MSD for Twin Plug

[randywebb]

How does that overcome the lack of energy alluded to earlier?

[RLJ]

That same old guy built my twin plug system and he used two boxes.





See the two boxes stacked on the left side and the two coils.



Randy Jones
1971 911 "Iris"

[tom1394racing]

All very interesting. I'll add another tidbit.

A while back I asked Jim Newton (Auto Associates) about the function of the push/pull switch just above the ignition switch on a real factory '73 RSR he had at his shop. He had restored it a number of years ago and won the Manhattan trophy at the Parade so it was really original. The switch I am talking about is used for the rear wiper on production cars of that vintage however the RSR did not have a rear wiper. The switch can be seen in this photo I took while the RSR was at his shop.



Jim told me that the switch on the factory RSR's was used to switch from one ignition system to the second in case of an ignition failure during a race. This would lead one to believe that the factory wired the second CDI as a back-up and that the original design (at least on the '73 RSR) had one CDI driving two coils. It would also indicate that the factory realized that durability was a problem with this set-up.

I'll check this with Jim next week and let you know his thoughts on this.

[Lorenfb]

"How does that overcome the lack of energy alluded to earlier?" - randywebb -

It doesn't. The total energy is now divided between the two spark plugs,
i.e. the total CDI energy is transferred to the single coil but the coil's output
now drives two plugs. Mercedes and others use an inductive discharge ignition
system where a single coil drives two cylinders, where one cylinder is on the
compression stroke & other is on the exhaust stroke. Some motorcycles use
the method also.

[klatinn]

Hi,

Loren's right, it does not. But the coil energy is divided equally between the plugs, not dependent on tight tolerance requirements between the coils. Remember, when one spark fires, the current path over the plug is (as far as the coil is concerned) almost a short, it's impedance transformed back to the primary side. Current goes the path of lowest resistance. So with 2 coils fired by the same box, it's very likely that only one plug actually fires under compression. With the waste-spark setup the spark gaps are in series. So any spark current flowing through one gap MUST also flow through the second gap. They will ALWAYS fire both, albeit with the total energy divided between them.

Regards,
Klaus

[randywebb]

That makes sense.

Let's suppose I thought I might set up the single box & 2 coils as a waste-spark system temporarily... at least for the rest of this year. What exactly does one do? How much labor is involved and what components (and their cost)? Is it worthwhile as a temporary expedient?

Do you just plug in a single coil with dual outputs in place of the 2 coils? And, the msd box 'likes' its own coil and doesn't 'like' say Bosch coils -- does msd make such a coil?

FWIW, RLJ's engine was done (or is it done the first time?) a year or two before mine... or maybe I misheard (doubtful) - anyway...

[klatinn]

Hi,

Do you just plug in a single coil with dual outputs in place of the 2 coils?

Yes.

And yes, MSD makes such coils. They are not only used on motorcycles and Mercedes, but many distributorless modern cars use waste-spark systems (Supra's for example). If you look at MSD's web-site, I'm sure you'll find some.

Regards,
Klaus

[randywebb]

Actually they have two but neither will work with the 6-AL boxes - they say they work with DIS ignitions only.

[Steve@Rennsport]

I spoke with that "old guy" today and he confirmed one coil for each CD box.

He was originally referring to the fact that the 3.6 dizzy can trigger both CD boxes, but each box (MSD-5, MSD-6, MSD-7 and OEM Bosch) needs its own coil. Newer DIS-series MSD's can use coilpacks for wasted spark ignitions with the right coil drivers.

[tom1394racing]

I spoke with Jim Newton today. Auto Associates have been doing twin plug single MSD applications for over 4 years now with no problems. Jim recommends .028" plug gaps.

He also informed me that the original factory set-up was two CDI's driving two coils. The switch I referred to was for the switching the connection to the tach.

[DW SD]

Tom,
let us know how it runs, when you have everything going. That should clear up the debate. Interesting thread!

Doug

[Steve@Rennsport]

Originally posted by tom1394racing
I spoke with Jim Newton today. Auto Associates have been doing twin plug single MSD applications for over 4 years now with no problems. Jim recommends .028" plug gaps.

He also informed me that the original factory set-up was two CDI's driving two coils. The switch I referred to was for the switching the connection to the tach.

Tom:

You can certainly use .028 plug gaps, but you will not be taking advantage of one of the main benefits of MSD's and you leave some power and drivability on the table,....

MSD's reliably fire plugs with no side electrodes at all,....LOL. Someday, I'll tell you a story about that.

Open those gaps to .040 and your car will run much better.

[tom1394racing]

Steve

I plan to run initially with the single MSD and the tighter plug gap. You can bet as I learn the car, I'll be trying the larger gap as well as many other tuning concepts.

Right now my short term goal is to get the engine in the car and get in running.

Thanks again for your expert feedback.

[randywebb]

Thx Steve - BTW, I got the 2 coils & 1 box from him...

Right now _my_ short term goal is to get the paint on the car and get to driving it.... but like I said, I'll be up North to the big city at some point not too far in the future...

[Steve@Rennsport]

Hi Randy:

I'd agree, its far more important at this moment to get the paintwork done and have it back on the road,...)

BTW, you can find the best buys on MSD's at either Jegs or Summit Racing. That will give you the opportunity to get that additional MSD 6-AL for later,........

[tom1394racing]

Here's some pics (as promised) of my twin plug dizzy as modified by Jerry Woods. The base was the original unit from my '78 SC core motor. The unit came back from Jerry with the vacuum advance mechanism still attached. I removed it and installed a cover plate over the opening.

[Nitrometano]

Really cool!!!

[DW SD]

Tom,
Any updates on your progress?
Hope things are going well for you and your project!

Doug

[randywebb]

BTW - what are U and L?

[Steve@Rennsport]

Randy:

Upper & Lower,....