Date: Fri, 20 Apr 2001 18:28:09 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Timing theory
In-Reply-To: <Pine.GSO.4.21.0104201431130.19206-100000@sunfish.math.utah .edu>
Content-Type: text/plain; charset="us-ascii"; format=flowed
At 04:39 PM 4/20/2001, Blake Thornton wrote:
>What determines when the plug fires? I had thought that when the
>distributor rotates and makes contact with the cap, then this is exactly
>when the plug fired. But now I read what David wrote and he said the the
>digifant engines the ECU controls the timing (i.e. the exact moment of the
>spark) and the distributor mearly points to which plug is going to fire.
Distributor has two sections, primary and secondary. Secondary is the
obvious part -- takes in high voltage in the middle of the cap and sends it
to one of the side terminals. It does this by passing a conductor near but
not touching the contacts in the cap -- that's the function of the rotor.
Primary i.e. low-voltage side provides timing info to the spark-generating
circuit via the Hall-effect generator (in Digi*), an optical encoder, a
variable-reluctance pickup, or points. This timing info may be modified by
centrifugal advance and vacuum advance/retard, and in the case of
points+condenser it also generates the actual impulse to fire the coil.
>Also, in my 84, I think can adjust the timing by rotating the distributor
>around. I thought that this would change the the firing point. I thought
>that the timing was adjusted in the digifant engines exactly the same way,
>but if the ECU controls the timing then there must be something different
>going on here.
The digifant ECU still takes basic timing info from the distributor, but it
generates the advance curve internally instead of using mechanical devices
in the distributor to do it.
>And, how about advances. My 84 has a vacuum advance built into the
>distributor, does the digifant (I guess I could look in the bentley).
No, it gives the same timing info regardless of other conditions -- much
simpler distributor mechanically.
> How
>about the centrifugal advance (which I think advances for higher rpms and
>is also built into the distributor)?
ditto
>And finally, surely there are cars without distributors. Do they have a
>separate ignition coil for each cylinder?
Actually they seem to have one coil for every two cylinders. I don't know
for sure but I strongly presume that they're paired so that one cylinder is
ready to fire and the other couldn't possibly...
david
>thanks,
>Blake
>84 Westy Vanagon
>
>I put in the vanagon so that this would make it through to those of you
>filtering mail with this word in it :)
David Beierl - Providence, RI
http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
