Should I go with the MSD MAG or Vertex?

I believe that MSD is not making their MSD 10+ anymore which is a shame. They do still make the MSD 8 which puts out 345 MilliJoules which is more than a ProMag 12 at 300 MilliJoules. A 7AL3 and 7AL2 only puts out 160 MilliJoules of spark power.

 

I think the limit would be based on the total compression which is your static compression, and your blower boost plus the cam. The limit would be hard to pin point because sometimes it is not possible to tell when the missfiring starts because you may not see it or hear it until it gets real bad. You just may not make the power your combination should but to the driver it would still appear to be fast. Then one day you put a mag on it it and it suddenly wakes up. In the case of trying to use a 7AL2 system the NGK Iridium plag maybe a good choice because it takes less spark power to jump from a small electrode than a big electrode thus the reason for the use of Iridium so they can make a small electrode last in a combustion chamber with a lot of spark power.

 

If it is an "analog limiter" which uses a chip then it is really harsh sounding and there is no method to their madness on which cylinder does not fire so it is possible to get one cylinder not fire many times in a row. This loads up that cylinder real bad. The software controlled limiters such as what is used in the Digital 7 ignition systems and the MSD 8973 digital PrMag controller are "Soft Touch" limiters and do not sound as harsh plus they do not ever not fire a cylinder twice in a row. Regardless of which one it is I still hold my breath and shut my eyes when the car is on a Two Step.

 

It just sounds scary and looks bad with all the unburned gases shooting out. Don't get me wrong because I have never had a problem with a car on a Two Step.

 

All magnetos have an inherent characteristic of retarding timing as the ignition load on the magneto increases. This retarding of the ignition is due to the inability to build and collapse the inductive field in the coil at a rate that will keep up with the load of the motor. The amount the magneto is behind or retarded equates to about four degrees total at high load conditions. If a standard magneto such as the Mallory points type is timed at 36* at idle the ignition will be retarded to around 32* during high load conditions. This means if you really wanted 32* under load one had to learn to set the points magneto at idle on 36*. Racers learned to live with that and they just knew where to set them to make the best power.

The points style magnetos under no load conditions such as idle can produce voltage at a fairly fast rate but when put under extreme loads the rise time of the voltage into the coil slows way down. The ProMag 44 has a much faster rise time for the voltage going to the coil so the collapse of the field in the coil also occurs at a faster rate. When under load this results in the spark being initiated at a much faster rate than the points magnetos. This faster build up of the spark results in the spark being able to jump the sparkplug gap faster when the cylinder pressure is higher due to extreme loading. This in turn initiates the flame front in the cylinder at a faster rate so the burn rate of the fuel in the cylinder occurs at a faster rate and less advance timing is required.

The MSD ProMag 44 magneto generator still has this characteristic retarding of the ignition as the load increases but MSD coupled it with the Electronic Points Box which contains a magneto timing compensation circuit to eliminate the retard. Automatic compensation is accomplished by the Points Box initially applying retard to the timing at idle and as ignition load is increased the timing is applied back as ignition advance at a rate equal to the amount being retarded. This automatic compensation offsets the ignition timing change by the magneto so the timing remains at one setting in all conditions. If you want 32* at high load conditions you could now set the ProMag 44 at 32* at idle and it would remain where set throughout the entire load range.

This ProMag 44 compensation feature and the improved characteristics of voltage rise time in the coil is why we now set the timing at idle at a lesser amount of approximately 4* on ProMag 44 equipped motors than we did before on the motors with the points magnetos.