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Twin plug msd question.

How would I ever know if one of my coils or MSD units were to stop working?
I have the 2 MSD units pigtailed to one of the pick ups on the distributor. The 2 coils of course run individually to their respective positions on the cap.

I noticed that if I unplug one of the MSD plug ins the car still idles. If I unplug the other it still idles, just a little rougher. Of course I put the first one back on first.

I was just wondering, how would I ever know, while driving, if one of my units failed?
Theoretically, I could be driving around for hundreds of miles on 6 plugs, detonating all over the place before I realized it. If I ever realized it.

I haven't tried to drive it on 6 plugs, and don't plan on experimenting. But just wondering if it would be noticeable or not.

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Old 04-07-2013, 05:01 PM
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You could have 2 switches on the dash that would allow you to turn one or both of them off - kind of like what Porsche did with the first RSRs.

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Old 04-07-2013, 05:04 PM
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One of the preflight tests on a Cessna is to swith each mag off to ensure the other is working. Sounds like maybe you are running on 5 when you switch off one msd off?
Old 04-07-2013, 05:25 PM
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Its actually just the opposite, if you loose one coil you will get a slower burn and since you already reduce the timing on a twin plugged engine setup it pretty much defaults to a safe mode.

And on our older planes we usually start it on only one mag, the one that is retarded, so it starts easier.
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Last edited by cgarr; 04-07-2013 at 05:53 PM..
Old 04-07-2013, 05:46 PM
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There is another recent thread on this so search around. Summary:

Part of the debate is two boxes two coils or one box two coils. MSD says either will work. Some engine builders swear by 2+2 (and claim more power) and others say one box and two coils is just fine. Jerry Woods does the latter, and he has plenty of time in Porsche engine dyno rooms.

I think what makes sense depends on some variables.

Jerry did explain how the factory switch deal works, but sadly I cannot remember. I do not remember it being goof proof.

MSD says you can trigger the two boxes from one pickup. So say it is magnetic, you can drive two boxes this way. Now you have two tach signals and you can rig a switch to your tach and switch back and forth from time to time. Uhm... okay.

Or you can have manual switched power to each box and turn them off one at a time, from time to time, for example in the paddock.

My engine builder drives one box from the distributor, and uses its tach output to drive the input of the second box. Then box #2's tach signal goes to the tach. Daisy chain. He has done this for years in many cars with many different configurations.

But MSD tech support says this won't work. Yeah right.

As an alternative, you can drive both boxes from one mag pickup. One tach output can go to the tach. The output on tach #2 could go to a simple $40 adjustable rpm switch. Set that switch for say 500 or 1000 rpm. Then connect a dash warning light which has a ground when the rpm switch's normally closed contacts are closed. So the light is on below that set point.

Then above that 500 or 1000 rpm set point, those normally closed contacts open up and the light goes out.

So you are driving along and the tach is working fine. You are above that 500 or 1000 rpm set point as shown on the tach (and via common sense). But the dash warning light is on. That means the second box's rpm signal is compromised, and does not trigger the NC contacts to open.

Another approach would involve rigging something up (a spark checking device or CHT gauge or ??) that could help detect lack or compromise of spark on a cylinder. Again, this is limited because unless you pick all cylinders... you get the idea.

Of course these schemes assume that the failure mode you are trying to detect causes a loss or compromise of rpm signal. Either box could have a different failure mode.

The single box two coil approach makes all of this unnecessary.

I decided on 2+2 with manual switches for checking in the paddock. I have an extra rpm switch and a spare dash warning light, so I might rig this to the tach output of box#2 as I have described. Easy and cannot hurt. But not a panacea.
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Last edited by Mahler9th; 04-07-2013 at 07:19 PM..
Old 04-07-2013, 07:17 PM
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Quote:
Originally Posted by Mahler9th View Post
There is another recent thread on this so search around. Summary:

Part of the debate is two boxes two coils or one box two coils. MSD says either will work. Some engine builders swear by 2+2 (and claim more power) and others say one box and two coils is just fine. Jerry Woods does the latter, and he has plenty of time in Porsche engine dyno rooms.

I think what makes sense depends on some variables.

Jerry did explain how the factory switch deal works, but sadly I cannot remember. I do not remember it being goof proof.

MSD says you can trigger the two boxes from one pickup. So say it is magnetic, you can drive two boxes this way. Now you have two tach signals and you can rig a switch to your tach and switch back and forth from time to time. Uhm... okay.

Or you can have manual switched power to each box and turn them off one at a time, from time to time, for example in the paddock.

My engine builder drives one box from the distributor, and uses its tach output to drive the input of the second box. Then box #2's tach signal goes to the tach. Daisy chain. He has done this for years in many cars with many different configurations.

But MSD tech support says this won't work. Yeah right.

As an alternative, you can drive both boxes from one mag pickup. One tach output can go to the tach. The output on tach #2 could go to a simple $40 adjustable rpm switch. Set that switch for say 500 or 1000 rpm. Then connect a dash warning light which has a ground when the rpm switch's normally closed contacts are closed. So the light is on below that set point.

Then above that 500 or 1000 rpm set point, those normally closed contacts open up and the light goes out.

So you are driving along and the tach is working fine. You are above that 500 or 1000 rpm set point as shown on the tach (and via common sense). But the dash warning light is on. That means the second box's rpm signal is compromised, and does not trigger the NC contacts to open.

Another approach would involve rigging something up (a spark checking device or CHT gauge or ??) that could help detect lack or compromise of spark on a cylinder. Again, this is limited because unless you pick all cylinders... you get the idea.

Of course these schemes assume that the failure mode you are trying to detect causes a loss or compromise of rpm signal. Either box could have a different failure mode.

The single box two coil approach makes all of this unnecessary.


Doesn't work, the first plug that fires "steals" the "juice" from the second coil.


I decided on 2+2 with manual switches for checking in the paddock. I have an extra rpm switch and a spare dash warning light, so I might rig this to the tach output of box#2 as I have described. Easy and cannot hurt. But not a panacea.
...
Old 04-08-2013, 09:56 AM
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I am not sure about the meaning of that statement. How much spark energy you have is one matter. How much spark energy you need is another matter.

I know and am aware of some serious engine builders that use the single box approach with pretty high budget builds where the customers want as much bhp as they can get.

And one other thing:

I tried to get MSD to indicate whether many or most failures that occur in 6A and 6Al boxes manifest in the tach output signal (like no signal). They wouldn't say.
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Old 04-08-2013, 10:39 AM
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Thanks for the responses. Cgarr has the one I like the best, it requires me to do nothing. Ha,ha.
But Andrew and Mahler, those make sense too.
I already have a kill switch hooked up and IIRC there is a wire on the MSD's that is used for a kill switch, I just connected those wires together and ran them to a single wire and ran that to my kill switch. It grounds out both units at the same time.

I wonder if I separated the wires and ran each of them to individual/side by side kill switches. If that would accomplish what you guys are saying works? Maybe? Then i could just click one off at a time and make sure everything is working.

Or do you think it is unnecessary to worry about these things and like Cgarr says, I would notice a performance loss and recognize something was wrong, before I drove around too long and did some damage.
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Old 04-08-2013, 06:45 PM
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did you find that older thread with Steve W.s comments?

you have 11:1 CR?
Old 04-08-2013, 07:29 PM
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Yes, I had it saved. But got talked into 11.1 instead. Not sure what the exact ratio is, but I'm happy with it.
Quote:
did you find that older thread with Steve W.s comments?



you have 11:1 CR?
Old 04-09-2013, 09:18 AM
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Cgarr is absolutely right. If you have retarded the timing properly (which is the whole idea with the twin plug setup), about all you are going to notice is a slight decrease in performance. It won't detonate. What it might do is start to overheat a bit, as any motor with retarded timing will. That might be your que that something has gone awry. It certainly won't hurt it, though.
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Old 04-09-2013, 02:11 PM
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Good, thanks. I always keep my eye on the temp. It rarely breaks 200/210.
Old 04-09-2013, 03:42 PM
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Quote:
Originally Posted by cgarr View Post
Its actually just the opposite, if you loose one coil you will get a slower burn and since you already reduce the timing on a twin plugged engine setup it pretty much defaults to a safe mode.

And on our older planes we usually start it on only one mag, the one that is retarded, so it starts easier.
Arent the mags timed together? I seem to remember that the c90 maybe had 2 degrees difference? Which probably means that in normal operation the fire gets lit from a single plug.
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Old 04-09-2013, 07:16 PM
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Old 04-09-2013, 07:21 PM
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your forgetting the impulse coupler or like a bendex, there is a separate start circuit energized when cranking the engine, but we are getting way off track

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Old 04-09-2013, 09:06 PM
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