Dwell calculations
Posted: Fri Mar 25, 2005 2:41 pm
Hi all,
I have used Bruce's formula for evaluating the dwell period on my ignition, one I had already tuned in the car (http://www.msefi.com/viewtopic.php?t=63 ... c&start=45).
Bruce's formula (I suppose it *may* have been derived prior to his use of it!) for the time it takes for an inductor to charge up to a certain current (assume t=0, I=0) is:
In my case, the values for my 7-pin HEI large cap set-up with red/yellow leads* are:
L = 6.18 mH
R = 0.4 Ohms
E = 12.0 Volts
I = 6.0 Amps
This gives a calculated T of 3.44 milliseconds.
In the car, I found that 3.4 worked, but set it to 3.5 to be sure. At 3.3 or less, I would get misfires, especially under 1000 rpm. So it certainly seems to confirm the theory.
I am now in the process of installing an 8-pin module/distributor and external coil. This external coil has an inductance of 4.285mH and a DC resistance of 0.4 Ohms. So for an initial coil dwell, I will try:
T = (-L/R) * ln( 1 - (R * I / E))
= (-4.285/0.4) * ln (1-[0.4*6.0/12.0])
= -10.7 * ln (0.8)
= 2.39 milliseconds
I will use 2.4msec to start, and let people know how this works out. <update> I ended up using 2.5 milliseconds, which is close enough to the theory!</update>
(Eventually I will add a dwell calculator based on Bruce's formula to the documents, as well as specs for as many coils as I can lay my hands on!)
Lance.
I have used Bruce's formula for evaluating the dwell period on my ignition, one I had already tuned in the car (http://www.msefi.com/viewtopic.php?t=63 ... c&start=45).
Bruce's formula (I suppose it *may* have been derived prior to his use of it!) for the time it takes for an inductor to charge up to a certain current (assume t=0, I=0) is:
Code: Select all
T = (-L/R) * ln( 1 - (R * I / E))
Where:
T = time (sec)
L = inductance (H)
R = resistance (Ohms)
E = voltage (Volts)
(ln is the 'natural logarithm, often available as 'LN' on calculators)
L = 6.18 mH
R = 0.4 Ohms
E = 12.0 Volts
I = 6.0 Amps
This gives a calculated T of 3.44 milliseconds.
In the car, I found that 3.4 worked, but set it to 3.5 to be sure. At 3.3 or less, I would get misfires, especially under 1000 rpm. So it certainly seems to confirm the theory.
I am now in the process of installing an 8-pin module/distributor and external coil. This external coil has an inductance of 4.285mH and a DC resistance of 0.4 Ohms. So for an initial coil dwell, I will try:
T = (-L/R) * ln( 1 - (R * I / E))
= (-4.285/0.4) * ln (1-[0.4*6.0/12.0])
= -10.7 * ln (0.8)
= 2.39 milliseconds
I will use 2.4msec to start, and let people know how this works out. <update> I ended up using 2.5 milliseconds, which is close enough to the theory!</update>
(Eventually I will add a dwell calculator based on Bruce's formula to the documents, as well as specs for as many coils as I can lay my hands on!)
Lance.