Hello,
A couple of questions/interogations here:
> Vacuum advance kicks in when vacuum is high, and tapers off as vacuum
declines.
> Mechanical advance kicks in when the RPMS come up
> In a system with strictly vacuum advance, the vacuum is generated by
the venturi effect (upstream of the throttle plate).
Venturi vaccum is generated by airflow, not by engine vaccum. Is the
puckup point in the venturi or no the sides (upstream or downstream of
the plates?) will change things..
> In a strictly mechanical system, advance increases as RPMs increase,
however this denies you the opportunity to advance timing with sudden
acceleration.
Yep, I get that one, the problem is how to "detect" suddent
acceleration...
My understanding is that if you pickup in manifold vaccum, upon sudent
acceleration, you open the plates and there is a suddent drop in vaccum
in the manifold as the air becomes restricted between the engine and the
next restriction point (venturi/air filter) (larger volume) rather than
between the engine and the plates and causes a suddent (but temporal)
drop in pressure. This can be detected and used to put an additional
advance (using a retard unit which will decrease the retard ie, increase
the advance) until pressure re-equalize.
This does not provide help under constant 'load'...
> In a system with both mechanical and vacuum advance, things are
generally set up so that the vacuum provides the early part of the
advance (has its pickup downstream from the throttle plate), about the
time the vacuum advance maxes out, the mechanical advance kicks in.
This gives a longer advance curve than either system could by itself.
This is completely at odds with the information that you find here:
http://shell-4000-rally.org/tdcperformance/tuning_guide.htm
At the same time, under a load (like climbing a hill at WOT), the vacuum
advance backs off (the open throttle plate combined with low airflow
mean low vacuum), as does the mechanical advance (low RPMS), preventing
pinging.
The upshot is, it seems to me you probably want to hook the vacuum up
downstream of the throttle plate.
Chris K.
On Wed, 19 Apr 2006 11:20:22 -0600, de Brebisson, Cyrille (Calculator
Division) wrote
> Hello,
>
> So what you are saying is that the vacuum 'timing tinkering' is
> designed to reduce the effect of the mechanical advance under heavy
> load to avoid pinging?
>
> My understanding was that it was to increase the advance in order to
> provide more power when the load increase...
>
> Or maybe it depends unpon the amount of mechanical advance provided by
> the distributor...
>
> One thing is sure, is that you always want to maximize the advance
> (just bellow knock zone), so if your mechanical advance is 'sloopy',
> you will want to use a vacuum advance to increase the advance under
> load to maximize power If your mechanical advance is too aggressive,
> then you will want to decrease the advance under load to minimize
> pinging...
>
> I guess what I need to see is a "normal" advance curve (let us say at
> every 500rpm) and compare it with what my car is doing...
>
> Does anyone have advance curves data around?
>
> Thanks, cyrille
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