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V3 MicroSquirt® - QuickStart Guide

Determining Your Maximum Dwell Setting

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:

  1. Calculate it: If you know the DC resistance and inductance of the coil's primary circuit (either from a spec sheet or from measuring these with an LCR meter), you can use the following formula for an idea inductor to calculate what the dwell should be. The formula for the time it takes for an inductor to charge up to a certain current (assume t=0, I=0) is:

    T = (-L/R) * ln( 1 - (R * I / E))


    (ln is the 'natural logarithm, often available as 'LN' on calculators)

    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.

  2. Oscilloscope: If you have access to an oscilloscope and have installed R43 on a V3 main board (and you are using the VB921 direct coil driver), you can use the voltage drop across this 0.01 Ohm resistor to monitor the current flow through the coil. You are aiming to keep the dwell down to the point of where it just starts current limiting, which you will see as a 'bend' from a rising voltage towards a flat line in the voltage trace on the scope..The time at which that bend occurs should be the dwell point (though this is subjective). That tells you what the max dwell is for the VB921, but this isn't necessarily optimal for the coil.

    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:

    1.9 division * 0.020 Volt/div / 0.01 = 3.8 Amps

    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:

  3. Trial and Error on the Vehicle: To set the dwell, you want to set it as low as you can without misfiring at idle and low engine speeds. Generally this is between 2.0 and 4.0 milliseconds. If you start at 3.0 and have no misfires, try reducing it a bit (0.1 milliseconds) at a time until you get misfires, then raise it 0.2 milliseconds. If you get misfires at 3.0, raise it a bit at a time until the misfires are gone (then add 0.2 milliseconds). If this seems familiar, it is very similar to the process of setting the PWM% for low impedance injectors.

  4. Other sources of information: Ask someone on the forums, look in a factory service manual, or Google to see what might be reasonable dwell values for your coil. For example, the GM 7-pin HEI large cap coil should be 3.5 milliseconds, the GM 8-pin HEI external coil should be 2.5 milliseconds.

If you have any questions or problems that can't be answered from the links above, or a search the MicroSquirt® manual:

you can ask questions at the MicroSquirt® support forum which is at: Click the links for more information.

MegaSquirt® and MicroSquirt® controllers are experimental devices intended for educational purposes.
MegaSquirt® and MicroSquirt® controllers are not for sale or use on pollution controlled vehicles. Check the laws that apply in your locality to determine if using a V3 MicroSquirt® or MicroSquirt® controller is legal for your application.
©2011 Bruce Bowling and Al Grippo. All rights reserved. V3 MicroSquirt® and MicroSquirt® are registered trademarks. This document is solely for the support of V3 MicroSquirt® boards from Bowling and Grippo.