An engine that has too much or too little ignition spark advance may lack power, tend to run hot, or overheat. An engine that has too much mechanical ignition spark advance for the octane of the gasoline may also suffer from a ping or detonation problem, which can lead to engine failure. The vacuum advance system can also supply an engine with too much or too little spark advance for the needs of the engine. The distributor's mechanical and vacuum advance curves must be correct for both the needs of the engine and the octane of the fuel being used or the engine's performance will suffer as well as the possibility of engine damage from detonation.

Initial Timing Guideline
The initial timing guideline we use to determine what initial timing an engine should use with unleaded gasoline of today is from the Barry Grant catalog and/or website in the Demon carburetor selection guide. The guideline recommends 10 to 12 degrees of initial timing for an engine with a stock/mild camshaft (duration is less than 220 degress at 0.050 valve lift), 14 to 16 degrees of initial timing with a high-performance cam (duration less than 240 degrees at 0.050), and 18 to 20 degrees of initial timing with a radical/race cam (duration less than 260 degrees at 0.050 valve lift). When you increase the initial timing, you should re-curve or modify the mechanical advance system of the distributor so it does not supply the engine with too much ignition spark advance for the needs of the engine

Tuning the Vacuum-Based Ignition Spark Advance for Fuel Economy
The ignition spark advance curve an engine needs for maximum power with the rich power air/fuel mixture the engine has during wide-open throttle acceleration is different than the ignition spark advance it needs for the leaner cruise air/fuel mixture the engine has when it is cruising at 65-75 mph on the highway. A vacuum advance-equipped distributor adds in the extra ignition spark timing advance the engine needs to fully burn the slower burner leaner air/fuel mixtures the engine sees during low load/high vacuum cruise driving conditions.

Back in the days of leaded gasoline, most tuners limited the total advance (initial, mechanical, and vacuum) of the typical overhead valve engine to 52 to 56 degrees but today's gasoline has a different burn rate than leaded gasoline, so we limit the total advance (including the vacuum advance) to the 46 to 50 degree range. The vacuum spark advance curve we use most will provide the engine with an additional 10 to 12 degrees of spark advance when the engine vacuum is above 10 inches. We have found whenever the advance from the vacuum advance exceeds 15 degrees the engine will often suffer from ignition misfire and/or ping problems at cruise speeds with the blend of reformulated gasoline we have here in California.

Tools for Checking the Ignition Timing Spark Advance Curves
The best way to check both the vacuum and mechanical advance curves of a distributor is with the use of a distributor test stand because you can check the spark advance curve at any rpm without fear of over-revving the engine. If you do not have access to a distributor test stand, an optional method that you can use to check the vacuum and mechanical advance curves is the use of a dial-back timing light. A dial-back timing can allow you to read the advance curve of an engine at different engine speeds but take care not to over-rev an unloaded engine. The vacuum advance curve can also be checked with the use of a hand vacuum pump to vary the vacuum supplied to the vacuum advance, just use the timing light to read the amount of advance given by the vacuum advance at different amounts of vacuum from 1 to 23 inches of vacuum.

Once you have the ignition spark timing properly tuned for the blend of gasoline you are using, the next step is the tuning of the carburetor's air/fuel mixture curves. In the final part of this tuning article we will explain to you how and why we tune a carburetor's idle, off-idle, cruise mixture, power mixture, and accelerator pump systems for the best power, driveability, and the lowest possible exhaust emissions with the blend of gasoline you are using.

OTC/SPX Corporation
John Bishop's Hot Rod Tuning
Ole's Carburetor & Electric Inc.
Bridge Analyzers
El Paso