Disclaimer: These ignition settings are generalizations and do not apply to every motor.
For the 351C with iron 4V heads Ford suggested a centrifugal advance curve of 20° nominal (17° to 22°) that starts at 1200 rpm and is all in by only 2000 rpm (OHO manual of 1972) or 2800 rpm (Muscle Parts catalog supplement no. 2 of 1970). Ford advised using 16° nominal initial advance for a total advance of 36°. However Ford also wrote that up to 40° total advance may be allowable.
Motors equipped with alloy heads featuring high swirl combustion chambers usually call for 28° to 32° total ignition advance. Motors equipped with quench combustion chamber iron heads and dished pistons will also call for a little less ignition advance, whereas motors equipped with pistons having pop-up domes will call for a little extra ignition advance.
I consider Fords OHO centrifugal advance curve too fast for street motors. I prefer a centrifugal advance curve that starts 200 to 400 rpm above the idle rpm speed and swings 10° every 800 to 1000 rpm, dependent on gearing; lower gearing will allow the faster curve. To determine how many degrees of centrifugal advance is needed subtract the degrees of initial advance from the degrees of total advance. Total advance is best determined on a chassis dyno, I shall describe how to optimize initial timing advance below.
To optimize the initial ignition advance setting:
(A) Connect a manifold vacuum gauge, an accurate tachometer or an exhaust gas temperature (EGT) gauge to the motor. Leave the vacuum advance hose connected if the motor has vacuum advance. Loosen the distributor clamp bolt if the motor is equipped with a distributor, start the motor and allow it to warm up to full operating temperature.
(B) While the motor is running at idle speed in neutral (manual transmission) or in drive with an assistant firmly pressing on the brake pedal (automatic transmission), and assuming the initial advance is in a retarded setting; begin slowly turning the distributor clockwise to advance the initial advance setting, or increase the initial advance setting of your distributorless ignition controller using a potentiometer or a lap top computer. As you do the idle speed shall increase, the intake manifold vacuum shall increase and the EGT shall decrease.
(C) Continue advancing the initial advance until you reach the setting where idle speed no longer increases, manifold vacuum no longer increases or EGT no longer decreases. The setting where the readings first stabilize is the optimum initial advance setting.
(D) Tighten the distributor clamp bolt if the motor is equipped with a distributor, and reset the motors idle speed, you are done.
(E) After optimizing the initial advance setting the amount of centrifugal advance will require adjustment to attain the proper total advance; this is very easy to do with an distributorless ignition.
High compression motors often have trouble hot starting with the ignition set at optimum initial advance. If your motor will not hot start after setting the initial advance as described above you have two recommended choices to resolve the problem:
(A) Install a new or more powerful starter and/or a battery with more cranking amperage
(B) Install an ignition module that retards the ignition during starting
And if all else fails, there's a third alternative. This solution is detrimental to the engines low speed performance, which is why it is not a "recommended" solution.
(C) Set the ignition for less initial advance and compensate by increasing the centrifugal advance.
Street motors should utilize vacuum advance for optimum efficiency during part throttle (high intake manifold vacuum) operation. Vacuum advance will more than improve fuel economy, it will prevent burning up the exhaust system during partial throttle (high manifold vacuum) cruising. The general consensus is to limit vacuum advance to about 10° to 12°.
Some people debate the superiority of either ported vacuum or manifold vacuum. Truth is either method can be used, each has its advantages.
Ported vacuum sets a higher limit to how low the advance will be retarded under low rpm/low manifold vacuum conditions; it allows a race motor type ignition calibration while taking advantage of vacuum advance for cruising (high manifold vacuum, part throttle) conditions. It is the method I prefer and the method recommended by aftermarket ignition manufacturers such as MSD. This method results in peppier throttle response. Since there is no "ported vacuum" at idle, the ignition timing is not worsened by a lumpy idling camshaft. It should never cause a problem if a car is geared low enough and if a motor is in a good state of tune. However, the manifold vacuum method is useful when a high compression motor has trouble hot starting using the ported vacuum method, because the static/initial advance is significantly retarded. It is also useful for high geared or heavy vehicle applications in which the motor "pings" with the ignition tuned for ported vacuum.
Assuming a motor's optimum total advance is 36°, its optimum initial advance is 16°, its vacuum advance is limited to 10°, and it idles at 1000 rpm.
16° initial advance + 20° centrifugal advance = 36° total
Centrifugal advance curve starts at 1200 rpm, ends at 2800 rpm (10° per 800 rpm)
Advance at idle = 16° (16° initial setting, no vacuum advance)
Advance gain above idle is a combination of centrifugal advance plus vacuum advance
The vacuum mechanism always supplements total advance
6° initial advance + 30° centrifugal advance = 36° total
Centrifugal advance curve starts at 1200 rpm, ends at 3600 rpm (10° per 800 rpm)
Advance at idle = 16° (6° initial plus 10° vacuum advance)
Advance gain above idle is solely due to centrifugal advance
The vacuum mechanism can either supplement or diminish total advance under various engine speeds and loads
Ignition advance can actually take a "dip" at throttle tip-in using the manifold vacuum method, whereas ignition advance will increase at throttle tip-in using the ported vacuum method; this is why ported vacuum provides peppier throttle response. As motor speed increases the centrifugal mechanism increases advance while decreasing manifold vacuum results in the vacuum mechanism decreasing advance, therefore the advance mechanisms buck one another using the manifold vacuum method, resulting in less advance during part throttle cruising compared to the ported vacuum method.
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If you use a 351C 4V powered vehicle for a grocery getter ... the eggs aren't going to make it home!