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Revision as of 08:09, 31 December 2011

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.

1 Initial Timing
2 Total Advance Comparison
3 Emission Control
4 Advance Characteristics
- 4.1 Mechanical Advance
  - 4.1.1 Non-EXHAUST Type
  - 4.1.2 EXHAUST Type
- 4.2 Vacuum Advance

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) but a lot at higher RPMs (typically around 35 degrees). This is called advance because the spark occurs before the piston approaches the top of the cylinder and the power stroke begins. This is called Before Top Dead Center (BTDC). The advance allows time for the combustion to get going to power the piston down with maximum force.

For the A12, you want a full 32° advance at high RPM. This can be verified with an advance-type Timing Light, or with a standard timing light and additional marks on the crank pulley showing where 32° is. Of course all mechanical advance measurement should be done with the vacuum advance disconnected.

The stock distributors have a mechanical advance tolerance of +- 2 degrees at the distributor. That's a huge difference and is why blueprinting your distributor is important. If your distributor is on the low side of the tolerance it can be 4 degrees retarded at the crankshaft from the optimal.

If the full timing is not achieved at high RPM, you can:

(better) can add more initial timing to get to the 32 mark
(best) recurve the distributor on a recurving machine

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.

1970 Centrifugal Advance - part D411-61
0° at ~550 rpm
11.5° +-1 degree at 2400 distributor rpm

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

Total advance is:

7° initial + 21° (10.5 x 2) mechanical = 28° total
to
7° initial + 25° (12.5 x 2) mechanical = 32° total

You want it blueprinted to the latter.

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).

EXHAUST controlled engine have lesser initial timing advance (5° compared to 7°) to reduce idle emissions.

1970 Centrifugal Advance - part D412-63
0° at ~550 rpm
12.5°+- 1° at 2,100 distributor rpm

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

Total advance is:

5° initial + 23° (11.5 x 2) mechanical = 28° total
to
5° initial + 27° (13.5 x 2) mechanical = 32° total

1973 Centrifugal Advance - part D412-80 for manual transmission
0° at ~600 rpm
12.5° +- 1° at 2,125 +- 25 distributor rpm

Mechanical (Centrifugal) advance is all in by 2150 distributor RPM = 4300 crankshaft RPM.

Total advance is:

5° initial + 23° (11.5 x 2) mechanical = 28° total
to
5° initial + 27° (13.5 x 2) mechanical = 32° total

1973 Centrifugal Advance - part D412-89 for AUTOMATIC transmission
0° at ~600 rpm
12.5° +- 1° at 2,325 +- 25 distributor rpm

Mechanical (Centrifugal) advance is all in by 2350 distributor RPM = 4700 crankshaft RPM.

Total advance is:

5° initial + 23° (11.5 x 2) mechanical = 28° total
to
5° initial + 27° (13.5 x 2) 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

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Spark Curve

Revision as of 08:09, 31 December 2011

Contents

Initial Timing

Total Advance Comparison

Emission Control

Advance Characteristics

Mechanical Advance

Non-EXHAUST Type

EXHAUST Type

Vacuum Advance