just pigtailed them off of each other. I was told to do this by a very well known Porsche race engine builder in Portland. It's not Steve (obviously). He's an elderly guy, so that might enter into it.

After I get the car out of the paint shop (if I ever get the car out of the paint shop), I will be paying Steve a visit and getting his group to take a look over the entire engine setup. This may put off my torsion bar buying for some time...

I only have a few hundred miles on this engine, so I'm not too worried at this point.

[JP911]

I see 2 MSD boxes in that pic. The second one can just barely be seen behind the left side of intakes.

[Early_S_Man]

The most likely component to fail with excess load current is the SCR ...

Good eyes, JP!!!

[steand]

If you consider the way a CDI ignition works electronically, you might end up with an answer to the question.

Having an education in electronic engineering, I'll explain.

The CDI works by Charging a capacitor to approx. 350V and then fire that charge into the coil. The coil acts as a transformer stepping up the 350V to approx. 30-40kV to the spark plug.

All off the energy in the capacitor is delivered to the coil every time a firing event occurs. The capacitor size is designed to hold a specific amount of charge energi to one spark.

If you connect 2 exactly similar coils to the CDI box, they have to share the energy in the capacitor, and they will both deliver a weaker (shorter duration) spark than with only one coil.

If you have an CDI unit that normally delivers 80 milli Joule energy in one spark, you end up with 2 sparks with only 40 mJ energy each. (not accounting for losses).

If the coils are not the same you end up with 2 different sparks.

So electrically there's no hinder to hook up 2 coils to an CDI unit; but the spark quality from the coils will suffer.

Some CDI unit's (i think it was www.mwignitions.com, not affiliated) have a setting for energy in the spark, so you can get say 150mJ energy instead of the normal 80mJ. If you have such a unit you might split to 2 coils with 75mJ spark.

The reason that i think it will work with the MSD6 is that the capacitor holds (½*C*V*V) = 61mJ and divided by 2 coils it's approx. 30mJ each, witch is enough in most applications to get a decent spark. Most "old school 12V coils" only delivers 20mJ or less and engines still run on them.

Maybe in an Twin ignition setup one could argue that the full 63mJ energy is delivered into one cylinder at each spark event, but divided into 2 sparks. Almost like if you got 2 injectors per Cylinder delivering half the fuel.

[tom1394racing]

Steen's reply would indicate that the durability of the MSD would not be compromised by driving two coils but that the spark energy from each coil would be lower than if I were to run two MSD's. Based on that, I think I'll try the single MSD option and see how the car runs.

BTW I haven't ignore the requests for photos of my twin plug dizzy. I'll post some detailed photos sometime this coming week under this post.

One again....You guys are amazing with your collective knowledge on all subjects relating to our 911 cars.

[Lorenfb]

As mentioned above, when two coils are used with a single CDI, all the spark
energy is divided between the two coils. This will probably result in insuffficient
energy to one of the plugs. The key factor, though, depends on how the coils
are connected to the CDI.

If the coils are connected in parallel, then the max voltage appears across both
coils but the energy is theoretically divided, but in practicality less. If the coils are placed
is series, the voltage to each coil is half the peak CDI output. The other factor is the
problem of spark rise time which is affected, most significantly when the coils are in series.

The key problem for the parallel coil connection is that the peak current for the CDI SCR &
capacitor is now 1.4 times the single coil connection. Remember, C X V X V = L X I X I
where, I is the peak SCR & capacitor current. This excessive current may shorten
the life of the CDI, especially the CDI capacitor. Also, the spark duration is now shorter
by .707 (30% less).

Porsche/Bosch were aware of the above and obviously chose to use two complete
CDI systems when dual plugs were needed. Using two coils with a single CDI results
in a overall marginal ignition system.

[randywebb]

tom - it will run - mine does and seems good. I've never compared it to one with twin msd's. Steen's comments are qualitative - e.g. for all we know the box has plenty of reserve capacity to drive 2 coils -- or 7. But...

Note again what Steve W. said above - he/Gamroth have extensive real-world _data_ (on most things Porsche) from racing at some pretty high levels, as well as more ordinary customers (and some not so ordinary). So, beyond the lower spark energy associated with the smaller plugs gaps he noted, his comments re duralbility should be observed - at least I plan to do that. To the extent that it is heat related, the ambient temperature your msd operates in shold be a factor.

Then there is the parallel factory design considerations and pracitce that Loren noted. Add it all up and divide by the cost of replacing burned out units - also include in the denominator how much fun it would be to get stranded in the boonies on a nice drive - say as a hail or rain storm is coming up with no cell service and aybe no other cars for some hours or days (depends on where you drive...).

And BTW, re my expert status - I'm not a mechanic, racer, or engineer. I'm a scientist (who used to do energetics and heat (cold) studies on animals, and now studies extinction phenomena to help endangered species). So I don't really have any expertise besides the sort of fundamental understanding that scientists have of reality - and being a 911 owner. [My lawyer education comes in handy for endangered species, and a bit of overlap into contracts, policing, traffic tickets, yadayada but is not relevant here).

So I just wanted to correct that appellation since it came up above.

[RPMClassic]

One benefit that I see to having twin plug - is having a complete " back up" system should one CD ignition fail during competition. Lets face it, MSD's are cheap $ compared to other porsche things we all spend our money on. With 2 coils, 2 CD's and the right wiring - you have one independent system for the "TOP" plugs and one for the "bottom". This way no matter what happens - I know that I will always be able to finish the race or get home on a weekend drive. On my 2.5 liter - running wider plug gaps made a " big" improvement.

Good luck Tom - the car looks awesome!

[klatinn]

Hi,

One solution is to have the MSD drive ONE coil with two HT outputs. Just use a coil designed for a waste-spark system. Then feed the two dizzys with the two outputs. This way the spark energy is balanced always and you don't run into problems with energy distribution between coils.

Regards,
Klaus