afaik a CDI is able to better control combustion. It can be by extending the spark duration or multi-sparks in the envelope.

question
- Is the Bosch spark duration fixed thru the whole rpm range?
- Is the MSD lower rpm time of multi-sparks = Bosch duration?
- Is the MSD upper rpm single spark duration = Bosch duration?

huh?

ok.. If mine I would first try a non-rev-limiting-Bosch rotor. Then if still toasted terminals I would also change the Beru plug terminal wires.

all of above is a prejudical rant.

- Is the Bosch spark duration fixed thru the whole rpm range? Yes


- Is the MSD lower rpm time of multi-sparks = Bosch duration?

The key is the length of the INITIAL spark! The other sparks occur TOO late (>2.5-5ms)
and they have NO value. There's no more fuel charge energy left when the next spark
occurs to produce any additional combustion and force on the piston. That's why Porsche
used dual plugs in some of their engines, e.g. 964 & 993.

Note: At 1000 RPMs a piston moves 6 degrees every millesecond. So buy the time
the next MSD spark occurs (5ms), the piston will have moved 30 degrees. At 3000 RPMs
the piston moves 90 degrees. The combustion process is completed in about 2ms.

- Is the MSD upper rpm single spark duration = Bosch duration? Yes

Come on guys. Think about what's really occuring in the combustion chamber
at ignition time. Porsche and Bosch have thought about it and have concluded
a multiple spark CDI is B.S.

Bottomline: There's thermodynamic/scientific theory which supports the B.S.
If you want a little better idle, gap the plugs at .045 - .050 with Bosch CDI.

Like other "no basis" mods, if it makes you feels better, do it!

burned rotor
 

Echlin. I am beginning to wonder if the problem lies in the shielded Beru wires, even though they are not solid core.

bplante,

If you have new OEM shielded wires, then they are, indeed, copper-core wires ... with an incoporated resistor in the Beru connector! Both ends are removeable ... just unscrew.

My suggestion is to gap your spark plugs at 0.040" maximum and most likely your burning rotor problem will disappear!

"The key is the length of the INITIAL spark! The other sparks occur TOO late (>2.5-5ms) and they have NO value. There's no more fuel charge energy left when the next spark occurs to produce any additional combustion and force on the piston. That's why Porsche used dual plugs in some of their engines, e.g. 964 & 993."

Thats odd, I have two seperate mags and twin plug, left mag top right mag bottom, on my engine, one timed at 24 and the other one at 26, why would I get an rpm drop when when I switch from left and right if everything is already burned?

Craig

"why would I get an rpm drop when when I switch from left and right if everything is already burned?"

Sounds like you have a real problem! You might put it on a dyno and vary each's timing
rather than guess at what's happening.

So using 6 degrees every millisecond @ 1000 RPMs, then 2 degrees (26-24) equals
.300 ms which is much less (1/10) than what the MSD can produce with
the next spark (2.5 - 5ms) - no real benefit.

Since OEM car manufacturers are continually under pressure to reduce emissions and
if the hype about a MSD system had merit for better combustion, don't you think they
would utilize some form of a simple mulitple spark system?

Not Getting the point!....
 

I won't mention any names, but someone here is just not getting the point! Why spend $400 (or more) for a "REBUILT" Bosch CDI unit when the problem can be rectified (pun intended) for $200 or less using quality components? Oh yeah, someone needs to keep the MYTH going that only Bosch can build a reliable ignition! Well, I won't tell if you don't!

Merry Christmas to All and to All a good night!

Fred Cook
'80 911SC coupe (MSD energized and loving it!)

"Why spend $400 (or more) for a "REBUILT" Bosch CDI unit when the problem can be rectified (pun intended)"

So after all the mickey mouse wiring mods. e.g. constant +12, and the time spent
to install it, e.g. modify the brackets, you have an engine compartment that no longer
looks stock Porsche, but one waiting for the Chevy engine install.

Bosch Humbug!....
 

That's ok, I'll laugh all the way to the bank! Besides, there is really nothing wrong with a hot small block chevy V8! There is an old saying that goes something like "If you are not growing, then you are dying." Sometimes it pays to look and see what is new in the world, even the Porsche world.

Fred Cook
'80 911SC coupe

I use the MSD-M marine version.. who knows?

I figure if it's reliable in an ocean racer how bad can it be? I always had to ride it them with bent legs 'cause those are pounding machines. A tight jock strap helps also. Their engine compartment are good for heating cold coffee also 'cause their so hot.. whatever floats your boat.

If you don't like the idea of an MSD install, don't do it. If you do it and then decide you don't like it, take it out. If you like it, leave it.
I can't believe the ammount of debate these things cause.

I can...

The endless quest for a deeper understanding of combustion is somehow like looking into the universe imo. The waves of energy, it's control, it's efficency, etc is the stuff motor-heads dream about. The blending of reversion lower energy electro-magnetic particles with higher energy particles is like stroking to the climax of power.

but you can't come without the necessary spark imo.

Merry Christmas Ronin. I guess that is what this debate is really about. The only reason I'm being blase is that:

1) I have MSD and am happy
2) I would be equally happy if I had CDI
3) I would even more happy if I could do a US road trip/911
5) ditto for being P. Car test pilot at the 'Ring
4) Combustion, petrol & electricity...MMMmmmmmmmmmmm

Ok, I understand...

Merry Christmas Matt

To fix your rotor burning problem, ditch the factory wires and get some Magnecores.

Like Warren said, those stainless braided wires are copper core with a resistor in the plug connector. You need a good set of 8mm resistor core plug wires. MSD specifically recommends against copper core wires.

BTW, I suspect it's not the voltage that's burning the rotor, but rather the amperage that's flowing through the rotor's resistor with the relatively low-impedence copper core wires.

I've fixed this very problem a number of times on customer's cars by swapping out their factory wires for the Magnecores.

I've had MSD on the P-car for over 2 years, no problems even with my slightly rich idle for smoother midrange power. I've also had MSD on my drag car, four trucks and two other street vehicles IIRC. Never a problem except one dead box. MSD equipment is used to fire Top Fuel drag motors and fire so hot they melt the plugs halfway down the track (magnetos), just about every form of racing uses MSD particularily when carbs are required by the sanctioning body. The single biggest gain of installing an MSD box is the soft touch rev limiter. I used 3 limiters on the drag car, one for total RPM, one for burn out and one for the starting line. Hold the button and push the pedal to the floor, the engine "stutters" against the limiter at the set RPM ready for launch, if it was good enough for a motor that cost me more than twice as much as my 3.0 SC motor, it's good enough for the P-car. I can think of a few times the bad synchro in my 901 box caused me to miss the next gear and the rev limiter kicked in saving the engine from an over rev and possible damage. I simple don't and won't trust mechanical limiters, having seen the results of a failure more than once. BTW my SC motor runs Zeniths with 36mm venturis, headers, stock dizzy, cap and rotor, and plug wires. I run slightly cooler plugs at a 0.050" gap.

Back to your question, are you changing the cap AND rotor? If the cap has too much clearance and you're only replacing the rotor that could be part of the problem. Also check the intalled height of the rotor, if it's somehow too low that can cause problems. Tyson's suggestion of Magnecors sounds good, that's one of my planned winter upgrades.

"BTW, I suspect it's not the voltage that's burning the rotor, but rather the amperage that's flowing through the rotor's resistor with the relatively low-impedence copper core wires."

I (current) = V (voltage) divided by R (resistance)

So it's also the voltage, right.

Bottomline:

Two choices;

1. lower the voltage, put a stock Bosch CDI on
2. increase the series resistance;
a. different spark connectors
b. use resistive wire

Copper vs steel vs XYZ metal (R<1-10 ohms) won't change the resistance that much.
The current is affected by:

1. spark plug connectors (500-5000 ohms)
2. coil connectors (500-5000 ohms)
3. rotor resistance (500-5000 ohms)
4. cap carbon wiper
5. the discharge ionization at;
rotor
spark plugs

Too much guessing on this thread! So what could be the problem if stock Bosch CDIs
don't burn ignition parts & MSDs claim more voltage, energy, & MULTIPLE sparks?

Now let's think hard!

There is only this much debate becuase Loren rebuids Bosch units and needs to bolster a market that is shrinking becuase many people prefer the performance and cost of aftermarket CDI's