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Twin plug pickup question.

Is there any advantage or disadvantage, to running both both msd boxes into a single pickup on the Jb 12 plug racing distributor?
There are two connections, for plugging in.
Currently I have them pigtailed into one of the pickups. The thinking is that if one pickup fails, you switch to the other.
I was looking at it the other day, and was wondering if both would be better, or is a single pickup better.
Or does it even matter.

Thanks in advance.


Last edited by dipso; 07-13-2019 at 07:46 PM..
Old 07-13-2019, 07:08 PM
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According to JB Racing the answer is no, there are no advantages to using both pickups. To simplify wiring in my race car, I use one pickup and split them at the MSD units.
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Old 07-13-2019, 10:14 PM
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That's how I run twin plug pickups as well. single trigger point split to each ignition.

For a race car I would consider to parallel the pickups through a switch network so that you could choose A or B channels for redundancy. Though I don't think I have ever seen a magnetic pickup go bad in normal use.
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Old 07-14-2019, 05:56 AM
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Why do you think the distributor has two connections coming out of it, if you are only supposed to use one?
If magnetic pickups rarely fail, it seems kind of silly to have it there as just a spare.

Mines been plugged into one for 7 years. No issues.
But now I'm on a mission.

Are they both timed the same?
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Old 07-14-2019, 06:16 AM
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What got me wondering is this.
I just got my car out of the shop. Transmission, clutch and axle work.
Car runs and sounds great.

My mechanic said something like "i think it is plugged into the correct one' as he pointed to the connections, since there is always an unused one dangling.
Then he said something like ' If its not right, don't adjust the timing, just switch plugs"

I thought that was a weird thing to say. I always thought they were the same.

Does that make any sense?
I asked him about using both connections and he said I should contact JB since he didn't know exactly.
But JB say using one is fine.

Why is there two?
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Old 07-14-2019, 06:27 AM
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The reason for two triggers in the JB distributors is redundancy, the problem we have seen in using both pickups and feeding 2 boxes is the timing event is close but not exact and if you have a problem with a trigger or box it will continue to run on half of the plugs and may do damage if it is unnoticed, especially if it is running on the lower set. I would test both triggers with a timing light and use the one with the tic more if there is a difference and if you need to swap you will be going safer until you can reset it. In NASCAR they have in car switching to change over the entire systems from A to B on the fly if they suspect a problem, Hope this helps
Mike Bruns
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Old 07-14-2019, 06:27 AM
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I think I understand.
I should hook up the timing light and check to see if there is a timing difference between the two connections. Unplugging the actual connection into putting it into the other one.
Then if there is a difference, use the one that is higher, more advanced.

Thanks.
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Old 07-14-2019, 06:42 AM
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Correct, check them both and see if there is any timing difference, pick the one with the higher value and set your desired total timing and if you need to test or swap to the other pickup at some point you will be safe, if you get a chance on a chassis Dyno watch your timing with a light all the way through the pull, we have seen the timing retard from a little to quite a bit at full RPM, it’s worth some power if that’s the case
Mike Bruns
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Old 07-14-2019, 07:00 AM
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I have a question, Mike. And I really don't understand how the whole system works, so bear with me.
If the MSD system fires multiple sparks at low RPMs and then converts to single spark at higher RPMs, because there is not enough time for multiple sparks.
And, if there is a slight difference between the timing on the the pickups.
Wouldn't I be getting multiple sparks at higher RPMs, if I used both. If there is difference and I know there is not supposed to be.

I'm figuring 5000RPM, MSD is single sparking on one pickup, 24 degrees BTDC.
If I am plugged into both pickups and there is a difference. First plug fires at 24 from the first pickup, second plug fires at 25 from the second pickup.
Wouldn't that be better?
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Old 07-14-2019, 07:44 AM
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^. Apart from redundancy and phasing issues already mentioned , occasionally we see a distortion in the signal when two boxes are used on one sensor. This can cause the signal to cross the zero point twice causing an extra ‘rogue’ spark and a misfire. This distortion typically happens around 5500 RPM. This happens because you have effectively halved the resistance of the CDI input circuit by putting two boxes in parallel.


For interest only:

The MSD spark interval is 1ms (1000 is). Even if the MSD could spark at 5000 RPM the second spark would occur 30 degrees after the first. Let’s say you have 24 degrees BTDC at 5000, the second spark would be at 6 deg ATDC. Pretty much useless for extra power. One of the limitations is the MSD coil which takes around 700us to ‘recover’ from the first spark event.

The original Bosch black coils by comparison are very quick to recover, around 250us. Because of this, when we designed our CDI+ system, we allow the user to specify the spark interval and we have no limitation on multi spark RPM. The coil recovery time is dependent on cylinder pressure. High boost Turbo motors need around 400us but this is still 2.5 times faster than MSD.
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Old 07-14-2019, 08:29 AM
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We did a lot of testing at JB with the MSD and timing, a lot of it changes with piston dome shape and bore size etc.., we cut windows in a distributor cap and watched the event happening and what the spark width at the rotor tip/cap contact when it changed from multi to single spark at around 5k, the intent of multi was to help keep the flame going when chamber design is poor and mixtures we’re not perfect, and at hi RPM there’s not enough time to keep it multi and probably not needed. As far as staggering the spark top to bottom I have not tested that but seems to me once the flame starts the second a bit later won’t do much for improving flame travel, depending on how big your engine is and how much dome you have you really need both happening at the same time. Are you using race fuel ?
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Old 07-14-2019, 08:32 AM
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I have a 2.7. 11 to 1 JE pistons.
Pump gas. 91 octane.
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Old 07-14-2019, 08:47 AM
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Are you positive on the CR? as in poured all the volumes and figured it out
Mike
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Old 07-14-2019, 08:55 AM
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Quote:
Originally Posted by MBruns View Post
Are you positive on the CR? as in poured all the volumes and figured it out
Mike
No. But when I had it built he said 11/1. I asked for 10.5 but he said with the right cams and all the other stuff he did. It will work.
He’s done it before.
It’s ran great for 7 years, still does.

I know. 11/1 sounds crazy.
Old 07-14-2019, 09:37 AM
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Jonny, you think two connections are better because of the ‘rogue’ spark?

Mike, you think one is better for safety?

It seems that most people are using one. I am currently.
Does anybody use two?

I think I used two in the beginning. I can’t remember why I switched.
Old 07-14-2019, 11:16 AM
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Quote:
Originally Posted by dipso View Post
Jonny, you think two connections are better because of the ‘rogue’ spark?

Mike, you think one is better for safety?

It seems that most people are using one. I am currently.
Does anybody use two?

I think I used two in the beginning. I can’t remember why I switched.
I would also advise using a single sensor as it reduces the chance of them firing at different times (as already discussed).

All I'm saying is that I have seen an issue with running two boxes off a single sensor so just be aware of it if you get odd behaviour. It will depend on the sensor type and the type of CDI box too.

We've done a few builds using the JB Racing dizzy off a single sensor with no issues. Last one we worked on was this:

https://www.youtube.com/watch?v=UtnrLj1pVF4&feature=youtu.be&fbclid=IwAR1X6oBp1wObnSTDj-WJq9o73ROe1MAPUvsLDa7zW6-mX78R6QKnhx0jE14

Mike, do you know Neil Bainbridge? He does a lot of work with the JB guys. It's his dyno in the video.
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Old 07-14-2019, 01:28 PM
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Well, it seems the experts agree.
Single sensor it is.

Ive got 15,000 miles on the rebuild and about 2 miles on the transmission rebuild plus ‘while your in there’ stuff.
New axles, boots, clutch disc, pressure plate, all the stuff that goes with it, new flywheel.

The car screams now. I was just wondering about the comment my mechanic said.

Thanks.
Old 07-14-2019, 01:48 PM
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Quote:
Originally Posted by Jonny H View Post
I would also advise using a single sensor as it reduces the chance of them firing at different times (as already discussed).

All I'm saying is that I have seen an issue with running two boxes off a single sensor so just be aware of it if you get odd behaviour. It will depend on the sensor type and the type of CDI box too.

We've done a few builds using the JB Racing dizzy off a single sensor with no issues. Last one we worked on was this:

https://www.youtube.com/watch?v=UtnrLj1pVF4&feature=youtu.be&fbclid=IwAR1X6oBp1wObnSTDj-WJq9o73ROe1MAPUvsLDa7zW6-mX78R6QKnhx0jE14

Mike, do you know Neil Bainbridge? He does a lot of work with the JB guys. It's his dyno in the video.
I watched the video, pretty cool.
I forgot I was streaming music through my phone and the engine roared throughout my house speakers.
Scared the hell out of my dog.
Old 07-14-2019, 06:25 PM
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Neil

Quote:
Originally Posted by Jonny H View Post
I would also advise using a single sensor as it reduces the chance of them firing at different times (as already discussed).

All I'm saying is that I have seen an issue with running two boxes off a single sensor so just be aware of it if you get odd behaviour. It will depend on the sensor type and the type of CDI box too.

We've done a few builds using the JB Racing dizzy off a single sensor with no issues. Last one we worked on was this:

https://www.youtube.com/watch?v=UtnrLj1pVF4&feature=youtu.be&fbclid=IwAR1X6oBp1wObnSTDj-WJq9o73ROe1MAPUvsLDa7zW6-mX78R6QKnhx0jE14

Mike, do you know Neil Bainbridge? He does a lot of work with the JB guys. It's his dyno in the video.
Jonny, I met Neil many years back when I was at JB Racing, he had some cool projects
Mike Bruns
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Old 07-15-2019, 03:52 AM
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Just an update. I was having a little trouble cold starting the car so I took off the pigtail and plugged both MSD boxes into both pickups. Just to see what would happen.
The car started up, no better than before, but it seems to run really smooth.

Took it out for a while, warmed it up, topped off the oil, it was a little low.
Adjusted the fuel pressure to 3.5, it was a little high. Closer to 3.75.
Checked the timing, it was a little high. Its really hard to tell exact timing numbers due to the serpentine pulley.
But it looked like it was 16/ 28.

I reset the timing to 14/26, using the top plug, and then checked the difference between the timing on the top and bottom #1 plugs, since I am using both pickups.

It looks like it is 13 on the bottom and 14 on the top.

I would say I'm now firing at 13 and 14 at idle, and maybe 25 and 26 above 3500.
I kind of like it.

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Old 08-04-2019, 02:25 PM
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