Spark Curve

(Difference between revisions)

Revision as of 11:44, 30 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.

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

B110 A12 USA 32° total
620 L18 USA 34° total

Emission Control

To reduce certain types of emissions, many Datsun engines introduce spark advance cut-out (TCAS). 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 modules were fitted to juggle emissions of one type vs another.

B110 A12

Advance Characteristics

For a factory stock USA-specification engine, including 1.5" exhaust, here is the curve. These are for emission-controlled engines.

Centrifugal Advance - part D412-80 for manual transmission

• 0° at 600 rpm
• 12° at 2,100 rpm

Vacuum Advance - part D412-80 for manual transmission

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

• Mechanical (Centrifugal) advance is all in by 2100 distributor RPM = 4200 crankshaft RPM.
• This is distributor angle. Multiply by two to measure 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.

The stock design is a nice performance curve. 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:

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