Spark Curve

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As we all know, more spark advance results in more power, until the point where the fuel starts pre-detonating (pinging). So the key in create a performance spark curve is more advance. This includes more initial advance, more vacuum advance and earlier centrifugal advance. In no case should the optimum total advance for full-throttle high RPM use be exceeded. This varies by engine type and cylinder head design. Since the total advance is a given, if you dial in more initial advance, you need less centrifugal advance to reach the total.

Initial Timing

All Datsun engines run OK on 5 to 10 degrees initial timing. which is not enough to cause starter problems.

Total Advance Comparison

Total advance is the initial + centrifugal at full throttle. Because it is full-throttle, the vacuum advance is zero.

1973 B110 A12 USA 5° + 13.5x2 = 32° total
1977 B210 A14 USA 10° + 14°x2 = 38° total
1974 620 L18 USA 34° total

Emission Control

To reduce certain types of emissions, many Datsun engines introduce transmission gear-selected spark advance cut-out (TCVAS). This is not good for performance, so generally only works in top gear after the car has already accelerated up to speed.

Each year, and in each region, different advance parts (both mechanical and vacuum advance) were fitted to juggle emissions of one type vs another.

Advance Characteristics

For a factory stock Datsun 1200, including 1.5" exhaust, here are the curves.

NOTES

• The charts show distributor angle in degrees. Multiply by two to measure degrees at the crankshaft pulley.

All A-series distributors have similar curves, but vary a little by year and model. Check your Factory Service Manual for your specific model and year to find the curve for your distributor. NOTE: on the side of the distributor is the model number. Mine is D412-63.

Mechanical Advance

Mechanical or centrifugal advance is varied solely by the RPM of the engine. Gasoline engines need a little advance when idling (typically 7 degrees or so Before Top Dead Center) but a lot at higher RPMs (typically around 35 degrees).

Non-EXHAUST Type

These are for engines that are not EXHAUST emission-controlled, such as JDM A12s. The engine may have other types of emissions controls such as EVAPO (fuel vapor controls) or CRANK, but those do not affect spark timing.

The factory advance is a nice performance curve for stock engines. But due to factory tolerances could be several degrees too low for best performance and economy. Blueprint your distributor to ensure it is right.

Mechanical advance is all in by 2400 distributor RPM = 4800 crankshaft RPM.

Total advance is:

7° initial + (13.5 x 2) 27° mechanical = 32° total

EXHAUST Type

These are for EXHAUST emission-controlled engines, such as USA-specification A12. The spark advance curves are changed to reduce exhaust emissions (unburned HC, carbon monoxide, etc).

1973 Centrifugal Advance - part D412-80 for manual transmission

• 0° at 600 rpm
• 12° at 2,100 rpm

Mechanical (Centrifugal) advance is all in by 2100 distributor RPM = 4200 crankshaft RPM.

Total advance is:

5° initial + (13.5 x 2) 27° mechanical = 32° total

Vacuum Advance

1973 Vacuum Advance - part D412-80 for manual transmission

• 0° at 6 inHg (inches of mercury)
• 6.5° at 9.5 inHg