As noted, using even 10 degrees @ 800 RPM initial advance
won't produce the needed advance at 5000 RPM! No wonder some internet
references suggest using 15 degrees advance @ idle on the 502/502.
Interestingly, data from Mercury Marine, who use the 502 engine, shows a
distributor curve about equal to that shown in the above table. The motor
uses
15 degrees initial advance at idle and have excellent performance, no pinging,
excellent throttle response etc.
The Chevy instructions also say to use "Ported Vacuum" to
connect to the
distributor vacuum advance diaphragm. This means there will be no vacuum
present at idle.
This port comes from above the butterfly's in the carb. Therefore until
the throttle is open, there is essentially no vacuum in this port to activate the distributor advance
mechanism.
However and
old Peterson publication entitled "Basic Ignition and Electrical Systems" reviews
the history of "Manifold" versus "Ported" vacuum. As they indicate, "Ported
Vacuum" came about during the start of pollution controlled vehicles
to reduce nitrous oxide emissions with lean air/fuel idle mixtures.
They state that by using "Manifold Vacuum" the spark at idle will occur sooner
and "less cylinder wall is exposed to the heat (of combustion), thereby lowering
coolant temperature." Interpreting their
statement, with more advance at idle, the combustion starts on the upper portion
of the cylinder on the compression stroke. Flame front travels very slowly
at the low cylinder pressure that occur when idling. Therefore the
cylinder head and upper cylinder walls will absorb most of the combustion heat
and the lower portion of the cylinder on the power stroke will be cooler.
This transfers less total heat to the cooling system.
In fact, on this 502/502 motor an additional ~
10 degrees advance occurs at idle when the "Manifold Vacuum" port is
connected to the distributor vacuum advance rather than the "Ported Vacuum"
location. This provides about 25 degrees advance in total when idling!
The engine idles considerably faster compared to when the "Ported Vacuum" port
location is employed. The throttle butterfly's have to be closed
about 1/2 turn on the idle adjustment screw to maintain the ~900 RPM idle speed.
I consider this decreased throttle opening to be a good indicator of more
efficient use of the fuel. All else being equal, less butterfly opening means
less fuel consumed which equates to less total heat to transfer to the coolant.
Quoting another source; "Ported
vacuum showed up when emissions became a factor - before that time, full
manifold vacuum was provided to the distributor.
Many emissions engines use a thermal vacuum control switch to deliver ported
vacuum to the distributor until the engine reached operating temperature,
then manifold vacuum to keep the engine cooler at idle."
Measuring the engine temperature on a very hot day (95 degrees in the shade,
with the air-conditioning off and the electric fans on) with the car idling
at 1100 RPM in PARK, it slowly rose to 220 deg F
when "Ported Vacuum" was used . Under the
same conditions using "Manifold Vacuum" for the distributor vacuum advance the
temperature maxed out at 210 degrees,
a 10 degree lower
coolant temperature!
The only apparent problem encountered when using "Manifold Vacuum" was starting when the engine was
warm. The first thought was the slow and erratic cracking observed was a
battery or starter issue. The battery checked out fine. The car started
numerous times and the Yellow Top Optima battery was still reading 12.7 volts.
When the vacuum was blocked to the distributor, the car cranked fast and started
fine. Apparently when the car was cold, the engine was not firing until it
cranked over sufficiently to pull in the proper air/fuel mixture. Cracking
speeds were high and it then ignited an ran fine.
When warm, the cylinders were apparently firing as soon as the engine turned and some of
the pistons were creating enough backward force, firing 25 degrees BTDC, to slow the cranking speed
and make it somewhat erratic. The solution,
an electric vacuum switch was installed which eliminates vacuum to the
distributor advance while the starter is engaged! Works fine
now! The "normally closed" vacuum switch was installed out of the engine
compartment using a tee fitting on the vacuum advance line. It opens to
the atmosphere when the starter is engaged.
One additional note on starting:
Subsequently it was
found some oxidation on the battery terminals and the chassis ground location
was causing slow cracking when the engine was cold. After cleaning all
connections and installing a new battery to ground cable all was fine and has
remained so for over 9 months. Perhaps the electric vacuum switch was an
overkill. However at the time it was installed it made a definite
improvement.
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