M48 dwell time at high rpm

[Bruce Bogtrotter]

I am going to build an ignition module from scratch for a friend to connect to his M48 ECU. This is going to be a microprocessor controlled "smart module" that measures the coil current and sets the dwell time accordingly. Say we set the ECU dwell figure for a fixed 6mS right across the scale, regardless of rpm and battery voltage, then I read somewhere that the ECU forces a minimum coil offtime of 0.3mS What I need to know though is this - as you hit that 0.3mS gap in the pulses, does that 0.3mS then just stay constant and the 6mS hi time reduce with rpm increase, or alternatively as you =approach= that 0.3mS gap does the 6mS hi time start reducing and the 6mS/remainder mark-space reduce proportionally till you get to the 0.3mS minimum offtime.

Unfortunately I don't have access to the ECU at the moment or I would rig it up on the bench and measure it myself.

[Bruce Bogtrotter]

* bump *

[Bruce Bogtrotter]

Wow! Lucky I didn't ask a hard question.

[SprinterTRD]

I guess most people are thinking why would you do this?
The point of the dwell time is to limit the coil current anyway. You just need to test the coil work out what it needs and then enter the values into the coil dwell table in the ECU.

Your adding another level of electronics that is not required.

[Bruce Bogtrotter]

Hi SprinterTRD.
With all due respect, what you are saying is only half correct. The coil we are using has a 5.5mH 0.6 ohm primary and with a 15 volt bench supply it takes 4.5 mS to go from zero to 7 amps. So far, real simple. What happens as revs start to climb is you have to turn the current back on before the spark has finished arcing and then all bets are off as regards a fixed dwell time. You get a situation mathematicians call a "right hand plane zero" http://www.google.com.au/#hl=en&biw=144 ... fe9960e500 where the coil current becomes very difficult to control. Achieving optimum dwell control at all revs is a non-trivial task.

"Your adding another level of electronics that is not required."
Not so. As the coil heats up it's primary resistance increases, giving the need for additional dwell time. e.g copper resistance increases by about 50% with a change in tepperature from 15 deg C to 100 deg C IIRC.

[SprinterTRD]

It must be a serious high reving engine if you need to start dwelling a coil 4.5mS after you just fired the coil!
Is there only one coil on a multy cylinder engine?
If it is then select a coil with lower dwell requirements or go multi coil.

[Bruce Bogtrotter]

A coil with lower dwell requirements means lower primary inductance, which for the same amps means proportionally lower stored energy in the coil. Anyway, if someone would just answer my original question I would be most pleased.

[SprinterTRD]

The dwell calculation is effected by which REF/Sync mode you are using.
In multy tooth mode both the off time and dwell time period is effected by engine RPM. So the off time is not a constant.

Possible products that may save you some time
http://www.iceignition.com/

[Bruce Bogtrotter]

Wow! They look pretty good. I just read somewhere else that their stuff was on the 1st and 2nd placegetters in the Engine Masters Challenge in the US this week.
http://www.popularhotrodding.com/engine ... to_05.html