Questions (mostly ignition)

[Jim Conforti]

On Mon, 20 Mar 1995, Bill Lewis wrote:

> While trying to decide what approach is best for my project, I've been
> wondering about a few things...
> 
> 4) Do Bosch Motronic systems that take pulses off the flywheel ring gear count
>    these pulses and trigger events at a particular tooth number?  Are these
>    pulses used to get a better idea of the engines acceleration?  A 911 has a
>    130 tooth ring gear.  That's a little less than 3 degrees per tooth.
>    Do they use a software PLL to synthesize 'extra' teeth?

  AHHH .. Finally a question I can answer ;) ...

  Motronic systems first compute the required ign. adv. thru a set of maps
  which are basically functions of conditions like rpm, load, engine temp,
  and battery voltage .. once the ECU has figured out what it wants to do,
  it sets a single variable, and lets the interrupt routines of the uProc.
  determine "how many teeth" should pass before firing the coil, injectors
  etc etc etc ...

  It really just counts teeth ... well, HALF teeth actually, both on the
  rising and falling edges ... so if your car has 130 teeth (you're SURE it's
  not 116 ?!?) ... then the resolution of the spark is ... 360/(130*2)
  or 1.38 degrees ...

  Some cars use a 60 tooth sensor wheel, that is missing 2 teeth ....

  On these cars, the ECU first counts teeth, and then counts TIME until
  the spark is needed to attain a resolution of 0.75 degrees or so ..

> 5) When the knock sensor kicks in, how much do factory systems typically
>    back off the timing?

  Depends ... some 3, some 6 degrees, some more than that .....

  But the basic premise is to retard a BUNCH and then slowly crank the 
  knock adaption to timing back up .... eventually, the system should
  find a happy medium ... in performance work it is BAD to hit the 
  knock sensor ... you can FEEL the power loss ...

> 6) What is a good minimum RPM for a system?  i.e. what is cranking speed?

  Cranking speed is anything over about 10rpms and below idle

  In fact, many ECU's detect "start" mode by hanging in a loop until 
  the "teeth" fly by within a certain time and then set a start bit ...

  This bit is then cleared when the rpms reach a certain temp dependent
  preset rpm which is usualy a bit below the idle ...


  Jim Conforti
  <jec at us.dynix.com>