gcc, 68K and ignition advance

[Wouter de Waal]

OK, I (finally) got gcc to work. I compiled it under DJGPP, which means
that it runs on a PC. So now I can develop code on my PC, then stick
it in my 68K system. After I went through all the pain of building
gcc, I heard that pre-ported versions are available from ftp.pooh.com.
So if anybody is interested, you can (definitely) get the stuff from me,
but it might not work 100% yet, or you can (maybe) get the stuff off the
net, but I assume that if it's posted, it'll work.

The car that is the target of all this fun is currently on blocks, I'm
doing some fibreglass work on it to increase airflow to the (rear
mounted) engine. The thing sucks +- 950 liters/second @ 6000rpm, so
it needs BIG air channels.

The first thing that I would like to start playing with is ignition
timing. I've been thinking about why ignition advance is required, and I
can't seem to make my understanding tie up with what is commonly done.
So would some of you that have thought about this more thoroughly please
enlighten me?

My thoughts are as follows:

* The air/fuel mix takes a constant _time_ to burn.
* The max. pressure point has to occur when the piston is on it's
  way down, and when the conrod has a decent leverage on the crank.
* Assume idle = 1000rpm, and advance = 10 degrees @ idle. 1000rpm =
  6000 degrees per second, so the spark occurs 1/600 second btdc.
* Assume that we want the maximum pressure at 10 degrees atdc. (Please,
  this is the wildest guess I've made in a long time)
* The spark occurs 1/300 second before the peak in pressure.
* At say 2000 rpm = 12000 degrees/second, 1/300 second = 40 degrees, so
  the advance should be 30 degrees.
* In the same way, at 6000 rpm the advance should be 110 degrees (!).
* Now obviously, you can't fire the plug before the inlet valve is closed.
  But this should have happened at say 160 degrees BTDC. So this is not
  why most advances seems to be limited to about 30 or 40 degrees. The
  obvious question is why _I_ think an advance of 100 degrees is fine,
  whereas my VW manual only allows 30 degrees, which is "all in" by
  3000rpm. Somebody please clarify!
* And the next tricky point - vacuum advance. The vacuum is an indication
  of engine load. But why is it a good thing to advance the timing when
  the engine is loaded? By how much should the timing typically be advanced
  for a given load? (And I mean _typically_, I know that the Bosch systems
  amongst others use complicated, dyno-derived maps. However, I am more
  interested in deriving the kind of curve a standard distributor gives.)

OK. If that's not enough, I have plenty more questions :-)

Thanks

Wouter