MSD for Twin Plug

chris_seven · 04-03-2012, 05:23 AM

I seem to remember that the User Instructions for the MSD 6AL had a diagram showing how to connect 2 x Blaster Coils. I am fairly sure that the two coils were connected in parallel.

I have checked on line and the instructions for the 6420 unit don't seem to contain this schematic.

As the 6420 is no longer being supplied I wondered if the replacement 6425 could still fire 2 x coils and if so would they still be connected in the same manner.

The type of coil is also of interest as I have had problems with Blaster Coils failing mechanically and would be keen to know what primary impedance of coil will work.

Any ideas please.

lindy 911 · lindy 911's Garage

You probably already know this but when you use two coils and one MSD box you're only getting half the spark voltage. It negates any benefit of using the MSD (high spark voltage) when you use two coils in parallel.

tom1394racing · 04-03-2012, 05:51 AM

Quote:

Originally Posted by lindy 911

You probably already know this but when you use two coils and one MSD box you're only getting half the spark voltage. It negates any benefit of using the MSD (high spark voltage) when you use two coils in parallel.

I am not sure the story is that simple.

I have been running one MSD and two coils wired in parallel for nearly 4 years on my RSR clone.

MSD for Twin Plug

Henry Schmidt · Henry Schmidt's Garage

Hi Cris
We've been running race engines with one MSD at a time (two installed, one for back up) for 15 years.
I have never seen an issue with this set up.
There is a safety feature in having one CD drive two coils in that when the CD fails you know its gone and with two separate CDs you can engine up running your engine seriously retarded because only one plug is firing.
On low compression engines it's not so bad but with a high compression engine you can cause serious damage.
When your racing finishing is all that matters but with these high strung street engines and amateur racing perhaps saving the engine has value.

lindy 911 · lindy 911's Garage

MBruns shows in the link Tom provided that a 230 hp engine losses 15 hp changing from two MSD boxes to one. I also ran a single box but moved to two; noticeable difference on top.

Mahler9th · 07-06-2012, 11:51 AM

I have been doing some research on this. Here is what I conclude thus far:

1. The "safety" benefit of running a single MSD 6AL and two coils for a twin plug application is obvious in that if there are certain types of MSD failures, there will be symptoms and you can respond accordingly. Many folks out there run this set up. It is obviously less expensive as well.

2. When running two MSD boxes driving two coils, the typical application is to drive both boxes from the same distributor trigger output.

3. It is possible in some of their race cars that Porsche decided to have an ability to switch between two CD box tach outputs with a dash mounted switch. In theory, if one signal was dead, then maybe the engine was only firing off of one CD box, and folks could respond accordingly. I can verify that with a visit to a local shop that does many historic car restorations.

4. In theory you could wire twin MSD box tach outputs to a switch, and emulate what the factory did in #3. If you suspected a problem, you could check to see if one of the tach signals was dead or compromised.

Mahler9th · 12:04 PM

5. Another approach to explore with two 6AL units might be to wire the distributor trigger to the first box. Then use its tach output signal (a 0 and +12 volt square wave) to the "points" input of the second box.

If this worked, then any problems with the first box that manifested in its tach output signal would also theoretically be manifest in the second box's output. And theoretically this would give you a heads up. If you wired the second box's tach output to the tach, then you'd be able to "see" any of problems with its performance that manifested in its tach output.

So if this worked, you'd have some indication of problems with either box, as long as the problems were manifest in the bad box's tach out signals.

MSD claim that the "points" input circuitry cannot be driven by the tach output square wave from another MSD box. They tell me that the "points" input circuits expect to see a signal that switches between ground and some voltage (like from ignition points in a distributor), while the MSD tach output is square between 0 volts and +12 volts. I got that from two different MSD support techs, but I am curious anyway. And of course you could probably "condition/change" an MSD tach out square wave signal into something that the "points" input circuitry could see properly. Basic electronics problem likely solvable.

Mahler9th · 11:53 AM

Finally...

6. You could run both boxes from a single trigger. Then run both tach outputs to rpm switches with normally-closed circuits, each connected to warning lights in your dash. Then set the RPM switches at some low value, like 2k rpm.

In normal operation above 2k rpm, these switches would be open, and the warning lights would be off.

If one of the tach outputs was dead above the 2k rpm set point, the NC contact would stay closed, and the dash warning light would light up. This would only work for an MSD box problem that manifested as a dead tach output. So maybe not all possible issues, but perhaps some could be caught with this approach. And of course this would work as long as the rpm switch was in good working order.

Mahler9th · 07-06-2012, 12:27 PM

Oops, sorry:

2. a. Running two MSD boxes and two coils is superior to a single box/dual coil set up from a performance standpoint. Many Porsche engines are set up that way. I haven't read about any engines damaged as a result of an undetected MSD box failure in a dual box/dual coil set up, despite apparent theoretical possibilities. Perhaps other folks out there have experience with this.

Mahler9th · 07-06-2012, 12:42 PM

One more "oops..."

5. There might be a small time delay if the first box tach signal was fed to the input of the second box. This could be significant for dual plug firing.

James Brown · James Brown's Garage

you don't need redundant systems like in piston aircraft, one will drive both coils as stated. you could run a second as a backup with a switch but not really required, keep a spare in the glove box. That schematic Henry posted is from MSD.

Mahler9th · 07-06-2012, 05:39 PM

I am fully aware of what can be done for some applications.

My application is racing, so all of the fuss is not about redundancy. It is about max performance. So two boxes and two coils. I think the benefits have been proven, depending on racing engine configuration.

Secondarily, I must consider ways that I can know whilst driving my race car that one of the boxes is not functioning properly. Hence all of the stuff I wrote.

I have found a technical report with a full set of schematics for the inside of a MSD 6A and I will study the topology carefully.

BURN-BROS · 07-07-2012, 06:14 AM

If I am not mistaken, MSD has phased out the analog box and has moved on to the digital 6AL. The new box may not be able to run in that configuration.

Has anyone tried running one digital box with two coils?

Mahler9th · 07-07-2012, 07:59 AM

I have a friend running dual digital 6ALs in a twin plug application. But I don't know of anyone running a single digital box and two coils. I am sure MSD can tell you whether it would work. Likely it would.

I could see running one box and two coils for a street car to save money. But I am not sure about the magnitude of the safety benefit. A single box/dual coil set up may not help you if there is a problem with one of the coils. Or the distributor. Right?

I started with my current car/engine with twin plug heads but running a single coil and box back 10 years ago. I have always carried a spare box. And I have used it for troubleshooting at the track. The first time, it turned out that my Blaster 2 coil failed. Upside down mounting is not ideal for these oil-filled coils.

Another time, it turned out that I had a problem in my 3 liter distributor. Jerry Woods rebuilt it and it is now for sale.

Ideally one could have a "warning system" that would identify a problem with either box, either coil or the distributor, before any potential for damage could occur.

Steve@Rennsport · 07-07-2012, 11:21 AM

Quote:

Originally Posted by Mahler9th

The first time, it turned out that my Blaster 2 coil failed. Upside down mounting is not ideal for these oil-filled coils.

Very true and that's why MSD offered the "High-vibration" version which can be safely mounted in any direction. Another good option are the "E-core" coils which are much more reliable and can be mounted anyway you wish. The HVHC version works nicely with the 6AL's.

Mahler9th · 07-07-2012, 11:56 AM

Well I have not had a Blaster 2 failure since I started mounting them right. But I carry spares anyway as do my friends.

Again, what would really be cool is a way to have a warning light if the top or bottom plugs have a firing problem. Need to think about this more.

afterburn 549 · 07-08-2012, 06:52 AM

dumb question- How are you directing spark with 2 coils?... 2 dizzys ?

Henry Schmidt · Henry Schmidt's Garage

Twin plug w/one MSD 6AL
2.45 with mod "S" cams and 9.5:1 compression 40 mm Webers stock exhaust.
180 RWHP @ 6400. Plenty more on tap just going easy on the new engine.

Same engine design different car, same results 180 @ 6400. single 6AL

winders · 07-08-2012, 12:07 PM

Quote:

Originally Posted by Henry Schmidt

Twin plug w/one MSD 6AL
2.45 with mod "S" cams and 9.5:1 compression 40 mm Webers stock exhaust.
180 RWHP @ 6400. Plenty more on tap just going easy on the new engine.

Same engine design different car, same results 180 @ 6400. single 6AL

Did you ever try the engine with two MSD units?

Scott

Mahler9th · 07-08-2012, 01:36 PM

Very nice and very tidy looking engine. I am sure it performs well.

I don't think that there is any further question about whether one box/dual coils will work, nor whether you can get more performance from a dual box/dual coil set up. Both are true I am certain.

Sure, the former gives you some safety margin for certain types of failures in certain circumstances. But I have designed a simple scheme for the latter to be wired to give you a similar safety margin... and you get the increased performance.

In my application, a 3.8 liter twin-plugged racing engine, the dual box/dual coil deal is clearly the way to go. I may implement the safety scheme which is pretty easy in my case. But I am not sure it is necessary. In any case, neither approach helps you if one coil goes bad, or if you are running a belt-driven dual distributor and have a belt failure.