Does anybody know the inductance value for 70s era Mopar ignition coils? I am trying to calculate dwell in milliseconds for the stock LA coil for Megasquirt ignition control.
Specifically, the coil is the stock one that came with my 73 Cuda.
Thanks,
Craig

I too am running a MSII and throttle body injection with ignition control on my 73 Cuda but chucked the standard ign coil in favor of an E-coil to help deal with underhood EMI since they are supposed to be better for that. Currently using a small dwell value of 2.5ms maximum for this coil. I don't know if you can go higher than that for the oil-filled stock coil but I suspect you could. Dwell (actually coil charge on the primary winding) is very dependent on battery/alternator voltage seen by the coil and thus the need for the correction table for dwell vs voltage in Megasquirt programs. I have had lots of problems with internal distributor cap flashover from too much coil secondary voltage (too much dwell) at higher rpm and spark advance values. The distributor rotor phase position must be perfect to cure this and a stock Mopar distributor is not easily re-phased. I gave up on the stock Mopar spark pickup and ign box and went with an aftermarket small cap GM distributor system with a GM pickup and hid the GM ign module cleverly in a gutted old mopar ign box mounted away from the hot engine bay. You can get small or large cap GM HEI distributors for Mopar applications but I suspect the bigger distributor cap ones (with their in-cap coils and better flashover prevention rotor features) will hit your firewall or wiper motor... That however is my next move before going distributorless which does away with cap phase and flashover concerns...

Thanks for the input. I guess ideally should go to coil on plug.
I did a spread sheet using values of inductance from 4 to 8 mH for the coil and the factory spec for the ignition of 1.9 amps and that ranged from 1.9 to 2.5 ms of dwell. There was a slight miss at 2.5. Went up to 2.9 and all is well.
Craig

Bought an LCR meter so if anyone else cares, the inductance of a stock coil, at least for 1973, but probably in the ball park for all Mopar ignitions of the era is 7.5mH.
With the factory spec of 1.9 Amps, and a total resistance of 2.1 ohms, the dwell would come in at 1.46msec. Rounded that up to 2.
Craig

For Info,
Here is what I measured on my 1995 model year stock Ram ignition coil a few years ago.

Number on side of coil: 56027966

Primary Resistance 0.99 ohms
Primary Inductance 6.25 milliHenrys
Secondary Resistance 12,250 ohms
Secondary Inductance 41.9 Henrys

I put 6.3 volts of 60 cycle AC on the primary of the stock coil and got 453
volts AC on the secondary. The turns ratio of the stock coil might be about
453/6.3 = 72

A lower turns ratio can be good in that it prolongs the 'burn time' of the
spark
especially at high rpms.

A higher turns ratio can provide a higher voltage to get the spark to jump
but most of the time a higher voltage is not needed. Exceptions are
when the plug is fouled with oil or gasoline, the tip edges have become dull,
or when the tips are cold when the engine is first started.

Above measured with a seventeen year old BK Precision LCR Meter model number
875A and a Fluke 87.

The 1995 FSManual says the coils are made either by Diamond or ToyoDenso.

For the Diamond the primary resistance is supposed to be between .97 and 1.18
ohms and the secondary 11,300 to 15,300 ohms.

For the Toyodenso the pri resis is supposed to be between 0.95 and 1.20 ohms
and the sec 11,300 to 13,300 ohms

Thanks for posting that. It may save someone a lot of work getting their Megasquirt running.
In 1973, the choice was between Prestolite and Essex. By part number, mine is a Prestolite.
BTW, my issue with timing and dwell was with the reluctor pickup wired backwards from the distributor. The timing signal was then out of phase.
Craig