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Sigurd's Avatar
 
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CDI and twin spark

Can I use one 6 pin CDI box to fire both coils on a twin spark engine or do I need two?
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Old 09-22-2016, 12:51 AM
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I am sure you need two boxes.

I believe that the RSR used an 8 Pin Box but if you have an SC dizzy with a mag pick up you can use 2 x 6 Pin Units.
Old 09-22-2016, 01:36 AM
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I have a twin plug set-up on my RSR motor. I use a single MSD to drive two coils wired in parallel. Very common and works well.
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Old 09-22-2016, 03:59 AM
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I have no idea how to make the stock CDI work.

There are two warring camps on the MSD Twin plug debate, some say 2 boxes, some say one is fine.

Pros and cons

Single MSD box
Pro if the box fails not a big deal, other than the tow home.
Con, you may lose a hair of HP if you use only one box, but this point is debatable.

Twin boxes
Pro Many say more HP
Con if one box was to fail you would be running the engine on single plug, on a high CR motor that could result in engine damage.

My thought is for a race car 2 boxes, but for a street car, where losing a couple of HP isn't a big issue, a single box is just fine.
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Old 09-22-2016, 06:53 AM
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One MSD box can fire two coils, however the output is halved and we see the result on the engine dyno.

For best power, one needs two ignition boxes and two matching coils.

Not that its relevant, but the '73-'74 RSR (and all 935's) used twin Bosch CD boxes and twin coils.
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Old 09-22-2016, 08:19 AM
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Steve,

How much difference does it make?

We ran 3 x Twin Plug Rally cars with single MSD and Twin Plugs, they produced about 265 BHP fro 3.0SC engines with GE60 cams and 10.31 compression.

We never tried them with two units and I would be interested to know what improvement we could gain.

We ran a fairly tight plug gap and used NGK BP8ES plugs.
Old 09-22-2016, 09:06 AM
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Quote:
Originally Posted by Steve@Rennsport View Post
One MSD box can fire two coils, however the output is halved and we see the result on the engine dyno.

For best power, one needs two ignition boxes and two matching coils.

Not that its relevant, but the '73-'74 RSR (and all 935's) used twin Bosch CD boxes and twin coils.
Yes but what do you see? 1-3% or 10-15%?
I believe you have been asked this before with no answer.
Is the torque numbers also down? As an engine builder torque has always been more important to me than HP numbers.

If it's 3% in a street car it will not be a hill of beans.
In a race car, sure, of course you want every spec of HP you can gain.

In my narrow body 914 3.0 street car, I should dyno over 250hp, I'm not going to really care if I'm losing 5 hp.
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Last edited by Mark Henry; 09-22-2016 at 09:22 AM..
Old 09-22-2016, 09:20 AM
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Quote:
Originally Posted by Mark Henry View Post
Is the torque numbers also down? As an engine builder torque has always been more important to me than HP numbers.
As the real issue with 911s running high compression pistons is plug shrouding which will prevent combustion crossing the top of the piston at high engine speeds the effect will be reduced peak cylinder pressure due to incomplete combustion and this must impact on the torque being produced.

It is less important at lower engine speeds.

Early 2 litre engines are mush more adversely affected due to the deeper combustion chamber and taller piston crowns.

It will also limit engine speed which will have a significant impact on Horsepower.

The torque vs horsepower argument will never end but in reality I think that they are just two sides of the same coin.

For a race car I would always chase Horsepower matched to optimum gearing.
Old 09-22-2016, 10:20 AM
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Gentlemen,

thanks for the inputs. I know several are using one MSD box, but I am thinking of the Bosch, original, unit. As with the MSD, can I assume there wil be no electrical problems due to the higher current needed for the additional coil?
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Old 09-22-2016, 11:05 AM
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I am running a M&W pro16 CDI. I LOVE it!!!!!
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Old 09-23-2016, 08:14 AM
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Chris,

Depending on CR and plug gaps (we run .045-.050 for more HP), I've seen 15-25 BHP differences between a single and dual CD boxes firing two coils.
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Old 09-23-2016, 10:09 AM
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if you run one of our CDI+ boxes you will see similar gains to a twin spark setup, especially on the early engines due to the exact reason chris_seven gave above.

It is a true multispark, up to 10K RPM, unlike MSD which reverts to a single spark at 3400 RPM on a six cylinder engine. The second spark further ignites the fuel as it swirls in the chamber producing more power.

Steve W, have you dynoed that box we sent yet???
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Old 09-23-2016, 02:04 PM
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Quote:
Originally Posted by Jonny H View Post
Steve W, have you dynoed that box we sent yet???
Not yet; been doing a run of big (3.6-4.0) motors this year.
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Old 09-23-2016, 03:51 PM
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[QUOTE=

Twin boxes
Pro Many say more HP
Con if one box was to fail you would be running the engine on single plug, on a high CR motor that could result in engine damage.
.[/QUOTE]

I run two Bosch 6 pins and two coils on my 914-6 twin plug motor. Because of this fear, I have two switches in the cockpit so I can turn on and off the power to each box independently to check operation after start up each time I take it out. Obviously a box can die during a drive, but it is reassuring to be able to check operation of each box from the cockpit at will.

Tom
Old 09-23-2016, 07:00 PM
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Might I ask . . . - Why not just run Electromotive and be done with all of it?

Simple design, full tuning capabilities and pretty much bulletproof .. ..

No distributor limitations, no moving parts to wear/fail etc etc

R
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Old 09-24-2016, 04:02 PM
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Quote:
Originally Posted by SoyRacer View Post
Might I ask . . . - Why not just run Electromotive and be done with all of it?

Simple design, full tuning capabilities and pretty much bulletproof .. ..

No distributor limitations, no moving parts to wear/fail etc etc

R
Posing factor! Coil packs does not looks as good as a twin spark distributor
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Old 09-24-2016, 11:14 PM
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Quote:
Originally Posted by Sigurd View Post
Posing factor! Coil packs does not looks as good as a twin spark distributor
"Look as good" ? .. .. .. different..... maybe


I've had great success with them on many builds.




It's all subjective - Many factors go into what works for some and don't work for others. For me --> they work well

R
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Old 09-24-2016, 11:44 PM
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Quote:
Originally Posted by SoyRacer View Post
"Look as good" ? .. .. .. different..... maybe


I've had great success with them on many builds.




It's all subjective - Many factors go into what works for some and don't work for others. For me --> they work well

R
Nice!
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Old 09-24-2016, 11:49 PM
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I am surprised that single and twin boxes make so much difference but have never tried to use a Twin Box.

Does the fuelling need to be changed when you add a second box?

I am not sure I understand why losing a plug causes engine damage, I can accept it may lose some power and effectively run a little rich but what else would happen?
Old 09-25-2016, 02:01 AM
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Quote:
Originally Posted by chris_seven View Post

I am not sure I understand why losing a plug causes engine damage, I can accept it may lose some power and effectively run a little rich but what else would happen?
Detonation
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Old 09-25-2016, 06:12 AM
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