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To control your coil for maximum performance and reliability, you must set the maximum dwell parameter for your MicroSquirt® controller. A couple of points are that few automotive coils are designed for more than about 6-6.5 Amps, and reduced coil resistance has been used to shorten dwell times as engine rpm limits have risen over the years (shorter dwells means higher rpms before the dwell is cut back), so as a result most coils are between 2.0 and 4.0 milliseconds, with a general trend towards shorter times for more recent designs and longer times for older designs (wasted spark and coil on plug can be different, of course, as they fire less often and thus don't get cut back until a much higher rpm).
There are four ways to set the dwell:
To save a bit of calculation, you can enter your coil resistance, inductance and current in the form below and press the "Compute Dwell" button (for the dwell at 12 Volts).
In one case, the values for a 7-pin HEI large cap set-up with red/yellow leads are:
The values give a calculated time (T) of 3.45 milliseconds.
In the car, it was found that 3.4 worked, but it was set to 3.5 to be sure of an optimal spark under all conditions. At 3.3 or less, this set-up would generate some misfires, especially under 1000 rpm. Note that the 'full DC current flow' would be 12/0.4 = 30 Amps.
The current creates a magnetic field ("charging the coil"), and a change in magnetic field creates a current (we use that to make the spark). In order for the current to increase, it must increase the magnetic field. But if the magnetic filed is already saturated, the current can't increase (without wasting the same amount of energy as heat), so the current increases more slowly (and makes the coil hot). So typically you see a quick rise while the field builds, then a fairly suddenly change in the rate of rise (but still rising) when the field is saturated.
If you plot some values from the equation below (for the 7-pin HEI example) you will see this effect:
Note that it takes 6.3 milliseconds to reach 10 Amps, but 17.0 (2.7x) to reach 20 Amps. Similarly, the coil reaches 6 Amps in 3.45 milliseconds, but takes 8 times as long to reach 4.16 times the current (25 Amps).
If the current was left to build, it would eventually reach the full DC value - if the coil didn't burn up first, of course. Note that the approach to 30 Amps is 'asymtotic', meaning it never quite gets there and you can't enter 30 Amps in the above example, you will get an error trying to find the natural logarithm. The magnetic field strength would not increase significantly though, and the spark would not be stronger.
To set the proper current limit for the coil, you can measure the voltage across R43 (with a scope, measuring the peak voltage at the top of the rise = peak current) and divide by R43's resistance (.01 Ohm) that's the current. For example, on an in-cap HEI coil and a dwell of 3.5 milliseconds, you would get:
The way to set the dwell is to measure this current and set the dwell to the point at which the current reaches the 'proper value' for the coil (from the spec sheet, etc.). Now the hard part is knowing the 'proper' value for the coil. Some coils will have this listed as a spec (try googling). For others, you might assume the correct coil current is between 3 and 5 Amps, though there's often little evidence to choose a figure like this. If you don't know the 'proper value' then you can use trial and error to determine it (next).
There is a good article on dwell settings, with 'scope shots, here: www.tonyfoale.com/Articles/Ignition/Ignition.htm.