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| Total advance is the initial + centrifugal at full throttle. Because it is full-throttle, the vacuum advance is zero. | Total advance is the initial + centrifugal at full throttle. Because it is full-throttle, the vacuum advance is zero. | ||
| + | 1970 B110 A12 CAN 7° + 12.5x2 = 32° total | ||
| 1973 B110 A12 USA 5° + 13.5x2 = 32° total | 1973 B110 A12 USA 5° + 13.5x2 = 32° total | ||
| - | 1977 B210 A14 USA 10° + 14°x2 = 38° total | + | 1977 B210 A14 USA 10° + 14°x2 = 38° total |
| + | * possibly the larger bore needs more advance? | ||
| 1974 620 L18 USA 34° total | 1974 620 L18 USA 34° total | ||
| + | * different combustion characteristics than A-series | ||
| + | |||
| + | NOTE: The distributor curves are listed in the | ||
| + | Datsun [[Factory Service Manuals]]. Some show a | ||
| + | graph, others have the curve specs in the text | ||
| + | (without illustration). | ||
| = Advance Characteristics = | = Advance Characteristics = | ||
Revision as of 09:11, 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.
Contents |
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.
Initial Timing
Most engines run OK on 5 to 10 degrees initial (base or static) timing. which is not enough far enough advanced to cause starting problems, and will still deliver decent power at the low idling speed.
Datsun 1200 distributors are designed for 7 degrees initial timing.
Because this initial figure is factored into the distributor advance curves, do not advance it further at risk of exceeding the maximum advance at high RPM.
The distributors for emission controlled A12s are designed for 5 degrees base timing. These distributors have more mechanical advance to make up for the lesser base timing, so you get the same Total advance at high RPM.
Total Timing
The total advance timing is the base timing + the mechanical timing. This tops out at 28-32 degrees at between 4200-4800 RPM. 32° is a safe amount of timing for the A12 at full throttle, and won't damage the engine. But if you exceed this it *may* (or may not) cause overheating and/or predetonation which can break the pistons.
Vacuum advance only works when there is high vacuum, so it does nothing at full throttle. It is not considered in the "total" timing, but adds more timing (and thus more power) at part-throttle conditions.
Total Advance Comparison
Total advance is the initial + centrifugal at full throttle. Because it is full-throttle, the vacuum advance is zero.
1970 B110 A12 CAN 7° + 12.5x2 = 32° total 1973 B110 A12 USA 5° + 13.5x2 = 32° total 1977 B210 A14 USA 10° + 14°x2 = 38° total * possibly the larger bore needs more advance? 1974 620 L18 USA 34° total * different combustion characteristics than A-series
NOTE: The distributor curves are listed in the Datsun Factory Service Manuals. Some show a graph, others have the curve specs in the text (without illustration).
Advance Characteristics
For a factory stock Datsun 1200, including 1.5" exhaust, here are the curves.
NOTE: The charts show distributor angle in degrees. Multiply by two to get 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, but depends on the cylinder head design and size of bore). 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. The stock distributors have a mechanical advance tolerance of +- 2 degrees at the crankshaft. That's a huge difference and is the reason why blueprinting your distributor -- even on a stock engine -- is important.
NOTE: If your distributor is on the low side of the tolerance it can be 4 degrees retarded and still be in specification!
Measuring Total Advance
Total advance 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.
NOTE: all mechanical advance measurement should be done with the vacuum advance hose disconnected and plugged.
- With a standard 138 mm crankshaft pulley, make a new mark exactly 39mm anti-clockwise from the stock TDC mark. You may use crayon or grinder to mark it. This distance is 32 x 1/360 the circumference of the pulley.
- Connect your timing light and start the engine
- Verify the Dwell and Intial timing is correct. See Timing
- Once the initial timing is confirmed, rev the engine to 4800 RPM and verify the new mark is matching with the TDC indicator.
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
If the full timing is exceeded, retard the base timing enough to compensate.
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.
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.
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.
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 A12 engines have lesser initial timing advance (5° compared to 7°) to reduce idle emissions. Because of this these distributors have additional mechanical advance to give you the full total timing.
Total advance is:
5° initial + 23° (11.5 x 2) mechanical = 28° total to 5° initial + 27° (13.5 x 2) mechanical = 32° total
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.
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.
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.
Vacuum Advance
1973 Vacuum Advance - part D412-80 for manual transmission
- 0° at 6 inHg (inches of mercury)
- 6.5° at 9.5 inHg