zero10 wrote: Sorry this is somewhat unrelated but I would like to hop in with a question about the GM HEI setup. In the documentation it is mentioned:
notes wrote: One solution is to not use the W terminal at all and provide a constant 1.4v bias to the pickup externally. The .2v extra bias helps during startup, and by disconnecting the W terminal, the 10k coupling resistor does not bleed off signal, which helps to increase the signal to the G terminal. This results in about 8ms dwell at 1500 RPM which decreases to about 2ms by 10,000 RPM. When a constant DC bias of .7v is used, the dwell remains at approximately 2ms to 2.5ms at 1500 RPM and stays relatively constant to 10,000 RPM where it ends up at about 2.1ms. However, .7v is not enough during start up, and a temporary 1.4v bias will be needed for start up.
The latter part of that text is for general notes on the HEI modules. With the stock KZ pickups and coils, you would use the two diodes with one resistor to get a 1.6 to 1.4v bias as mentioned in the earlier part.
I mentioned the .7v bias as that would be useful when using very low resistance coils, like ones found on cars.
zero10 wrote: Then checking the schematic there are 2 diodes in series to provide a static voltage for the stator's negative terminal.
Correct. But it should be "reluctance pickups", or just "pickups", negative terminal. I think stator would imply a rotating magnetic field, but in this case the field is not rotating. The rotor, by rotating, brings the static field closer and farther from the pickup. This produces a pulsating magnetic field, but not necessarily a rotating one. The signal from a rotating field would look different. I think it would have two peaks and two valleys per crank revolution, whereas the pulsating field has one peak and one valley. That way, there is always only one transition from positive to negative.
zero10 wrote: I don't have a spec sheet handy for the diodes mentioned but what is their forward voltage drop? I would expect around 0.7V each for voltage of approximately 1.4V on the negative terminal for the stator.
Correct, but it will be somewhere between 1.3 and 1.4v. Those are rectifying diodes meant to handle a decent amount of current (1A), but when running lower current (10 to 14 mA) , the voltage on one diode is a little lower than .7v. Temperature will also affect the final voltage, but the voltage is not super critical. Probably anywhere from 1.2 to 1.4v would work.
zero10 wrote: That means you will be in the situation where the dwell is down to 2ms at 10,000RPM, correct?
Yes.
zero10 wrote: If so, does that have any impact on how the bike runs?
Not on a stock bike.
If the compression is much higher than stock, or the spark gap is wider than stock, and you are using higher resistance coils, like 3-ohms or higher, then you might lose ignition at higher RPMs with wider throttle openings. It will stumble and possibly not fire at all. But will likely idle and run just fine when the rider is being real easy on RPM and throttle.
zero10 wrote: How does that compare to the stock ignition's dwell at those speeds?
Same as stock at high RPM, but slightly more efficient at lower RPM's, by having less excess dwell.
The stock coils need about 6 msec to reach 90% current saturation.
3-ohm coils need about 12 msec to get to 90% current saturation. (Accel super coils were tested. Dyna's are likely similar.)
90% is somewhat arbitrary, but is likely a reasonable target for these type of coils. (You would not use 90% on low-resistance coils, as that will likely damage the coils.)
Stock dwell goes from about 100 deg at idle and jumps to 120 degrees off idle, then stays at about 120 through redline. That is about 14 msec at idle, and 2 msec at 10k RPM.
The HEI does better at idle than stock. It can be as low as 60 deg at idle, but is usually closer to 80 deg. That is about 11 msec at 1200 RPM. (It is still 120 deg at 10K RPM, which is 2 msec.)
