ignition coils?

[Mike Blakey]

Sounds like a good DIY project!

At the moment I use the flipflops such that the Hall switch
turns "on" the "current" cylinder and turns "off" the "previous" cylinder.
That means I have 60 degrees for coil charge and discharge to occur.
Unfortunately, there's not enough time at high RPM to get a full charge, so
I have to be able to select a coil and keep it charging thru 120 degrees
instead...)

Why not use the "previous" -1 to turn on the coil, then you will have 120deg to 
'charge' the coil.

The only other thing to be careful of is too much dwell, the small individual 
coil-per-plug coils will not tolerate a high DC current long after being saturated.
If you can 'advance' your timing a further 1.2ms, then trigger a monostable to 
*just* charge the coil for 1.2ms. you will also reduce the load considerably too.

Good luck, with the project.

Mike.





hyc at highlandsun.com on 15/10/2001 11:13:26
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Subject:	ignition coils?

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Sorry if this is off-topic, but you guys seem to be the ones who'd know -
I'd like to get hold of a set of pencil ignition coils, to try to use them
in my '97 Probe GTS. I.e., I want to convert my current electronic
distributor ignition to direct ignition. I've worked out my trigger circuit
but I haven't found anyone who sells the actual coils at a rational price.
Do you know where I might be able to order these things?

I was originally planning to switch the output from a CDI kit into the
individual coils,  because then I wouldn't have to worry much about dwell
time at high RPM. But I'm not confident that I can switch a +/-400V output
reliably (using TRIACs?) so I think I'm going to have to go back to an
inductive design.

(The basic plan here - mount 6 Hall switches in my distributor cap, and a
small permanent magnet on the end of the rotor. Disconnect power from the
distributor, remove the internal coil. Using Allegro A3141 Hall switches,
triggering CD4044 flipflops to turn on a transistor to select an individual
coil. The stock ignition trigger signal from the ECU still controls
charging/discharging the selected coil. Except, if I scrap the CDI kit, my
switching logic gets more complicated. The A3141 switch is
momentary-on. At the moment I use the flipflops such that the Hall switch
turns "on" the "current" cylinder and turns "off" the "previous" cylinder.
That means I have 60 degrees for coil charge and discharge to occur.
Unfortunately, there's not enough time at high RPM to get a full charge, so
I have to be able to select a coil and keep it charging thru 120 degrees
instead...)

While browsing thru the archives for this list I noticed a discussion on
replacing an AFM with a MAF. This has been on my mind too, since this Mazda
KL engine also uses a pretty restrictive volume measurement sensor. The MAF
and VAF transfer functions are pretty much completely opposite in slope. I
decided to try just going MAF -> ADC -> EEPROM -> DAC. No
processor/controller of any kind, just a fixed mapping in the EEPROMs.
Unfortunately this setup doesn't take temperature into account, and the
stock ECU uses the IAT to convert the VAF input to mass. I started looking
at analog multipliers to take this into account, but the IAT's output is
only linear for part of its range, which tells me I need another EEPROM to
map that out, and a CPU of some kind to do the compensation. I've left
things alone for the time being; I think it will make more sense to just
reprogram the stock ECU to accept a MAF input. Does anyone have any good
info on programming a Mazda OBDII ECU?

  -- Howard Chu
  Chief Architect, Symas Corp.       Director, Highland Sun
  http://www.symas.com               http://highlandsun.com/hyc

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