Questions (mostly ignition)

[Peter Wales]

The more I read questions from people in this group, the more impressed I
get with the different way of looking at things people seem to have. 


At 09:28 AM 3/20/95 -0500, Bill wrote:

>While trying to decide what approach is best for my project, I've been
>wondering about a few things...
>
>1) Just how sensitive is the ignition timing?  Can you tell a difference
>   between 1 or 2 degrees?  What about 1/2 a degree?

When reprogramming vehicles for power, the biggest effect comes at peak BMEP
usually around 4000RPM. At that point, changes of about 1 degree affect the
performance. Above and below that point, changes of 1-2 degrees are needed
to do anything. This is on full throttle only of course.

>
>2) I've always thought the whole idea of spark advance was to give the mixture
>   time to get really burning.  And that the idea of advance in degrees BTDC
>   was an artifact of the distributor - what you really wanted was time BTDC.
>   
>   Why do centrifugal advance curves typically max out well below the redline?

Could the anser be that you need to have the ignition advance increase upto
peak BMEP and then holding it is good enough?

>   Do we really want the ignition point in terms of crankshaft degrees, or
>   would it be better in terms of time?

Good question!


>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?

I can only answer the last part. No. Consider the 4 tooth unit fitted to
Weber Marelli systems. The tooth at 90 deg BTDC is the one that the CPU
waits for, then it triggers the counter with the exact timing delay in it.
Which one triggers the timer in the Bosch system I do not know.


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

8 Deg

The other questions, other people will be able to answer better than I


Peter