|Quote, originally posted by the12for12 »|
However, the MSD has a nice built in feature that is not found while reading the instruction manual. By reversing the magnetic leads, the MSD can give you a nice timing advance curve. You simply set the ignition at you want at idle (stock is around 10-15 degrees BTDC). As you blimp the throttle, you will find that the timing jumps to around 20-35 degrees BTDC. ,
I found this in a search, and I thought I would comment.
I don't know for sure what is going on inside the MSD box to cause this, but I have a guess. If my guess is right, this probably isn't the best idea for controlling advance.
As the trigger wheel aproaches the sensor, a positive voltage is generated. As the trigger nears the point where it is closest to the sensor, the voltage rapidly drops and goes negative. (This is the event you want to capture) As the wheel moves away from the sensor, the negative voltage increases as it tends toward the steady state of about 0V.
If you switch the wires, then you get a voltage response that goes negative, jumps positive, then settles to zero. The MSD unit is probably triggered by a negative going zero crossing. If you have it connected correctly, this corresponds to the point where the trigger wheel is closest. This part of the voltage output curve is very steep and doesn't vary much with RPM -- so the output from the detection circuit is pretty stable.
Switching the wires means that the detection circuit will *not* trigger on the nice crisp transition because it will be a "positive going" transition (starts negative, and goes positive). Instead the circuit will trigger sometime later as the voltage settles back down to near zero. Unfortunately this part of the curve both varies with RPM and isn't very steep. The "varies with RPM" part is what makes it advance the timing. The fact that it isn't a steep curve means that slight variations (including the temperature of the electronics, external "noise", etc.) can have a significant effect on when the circuit will trigger.
This may be a bit far afield for a carb FAQ, but I thought I would add it. Keep in mind that this is again just my guess based on my understanding of how the ignition system works. And while I'm pretty confident that switching the polarity of the wires is *not* a standard operating mode -- if it works, and it's consistent..go for it. Just be aware of potential problems (I would expect inconsistent advance values)