MicroSquirt.com read only forum (Jan.15,2018)

Would the resistance and inductance be measured off the Primary Winding or the secondary winding?

I'm trying to use Bruce's formula to compute the dwell setting for my ignition coil. Here are the values for my coil.

L=9.5 mH
R=1.55 Ohm

I have input several values of I. Here are the corresponding values for T.


I_______T
(amps) (ms)

1______.85
2______1.83
3______3.00
4______4.46
5______6.36
6______9.14
7_____14.37
7.7____32

How do I decide which pair of I and T is the best to use?

I graphed it in Excel, but there is no obvious point of inflection on the curve.

And for future reference, what is the right way to put a simple chart (like the one above) into a post?

Thanks for your help and advice.

Blue wrote:I'm trying to use Bruce's formula to compute the dwell setting for my ignition coil. Here are the values for my coil.

L=9.5 mH
R=1.55 Ohm

I have input several values of I. Here are the corresponding values for T.


I_______T
(amps) (ms)

1______.85
2______1.83
3______3.00
4______4.46
5______6.36
6______9.14
7_____14.37
7.7____32

How do I decide which pair of I and T is the best to use?

I graphed it in Excel, but there is no obvious point of inflection on the curve.

And for future reference, what is the right way to put a simple chart (like the one above) into a post?

Thanks for your help and advice.

For this coil it appears to have a slow charge time. You want a primary current around 5 - 6 amps before spark, but this leads to a dwell time of close to 9ms which will limit your upper RPM. You may want to see if 4 amps (4.5ms) works for your setup..

BTW, your chart was fine

- Bruce

Thanks Bruce. I'll try that and post my results back to the forum.

Hi, i tried using this formula but the actual dwell time vary a lot with theoritical values do u have any correction factors for this?