Spark timing,points/Hall effect

[Mario Tito]

>	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>
>
>----------
>> 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
>


I didn't really answer my own question -the cali version did have the 
cat, and the dual-chamber vacuum thingy and the idle timing set at 5 deg 
atdc, Hall-Effect ignition.  But, the model before mine did not have cat, 
nor Hall, but was timed similarly to this one. My model has points, a 
single vacuum advance, but it is set at 7.5 deg btdc at idle. What am I 
missing? I am thinking of a computer ignition, and I want to understand 
more of what I need to do- without using a 'advance-till-it-knocks' 
possibly destructive method. Thanks everybody.

Mario T.
mailto:Freshmar at aol.com-----'76 VW Camper FI A/T,,'79 Fiat X1/9