Ignition Timing

Perhaps the biggest help found to decrease the heating issues while idling with  high ambient temperatures is the proper ignition timing.  First lets review the setting of the static timing, i.e. the initial timing set with no vacuum advance and essentially with no mechanical advance since it is not active at idle.  The instructions for setting the timing for a ZZ 502 from Chevrolet state, "Set at 8 degrees before top dead center (BTDC) @ 800 RPM and 36 degrees total advance @ 5000 RPM."  However  some references stating use 15 degrees BTDC at idle.

Checking the timing with a dial "set back" timing light with the vacuum advance disconnected, the following was obtained:

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.  This provides about 25 degrees advance in total when idling or when cranking!  The engine idles considerably faster compared to when the "Ported Vacuum" port 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.  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 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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