The MSD Ignition - 'Exposed'
05-07-2012, 02:36 PM
#1
Race Car
Thread Starter
The MSD Ignition - 'Exposed'
A technical evaluation of the MSD capacitive discharge ignition (CDI) system
indicates a number of shortcomings compared to the original Bosch CDI used
by Porsche in the 911 up until the 911 3.2 with the inductive discharge ignition
system.
The MSD CDI's key marketing feature is its multiple sparks which produce
a maximum of nine sparks at about 100 RPMs while cranking to a single spark once
the RPMs reach 3000 RPMs. The time between sparks is about 1.5 milliseconds
which contributes basically nothing to enhance the combustion process as the
significant part of the fuel charge burn time is less than 1.5 milliseconds.
Additionally, the spark pulse width of 40 microseconds is half of the Bosch CDI
which results in less time to fully facilitate complete fuel charge ignition resulting in
potential mis-fires and greater emissions.
The MSD CDI produces a primary pulse voltage over 450 volts compared to the
Bosch CDI's pulse of about 300-350 at typical engine RPMs, thereby stressing the
ignition system wiring, the coil, and the rotor & cap. This further shortens the life of
these components beyond what the multiple spark effect does, i.e. at idle the MSD
is producing four sparks, versus one for the Bosch CDI, resulting in significantly
shortened ignition component life.
To implement the multiple spark system, the MSD CDI requires a significant number
of components compared to the Bosch CDI and thereby significantly reduces its
reliability compared to the Bosch CDI, as the reliability of any system is reduced
as its component number is increased. Additionally, the MSD CDI requires additional
wiring for power because of the its high current requirement to produce the multiple
sparks, whereas the Bosch CDI has a simple power and ground.
Since most all the torque is developed at RPMs greater than 3000 RPMs and the MSD
CDI produces just one spark there, it operates in a mode the same as the Bosch CDI
and thus has no real advantage in performance, even if the multiple spark had a
benefit. So given the insignificant benefit of the multiple spark effect with the
additional complexity and number of components, and a shorter initial spark, a less
desirable ignition system results with the MSD CDI than with the Bosch CDI ignition.
Check here under 'Ignition Systems' for additional info: http://www.systemsc.com/technical.htm
Included are images of the oscilloscope trace of the 9 sparks at 100 RPMs, the inside
of the MSD CDI showing it two ICs (PIC processor, & driver IC), 7 power semiconductors,
4 electrolytic capacitors (bad choice for high temps in an engine compartment), and the
extensive number of other components, and the very simple and reliable Bosch CDI.
indicates a number of shortcomings compared to the original Bosch CDI used
by Porsche in the 911 up until the 911 3.2 with the inductive discharge ignition
system.
The MSD CDI's key marketing feature is its multiple sparks which produce
a maximum of nine sparks at about 100 RPMs while cranking to a single spark once
the RPMs reach 3000 RPMs. The time between sparks is about 1.5 milliseconds
which contributes basically nothing to enhance the combustion process as the
significant part of the fuel charge burn time is less than 1.5 milliseconds.
Additionally, the spark pulse width of 40 microseconds is half of the Bosch CDI
which results in less time to fully facilitate complete fuel charge ignition resulting in
potential mis-fires and greater emissions.
The MSD CDI produces a primary pulse voltage over 450 volts compared to the
Bosch CDI's pulse of about 300-350 at typical engine RPMs, thereby stressing the
ignition system wiring, the coil, and the rotor & cap. This further shortens the life of
these components beyond what the multiple spark effect does, i.e. at idle the MSD
is producing four sparks, versus one for the Bosch CDI, resulting in significantly
shortened ignition component life.
To implement the multiple spark system, the MSD CDI requires a significant number
of components compared to the Bosch CDI and thereby significantly reduces its
reliability compared to the Bosch CDI, as the reliability of any system is reduced
as its component number is increased. Additionally, the MSD CDI requires additional
wiring for power because of the its high current requirement to produce the multiple
sparks, whereas the Bosch CDI has a simple power and ground.
Since most all the torque is developed at RPMs greater than 3000 RPMs and the MSD
CDI produces just one spark there, it operates in a mode the same as the Bosch CDI
and thus has no real advantage in performance, even if the multiple spark had a
benefit. So given the insignificant benefit of the multiple spark effect with the
additional complexity and number of components, and a shorter initial spark, a less
desirable ignition system results with the MSD CDI than with the Bosch CDI ignition.
Check here under 'Ignition Systems' for additional info: http://www.systemsc.com/technical.htm
Included are images of the oscilloscope trace of the 9 sparks at 100 RPMs, the inside
of the MSD CDI showing it two ICs (PIC processor, & driver IC), 7 power semiconductors,
4 electrolytic capacitors (bad choice for high temps in an engine compartment), and the
extensive number of other components, and the very simple and reliable Bosch CDI.
Last edited by Lorenfb; 10-06-2012 at 01:07 PM.
05-07-2012, 11:26 PM
#2
Rennlist Member
I think the first guy to run this past me was Lonnie (?) who bought-sold-rebought (?) Permatune 20+ years ago. My cousin (the guy who's now doing the 910/907 stuff) developed a crank trigger ignition + software to drive it off the stock box. We were at SEMA, and Lonnie and he were trying to do a deal.
05-13-2012, 02:03 AM
#3
for those who do not know, Loren competes against the MSD boxes
He has yet to point out any real world data showing the superiority of the Bosch boxes.
http://forums.pelicanparts.com/porsc...n-exposed.html
He has yet to point out any real world data showing the superiority of the Bosch boxes.
http://forums.pelicanparts.com/porsc...n-exposed.html
05-13-2012, 09:51 AM
#4
Team Owner
I dont have a dog in the fight but i must say I see quite a few threads in general stating "my msd ignition isnt working". But i cant even recall the last time i heard a thread about the Bosch system isnt working. To me thats a huge advantage all other things being equal.
Loren do you have any info of what route cause failures you see on the Bosch compared to the MSD ?
The rest i think is coke or pepsi .. i sure wouldnt consider the MSD an upgrade. The onus isnt on anyone to proove the advantage of the Bosch system as it comes in the car and it works. The onus is on MSD to proove they are far superior as you are paying to put it in.
just my 2c
Loren do you have any info of what route cause failures you see on the Bosch compared to the MSD ?
The rest i think is coke or pepsi .. i sure wouldnt consider the MSD an upgrade. The onus isnt on anyone to proove the advantage of the Bosch system as it comes in the car and it works. The onus is on MSD to proove they are far superior as you are paying to put it in.
just my 2c
05-13-2012, 02:14 PM
#6
Race Car
Thread Starter
"Loren do you have any info of what route cause failures you see on the Bosch compared to the MSD ?"
The MSD is an overly complex design with over 5X as many components
as the Bosch unit which makes it inherently less reliable. The Bosch unit
failures mainly result from; an over-voltaging alternator, bad troubleshooting
(coil shorting), and reverse jumping of the battery.
"He has yet to point out any real world data showing the superiority of the Bosch boxes."
As was posted here (http://forums.pelicanparts.com/porsc...n-exposed.html), the point
is that MSD unit is just another basic CDI unit as designed in the 60's like the Bosch
to over come firing of fouled plugs with many less desirable aspects than the original
Bosch units have, e.g. the gimmick of multiple sparks. The MSD has no real benefits
over the Bosch CDI or for that matter any other ignition whether a CDi or an inductive
discharge ignition which Porsche started using with the 911 3.2.
The MSD is an overly complex design with over 5X as many components
as the Bosch unit which makes it inherently less reliable. The Bosch unit
failures mainly result from; an over-voltaging alternator, bad troubleshooting
(coil shorting), and reverse jumping of the battery.
"He has yet to point out any real world data showing the superiority of the Bosch boxes."
As was posted here (http://forums.pelicanparts.com/porsc...n-exposed.html), the point
is that MSD unit is just another basic CDI unit as designed in the 60's like the Bosch
to over come firing of fouled plugs with many less desirable aspects than the original
Bosch units have, e.g. the gimmick of multiple sparks. The MSD has no real benefits
over the Bosch CDI or for that matter any other ignition whether a CDi or an inductive
discharge ignition which Porsche started using with the 911 3.2.
05-15-2012, 07:13 PM
#7
Drifting
While the Bosch WAS designed in the sixties it should be pretty clear to anyone that the MSD design is a much more modern one. More components, yes, but likely with a computed MTBF far above that of the long (market) obsolete Bosch. Additionally, I doubt if the designers of the Bosch planned for the battery voltage/spike "sink" to be so remoted from the alternator output as it is in our 911's.
Speaking of which, after Loren repaired mine, failed due to alternator over-voltage, I added a few components to limit the incoming voltage to less than ~14 volts.
With regards to the MSD multi-spark issue there is good, sound, reason to believe that multiple sparks provides some of the same functionality as a twin plug system. The primary advantage of a twin plug system is that you start, initiate, 2 advancing flamefronts simultaneously, resulting in reaching peak cylinder pressure quicker. Reaching peak cylinder pressure just as quickly but without the detrimental effects of using highly advanced ignition timing otherwise required of a single plug.
Given enough SWIRL of the A/F mixture as the piston reaches TDC the follow-on sparks would undoubtedly start "new" flamefronts.
Loren has succeeded in convincing me that should my '78 CDI fail yet again (unlikely given the new input voltage "clamp") it will be junked rather than paying him for another repair.
Speaking of which, after Loren repaired mine, failed due to alternator over-voltage, I added a few components to limit the incoming voltage to less than ~14 volts.
With regards to the MSD multi-spark issue there is good, sound, reason to believe that multiple sparks provides some of the same functionality as a twin plug system. The primary advantage of a twin plug system is that you start, initiate, 2 advancing flamefronts simultaneously, resulting in reaching peak cylinder pressure quicker. Reaching peak cylinder pressure just as quickly but without the detrimental effects of using highly advanced ignition timing otherwise required of a single plug.
Given enough SWIRL of the A/F mixture as the piston reaches TDC the follow-on sparks would undoubtedly start "new" flamefronts.
Loren has succeeded in convincing me that should my '78 CDI fail yet again (unlikely given the new input voltage "clamp") it will be junked rather than paying him for another repair.
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05-15-2012, 07:25 PM
#8
Drifting
"Loren do you have any info of what route cause failures you see on the Bosch compared to the MSD ?"
The MSD is an overly complex design with over 5X as many components
as the Bosch unit which makes it inherently less reliable.
Loren, do you have any idea of the ratio of the number of "transistors" in a modern Quad-Core X86 processor vs the 80486....
And speak of an overly complex design I think even you will admit that the 5 components I added to my Bosch CDI to limit the imput voltage after you repaired it should have been included in the original design. Either that or the Bosch components that fail, become HEAT sensitive, under over-voltage stress should have been a bit more "robust".
The Bosch unit
failures mainly result from; an over-voltaging alternator, bad troubleshooting
(coil shorting), and reverse jumping of the battery.
"He has yet to point out any real world data showing the superiority of the Bosch boxes."
As was posted here (http://forums.pelicanparts.com/porsc...n-exposed.html), the point
is that MSD unit is just another basic CDI unit as designed in the 60's like the Bosch
to over come firing of fouled plugs with many less desirable aspects than the original
Bosch units have, e.g. the gimmick of multiple sparks. The MSD has no real benefits
over the Bosch CDI or for that matter any other ignition whether a CDi or an inductive
discharge ignition which Porsche started using with the 911 3.2.
The MSD is an overly complex design with over 5X as many components
as the Bosch unit which makes it inherently less reliable.
Loren, do you have any idea of the ratio of the number of "transistors" in a modern Quad-Core X86 processor vs the 80486....
And speak of an overly complex design I think even you will admit that the 5 components I added to my Bosch CDI to limit the imput voltage after you repaired it should have been included in the original design. Either that or the Bosch components that fail, become HEAT sensitive, under over-voltage stress should have been a bit more "robust".
The Bosch unit
failures mainly result from; an over-voltaging alternator, bad troubleshooting
(coil shorting), and reverse jumping of the battery.
"He has yet to point out any real world data showing the superiority of the Bosch boxes."
As was posted here (http://forums.pelicanparts.com/porsc...n-exposed.html), the point
is that MSD unit is just another basic CDI unit as designed in the 60's like the Bosch
to over come firing of fouled plugs with many less desirable aspects than the original
Bosch units have, e.g. the gimmick of multiple sparks. The MSD has no real benefits
over the Bosch CDI or for that matter any other ignition whether a CDi or an inductive
discharge ignition which Porsche started using with the 911 3.2.
05-16-2012, 12:40 AM
#9
Race Car
Thread Starter
"Additionally, I doubt if the designers of the Bosch planned for the battery voltage/spike "sink" to be so remoted from the alternator output as it is in our 911's."
Please try and obtain the Bosch CDI schematic and review it before making comments
about it, as Bosch has included voltage protection circuitry for over-voltage.
Please try and obtain the Bosch CDI schematic and review it before making comments
about it, as Bosch has included voltage protection circuitry for over-voltage.
Last edited by Lorenfb; 05-17-2012 at 12:53 AM.
05-24-2012, 11:13 PM
#10
Drifting
"Additionally, I doubt if the designers of the Bosch planned for the battery voltage/spike "sink" to be so remoted from the alternator output as it is in our 911's."
Please try and obtain the Bosch CDI schematic and review it before making comments
about it, as Bosch has included voltage protection circuitry for over-voltage.
Please try and obtain the Bosch CDI schematic and review it before making comments
about it, as Bosch has included voltage protection circuitry for over-voltage.
You may note that the 2N3055HV is forward biased using the raw "unfiltered" alternator output voltage as the source.
The only source of reverse biasing, switching the 2N3055HV off, is the secondary winding of the step up transformer. Other than that reverse biasing ability occuring as the 2N3055HV collector current rises, there is basically no limit to the level the collector current can rise.
I assume you will see, as I do, that with increased supply voltage, the harder it will become for the "transformer" reverse bias voltage source to switch off the 2N3055HV. So, the higher the supply voltage, the higher the 2N3055HV's collector current will rise for each "switch" cycle.
Let's assume that with a 12 volt supply source the collector current never rises above 6 amps. That would mean with a 16 volt source the collector current will rise to 8 amps. 8 amps into the primary of the step up transformer instead of only 6 amps....
Just so we have an understanding. The capacitor in parallel with the collector emitter of the 2N3055HV simply serves to limit, slow, the primary transformer inductive circuit risetime(***). The 82 volt 10W (24 ohm) Zener is there to protect the 2N3055HV. Once the ignition capacitor is fully charged there will be no load on the transformer secondary to be reflected to the primary so the peak rise of the collector voltage as the 2N3055HV is switched off must be limited.
*** The size of this capacitor, ~1MFD, seems extraordinary. Does this capacitor, along with the transformer inductance, form a resonant circuit at some specific, helpful, frequency?
So, now that we know a bit more about the operation of the Bosch CDI, why, how, did mine fail...?
There can be no question that the base causative reason was overcharging of the battery.
1. ) Obviously that might lead to overheating of the 2N3055HV.
2. ) Or it might lead to excessive voltage charge on the ignition capacitor resulting in self-firing of the SCR.
3. ) Lastly, the zener might be forced to clamp the transformer's inductive "kick" much more often.
But there remains one more clue to the failure of my CDI. Even once the source voltage problem was corrected my CDI proved to be to overly sensitive to heat, heating.
We could drive the car several miles before it would die. We could then let the CDI cool on it's own or we could "hit" the CDI case with a spurt or 2 of cool spray and then the car would again start right up. Repeatable.
I'm voting for the 2N3055HV to have been overheated to the level that an internal failure resulted in to much collector leakage current. Collector current flow even with no forwad bias at the base.
What say you, Loren. Do you have records? How did you fix my CDI..?
05-24-2012, 11:41 PM
#11
Drifting
Loren:
Looking back to events leading up the failure of the CDI in my '78 targa.
1. The e-brake light would intermittently "latch up" and could only be turned off if the battery was briefly disconnected.
2. The engine would just up and die, but could be restarted via switching the ignition completely off while counting to 10 slowly. A new battery seemed to abate this for a period.
3. The tach would "jump" intermittently.
4. The "just die" episodes turned out to be the result of the rev limiter latching up and opening the fuel pump circuit. Switch the ignition completely off, the power was removed from the rev-limiter....no latch up...
5. ) The CDI became overly sensitive to heat and would fail to operate unless cooled.
So, at least 4 problems directly related, seemingly, to battery overcharging.
I just had to Google "Lucas Electric LTD" in case it was aquired by Bosch back about the time these circuits were being designed. Other than the possibility that Bosch hired a bunch of Lucas engineers as Lucas was failing I didn't find anything.
How is it possible for Bosch or Porsche to have designed so many flawed circuits, subject to excessive supply voltage...? Especially given that the most common failure mode of mechanical voltage regulators was a stuck, welded together, regulator "relay" contacts, always resulting in battery overcharging.
Looking back to events leading up the failure of the CDI in my '78 targa.
1. The e-brake light would intermittently "latch up" and could only be turned off if the battery was briefly disconnected.
2. The engine would just up and die, but could be restarted via switching the ignition completely off while counting to 10 slowly. A new battery seemed to abate this for a period.
3. The tach would "jump" intermittently.
4. The "just die" episodes turned out to be the result of the rev limiter latching up and opening the fuel pump circuit. Switch the ignition completely off, the power was removed from the rev-limiter....no latch up...
5. ) The CDI became overly sensitive to heat and would fail to operate unless cooled.
So, at least 4 problems directly related, seemingly, to battery overcharging.
I just had to Google "Lucas Electric LTD" in case it was aquired by Bosch back about the time these circuits were being designed. Other than the possibility that Bosch hired a bunch of Lucas engineers as Lucas was failing I didn't find anything.
How is it possible for Bosch or Porsche to have designed so many flawed circuits, subject to excessive supply voltage...? Especially given that the most common failure mode of mechanical voltage regulators was a stuck, welded together, regulator "relay" contacts, always resulting in battery overcharging.
