Spark timing,points/Hall effect

[Gary Derian]

	You answered your own question.  The California engine has a catalytic
converter.  The dual diaphragm Bosch distributors used ported vacuum for
the advance but manifold vacuum for the retard.  At idle, the distributor
is in full retard.  At cruise, the retard diaphragm has no effect because
its function is to move the zero vacuum stop in the vacuum can assembly
towards the retard side.  When there is vacuum pulling some advance, the
position of the stop does not affect the advance.  When there is no vacuum
advance (idle) moving the stop does provide more retard.

	Non catalytic cars had retarded spark and very lean mixture to get low
emissions.  When a catalyst was added, emissions were even lower and the
engine could be tuned for efficiency.

Gary Derian <gderian at cybergate.net>

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> From: Mario Tito <mario at swingbridge.com>
> To: fi discuss 2 <diy_efi at coulomb.eng.ohio-state.edu>
> Subject: Spark timing,points/Hall effect
> Date: Wednesday, 17 September, 1997 12:53 PM
> 
> 
>    I have a question in stock spark timing. Looking through the Bently 
> manual for my 2.0L VW air-cooled, compression 8.5:1, 125psi(looks low, 
> compared to others on the list), and there are a few different 
> distributors over the years of engine changes, like size change, 
> different enviroment laws, etc., with different advance curves, and idle 
> timing, which all make some sense. BUT in the California engine, they 
> used a Hall effect sensor ignition, the ignition timing was changed, what

> seems to me, drastically. My point- and-condenser system works best at 
> stock 7.5deg BTDC at idle, ported vacuum advance (I'm not sure of this 
> term- the vacuum port is blocked by the throttle plate at idle position, 
> and uncovers just off idle. Right?). The earliest timing at WOT (no 
> vacuum), 3400 RPM, is 28-32 degrees BTDC. Ported vacuum starts at 80-130 
> mmHg, ends at 200mmHg, advancing 8.5-11 deg.
>    The Hall Effect Ignition is set at 5 deg ATDC at idle, with vacuum 
> advance and retard diaphrams. Total advance at WOT (no vacuum), at 3600 
> RPM is only 16-20 degees BTDC. The Bently manual is suprisingly lacking 
> in where these vacuum signals come from. But WOT has no vacuum, right? 
> Vacuum advance and retard both start at 60-120mmHg, ends at 240mmHg, and 
> both change timing 11-13 deg. The Advance has a 'cut- off' which enables 
> it only in fourth gear or if air temp is below 54F. This cut- off system 
> is described only in the older dual- vacuum point system, but I guess it 
> applies to this as well.
>    The year prior to mine, with points and two vacuum chambers, is 
> roughly the same as the Hall- Effect system. 
>    Does anyone have a clue as to why in a mostly equal engines (the cali 
> does have a cat converter), this difference exists? Am I missing 
> something?
>   Perhaps a discussion will help Tom C. as well.
>   Thanks
> 
> Mario T.
> mailto:Freshmar at aol.com-----'76 VW Camper FI A/T,,'79 Fiat X1/9