Originally Posted by scarceller (Post 8006874)

When I say 2 sparks I don't mean dual plug, I mean firing one spark after another just in case the first one did not light the fuel. This is simply not very helpful since it masks some other issue with the ignition. Bottom line is that the first spark has to do the job and hoping a 2nd spark will help just hurts power and generates un-needed heat into the exhaust.

Dual plugs are used to help light the flame front from both side of a Hemi cylinder at the very SAME time. When using dual plugs vs single plug in a Hemi cyl it allows you to not need so much ign advance to generate peak torque conditions (max cyl pressure). The down side of the hemi single plug is that the plug is offset far to one side of the cyl and this results in needing a lot more ign advance than with dual plug. The more ign advance results in fighting the piston on the up stroke before TDC. It's the trade off needed to achieve max torque. This is why the very same exact engine build with dual plug vs single plug, the dual plug will generate more torque on the dyno.

Originally Posted by mysocal911 (Post 8007059)

Hopefully energy of the spark and voltage of the spark are not being confused.

The system spark voltage determines maximum total gaps (plug & cap/rotor)
and cylinder pressures under which a spark can be produce. Systems producing
a minimum of 250/300 primary volts is more than adequate. The system energy
is the energy available in the spark to fully ignite the fuel charge which is typically
30 milli-joules. Most OEM systems produce 2-3 times this value.

Originally Posted by scarceller (Post 8007477)

100% agree here. Here's my simple explanation comparing CDI to Inductive ignition:
- Inductive builds up energy over time in the actual coil windings and then releases this energy stored when the field collapses because voltage is removed from the primary winding. This results in a long release of energy but the voltage typically is not as high as CDI. The good thing about Inductive is that the spark lasts considerably longer than CDI but not as strong (less voltage). This style ignition does very well in cylinders that don't have extreme pressures.

- CDI uses the coil as step transformer and NOT a holding device, coils can be used either way (CDI voltage stepping or Inductive energy holding) but they are different. In CDI the control module typically has large storage capacitors that charge to 200-500volts and then when commanded the capacitors release this voltage across the coil's primary winding. If the coil has a 100:1 wire ratio then the 500vdc gets stepped up to 500X100=50,000vdc at the output. This results in a very strong spark event compared to Inductive but it does not last as long. The spark is brighter hotter but not as prolonged in length. CDI works very well in Forced induction where cyl pressures are much higher and it difficult to jump the gap without sufficient voltage. Here the chances for not jumping the gap are more likely and is why CDI often fires multiple spark events back to back.

In summary: Inductive ignition has 20,000-35,000 volts with spark durations in the 1000-2000micro seconds. CDI has 50,000+ volts with spark durations in the
400-600micro seconds.

This site has a great detailed writeup about the 2 system and worth reading.
Inductive Ignition | GILL

Originally Posted by mysocal911 (Post 8007484)

Your quote from the internet, sums it up nicely.

The CDI system was used by Porsche for the fast spark rise time for fouled plugs
which wasn't available in the '60s with inductive discharge systems. Now fast
spark rise times can be achieved with today's advanced semiconductors resulting
in all OEMs using inductive discharge systems.

Originally Posted by scarceller (Post 8007515)

So if you are going to replace the ignition system maybe are are better served replacing it with a modern day electronic triggered inductive system instead of CDI. I see a lot of folks going the CDI route but I feel that a good inductive system would better server these motors. BTW - the 84-89 3.2L using an electronic triggered inductive system that's very reliable and seldom gives problems. Many of these cars still have the factory coil that came with the car!

Originally Posted by mysocal911 (Post 8007481)

What the resulting timing turns out to be is of little value as long as the
engine avoids detonation and achieves maximum torque.

Originally Posted by mysocal911 (Post 8007481)

"When using dual plugs vs single plug in a Hemi cyl it allows you to not need so much ign advance to generate peak torque conditions (max cyl pressure)."

It's just all about avoiding detonation while achieving maximum torque!

What the resulting timing turns out to be is of little value as long as the
engine avoids detonation and achieves maximum torque.

"timing.....of little value....."

Sorry, you simply do not get optimum torque AND avoid detonation without PROPER timing.

As an example, the Porsche 996 with its center spark plug runs advance timing greater than the air-cooled 911s. And the direct-injected 997 runs even greater
timing advances than the 996. Furthermore, both the 996 and the 997
have higher CRs than the air-cooled 911s.

Originally Posted by mysocal911 (Post 8007484)

Your quote from the internet, sums it up nicely.

The CDI system was used by Porsche for the fast spark rise time for fouled plugs
which wasn't available in the '60s with inductive discharge systems. Now fast
spark rise times can be achieved with today's advanced semiconductors resulting
in all OEMs using inductive discharge systems.

Originally Posted by Orange911S (Post 8007916)

I think it was the cap. I have not gotten to the timing yet.
I don't think it is a fuel issue. This all started with a day a couple of weeks ago where I got crank, but no start trying to get home from work. The car was running great before that.

I am not as experienced as many of you guys, but I am comfortable swapping parts. I checked with the forum after that incident and the overwhelming push was to look at the CD box and coil. From there, Wayne's MSD upgrade article made sense. I started with the coil swap to the MSD and that did not solve the problem. I then swapped the OEM perma-tune box for the MSD and it was running fine, with a few misses. This escalated into the backfire situation that started this thread a few days ago. I will put some miles on over the next few days and see how it goes, but so far so good.

Originally Posted by dicklague (Post 8007948)

I read the GILL report. They make inductive igntions. They make NO CDI ignitions. So what do you think they are going to say?

Some of what they said is misleading at best and possibly wrong. For example, a direct quote:
"Typically a capacitive discharge system will deliver a maximum of 10 millijoules of energy compared to an inductive ignition system delivering more like 50 millijoules of energy and potentially in excess of 100 millijoules. This large difference in supplied energies will mean an inductive system can provide spark duration of 2000 microseconds or more in a single spark, compared to 600 microseconds for a capacitive system."

The Daytona-Sensors CD-1 that I have in my 2.7l MFI 1973 puts out 135 mJ [millijoules] of spark energy, NOT 10!

Most MSD models put out in the 75 to 85 mJ range.

So once I read that and realized that GILL makes and sells ONLY inductive ignition systems, I can't put any credibility to what they wrote in this report.

I know inductive systems to a great job but stick to the facts GILL!

I do know that a properly installed high quality CDI system in the form of the Daytona-Sensors CD-1 has made a great difference in my 911. It comes with a matching coil so that your are sure the system will perform to its maximum.

I have to comment that some posters on this thread rebuild Bosch CDIs as a business and they have been critical in the past of anything but the original Bosch CDI being used in cars that are so equipped.

Reading through posts here there is a lot of history of problems with aftermarket CDI whether caused by faulty installation, other components not being upgraded or just plain failure of components.

I am convinced that if you choose a good aftermarket CDI, install it properly, and make sure your cap, rotor, and wires are up to the task, you are going to get increased performance out of your early carb of MFI 911.

Originally Posted by dicklague (Post 8007948)

I read the GILL report. They make inductive ignitions. They make NO CDI ignitions. So what do you think they are going to say?

Does Daytona sensors make inductive ignitions...???

Some of what they said is misleading at best and possibly wrong. For example, a direct quote:
"Typically a capacitive discharge system will deliver a maximum of 10 millijoules of energy compared to an inductive ignition system delivering more like 50 millijoules of energy and potentially in excess of 100 millijoules. This large difference in supplied energies will mean an inductive system can provide spark duration of 2000 microseconds or more in a single spark, compared to 600 microseconds for a capacitive system."

The Daytona-Sensors CD-1 that I have in my 2.7l MFI 1973 puts out 135 mJ [millijoules] of spark energy, NOT 10!

Taking the "word" of an unbiased vendor source?

135 mj of capacitor energy storage or 135 mj of energy dissipated in the HV circuit during spark sustainment? Put an O'scope on the coil primary and you will discover that a significant portion of the energy output of a CDI is RETURNED to the capacitor once the arc extinguishes.

Which number, unbiased number, is Daytona quoting?

Most MSD models put out in the 75 to 85 mJ range.

And then "take back" what proportion, %....??

So once I read that and realized that GILL makes and sells ONLY inductive ignition systems, I can't put any credibility to what they wrote in this report.

And just what gives Daytona-sensors more credibility...???

I know inductive systems to a great job but stick to the facts GILL!

I do know that a properly installed high quality CDI system in the form of the Daytona-Sensors CD-1 has made a great difference in my 911. It comes with a matching coil so that your are sure the system will perform to its maximum.

No reason to question this purely subjective statement, but how do you know that a modern inductive system wouldn't perform just as well, or maybe even better... subjectively speaking?

I have to comment that some posters on this thread rebuild Bosch CDIs as a business and they have been critical in the past of anything but the original Bosch CDI being used in cars that are so equipped.

And just why would you not expect that given your above vendor biased conclusions?

Reading through posts here there is a lot of history of problems with aftermarket CDI whether caused by faulty installation, other components not being upgraded or just plain failure of components.

I am convinced that if you choose a good aftermarket CDI, install it properly, and make sure your cap, rotor, and wires are up to the task, you are going to get increased performance out of your early carb of MFI 911.

Originally Posted by scarceller (Post 8007988)

Dick,

No question about it, CDI system works as does a good electronic inductive system. Either would be a good choice to replace the older points.

BTW - I ran a pro MSD setup in my 84 3.2L and it worked just fine but it did not make any difference compared to the stock ignition so I took it out and went back to the simple stock setup. Used MSD 6200 controller with MSD 8250 coil this is the pro version from MSD same used in NASCAR. Nice unit did the job well but had no effect on performance or running over the stock setup.