Re: So Slow (timing advance)

[Audial@aol.com]

Eric J. Fluhr writes:

>Hmmmm...then why the claim that advancing the ignition timing on a n/a
engines helps in the low end but hurts in the high end? ...  And why the
vacuum advance feature on the distributor?  You are likely to be at or near
full throttle when in the higher rpm ranges, hence minimal vacuum.

Exactly correct, but there are a few other things too:

The timing advance that is supplied due to increasing RPM and the timing
advance that is supplied as a result of part-throttle operation are two
independently controlled variables.  You are correct when you say that the
vacuum advance mechanism on a distributor adds more advance at low RPM, and
this is because:

For fuel-economy reasons, automobile manufacturers often make part-throttle
(low load factor) mixtures lean.  These lean mixtures take longer to burn,
requiring a separate advance mechanism that is sensitive to engine load (part
throttle operation meaning relatively low engine load relative to WOT).
 Therefore they incorporate a vacuum advance mechanism on the distributor
that is sensitive to vacuum downstream of the throttle plate, which senses
engine load.  One thing to remember is that the vacuum sensed in this manner
does _not_ decrease linearly with respect to throttle movement - in fact, it
initially increases as you begin to open the throttle  (at a given engine
speed) and then begins to fall in the region of WOT with increasing engine
rpm.  If you graph vacuum vs. engine load and throttle position, you will see
this. 

So, as you crack open the throttle under a given load (suppose you are going
up a hill) and the engine RPM remains constant, the manifold vacuum
increases, and the spark advance increases to compensate for the fact that
the mixture is relatively lean and takes more time to burn.  If it increases
too much, or you are running cheap gas, or have deposits in your combustion
chambers, the engine will knock.  This is why many economy-tuned cars that
have gone slightly out of whack will knock on slight upgrades under partial
throttle.

Note that this lean mixture that I am referring to is there because car
manufacturers are concerned about fuel economy, and because most people drive
their cars at part throttle (I know, not many of you on the Quattro list).

On my '68 Corvette, I don't worry about vacuum advance mechanisms.  I don't
have to because it is by my definition an impractical performance car and I
have the carburetor jetted so that I get a slightly rich mixture for maximum
power.  All I have to worry about is centrifugal advance to compensate for
the decrease in "burn time" caused by the increase in engine RPM.  Remember
that complete combustion from the time of the spark to the time that the
flame front has traversed the entire A/F mixture takes approximately 2ms,
regardless of engine speed, which is why you _have_ to advance the timing as
rpm increases.  This is why the 'vette has a dual-point, strictly
mechanical-advance distributor, and also contributes to its 7mpg "fuel
economy".

Also remember that most vacuum-"advance" mechanisms on distributors are
dual-diaphragm advance/retard mechanisms which actually retard the ignition
timing under some conditions like idle or trailing throttle.  However, there
are relatively few operating conditions where the spark is truly retarded
with regard to compression-stroke TDC.  In fact, the part-throttle operation
ignition advance is usually greater (as a result of the combined action of
the centrifugal and vacuum-advance systems) than the maximum (centrifugal
alone) advance at WOT.

Please note that most of this description applies to relatively obsolete
technology: with a modern engine-management system, it is possible to
precisely manage the spark advance based upon engine load, throttle position,
engine rpm, temperature, desired economy, etc.  Which is exactly why you
don't see very many vacuum-advance distributors on modern cars.

Best Wishes,

Alex
'86 5KCSTQ