MSD questions

kent olsen · kent olsen's Garage

I started out with a Mallory MSD and Mallory Coil maybe 6+ years ago. Then when I built my 3.0L with twin plugs I added a MSD/MSD and MSD Blaster coil to run the bottom plugs.

No problem for 4-5 years. Now the last year I have burnt out my JB racing twin plug distributor twice. Shorts out the rotor and this time damaged the cap as well.

I've ordered another 6420 analog MSD/MSD and blaster coil so I'll have both top/bottom plugs run from MSD/MSD and Blaster coils.

Has anyone else had issues with these units as they get older???

Ayles · 10-28-2016, 08:34 PM

I had an msd up and die, but it was much much older than that. I think it was 16yrs old. It just up and quit one day. No strange behavior or anything leading up to failure.

Jonny H · 10-29-2016, 12:20 AM

IMO the voltage output by MSD units is far too high (500V vs 300v on the original system). This can result ~50kV passing through the distributor (~25kV is usually more than adequate to fire a spark plug in most condiitons).

The higher the voltage, the more likely the spark will find another path to ground, other than through the spark plug. This is why MSD recommend you change to their coil and use Magnecor leads etc. Any resistance in the electrical path to the plug, will also generate more heat.

A high voltage is great in theory, but the components must be in top shape. Your leads might be deteriorating or you might have some moisture in the cap or a hairline crack / tracking. You should also avoid running a rotor with an internal resistor.

Run the engine in the dark and look for sparks around the distrbutor/leads.

Fred Winterburn · 10-29-2016, 05:17 AM

The advertised voltage of the old MSD6A series is very high, but the two I own (one from the early 80s and the other from 2009) won't reach advertised voltage unless 18V is applied which is well beyond normal charging circuit output on a car. The voltage output of the MSD is directly proportional to battery voltage with the early MSD 6A design. I can't speak for later designs. There has to be more to the damage issue than just voltage with that design, unless there is considerable variation from unit to unit and I just happen to own two weak units. Certainly multiple sparks occurring 1mS apart are an invitation for cross-fire within the cap if the rotor phasing is off, along with any extra erosion and voltage stress it gives in the long term. The multiple sparks will occur up to 4000rpm (8 cylinder, so over 5000rpm with a 6 cylinder) with the old MSD6A design. There is no programmed rpm cutoff for the multiple sparks as it is based solely on the ability of the power supply to keep up with the rpm demand. Excessive voltage is certainly detrimental with a CD design, but I think in the case of the MSD it is death by a thousand cuts and possible cross-fire as the distributor cap degrades from multiple sparks spaced too far apart.
Yes that rotor resistor is a weak link with any CDi and can fail with the standard Kettering too. Fred

Quote:

Originally Posted by Jonny H

IMO the voltage output by MSD units is far too high (500V vs 300v on the original system). This can result ~50kV passing through the distributor (~25kV is usually more than adequate to fire a spark plug in most condiitons).

The higher the voltage, the more likely the spark will find another path to ground, other than through the spark plug. This is why MSD recommend you change to their coil and use Magnecor leads etc. Any resistance in the electrical path to the plug, will also generate more heat.

A high voltage is great in theory, but the components must be in top shape. Your leads might be deteriorating or you might have some moisture in the cap or a hairline crack / tracking. You should also avoid running a rotor with an internal resistor.

Run the engine in the dark and look for sparks around the distrbutor/leads.

Jonny H · 10-29-2016, 07:41 AM

^ The MSD published cut off for multi spark with six cylinders is 3400 RPM.

CDI+ always puts out two sparks (at typically 200 - 400us intervals), at all RPM, but we have a constant output voltage of 300V. With a locked dizzy, the phasing is such that the rotor is centred on the post at max RPM. No issues. We've got several cars out there running beta firmware that is firing up to 8 sparks dependent on RPM and these also have no issues. These cars are also running their old rotors, caps and leads. That's why I think it is voltage related, rather than multi spark related.

OP, have you locked the dizzy or are you still using the weights and springs for timing? Which rotor arm are you using?

kent olsen · kent olsen's Garage

I'm runny a JB Racing twin plug distributor. It is Jaguar 12 cyl distributor modified by JB Racing in Florida. They have been using this for many year and have good luck with it.

The first time it shorted out I found lots of moisture in the cap, I live in Oregon, and that probably had a lot to do with it. The second time no moisture but I'm changing out the Mallory as it is the oldest unit

The new MSD units are now digital vs my old one is analogue so I purchased the older 6420 analogue unit from EBAY. I was told if you mix the 6420 analog with the 6425 digital you may have a 4 deg phase difference, hence my purchase of an older 6420.

Should have the MSD and coil by Monday, we'll see.

Fred Winterburn · 10-29-2016, 09:31 AM

Jonny,
I wasn't knocking your multispark ignition. The one I make is multi-spark too, but by a natural oscillation enhanced by the power supply. That MSD multi-sparks at one millisecond intervals makes it rather useless in my opinion and with up to 3 multiple sparks at over 4000rpm on a 6 cylinder (and more at lower rpm where there are up to 5 sparks), cross-fire does become more of a possibility. Also, despite the cutoff for a 6 cylinder that they state, I have two units (MSD6A) that multi-spark to 4000 rpm on an 8 cylinder. That puts the multi-sparking to 5333 rpm on a 6 banger, despite what MSD advertises. They also advertise spark energy based on energy stored in the capacitor which is inaccurate for several reasons, but then so does everyone else.
I do agree that excessive voltage is usually the culprit when it comes to damage and with the ability for a CDi to puncture insulation more easily, keeping the voltage in check is extremely important. If we are comparing numbers, my unit puts out 340V at cranking speed, dropping immediately to just under 300V at idle and then dropping very slowly to the maximum spark output of 533 sparks/second. 25kV is available to the maximum spark rate with a conventional coil of 100:1 turns ratio. Of course, voltage is only part of the equation but it does need to be controlled which is very uncommon on most units. I'm glad you have also designed that into your unit. Its importance can't be understated. Fred

Quote:

Originally Posted by Jonny H

^ The MSD published cut off for multi spark with six cylinders is 3400 RPM.

CDI+ always puts out two sparks (at typically 200 - 400us intervals), at all RPM, but we have a constant output voltage of 300V. With a locked dizzy, the phasing is such that the rotor is centred on the post at max RPM. No issues. We've got several cars out there running beta firmware that is firing up to 8 sparks dependent on RPM and these also have no issues. These cars are also running their old rotors, caps and leads. That's why I think it is voltage related, rather than multi spark related.

OP, have you locked the dizzy or are you still using the weights and springs for timing? Which rotor arm are you using?

Jonny H · 10-29-2016, 12:58 PM

^ Hey, I didn't take it as a 'knock' at all.

Kent, what a small world. We work with BS Motorsport in the UK who have prepared quite a few of JB Racing's cars for the track. Recently, a BMW CSL that raced at Monterey and there is a '67 911 that will most likely be getting our electric air conditioning.