Which came first, the chicken or the egg? It's an ageless debate, lacking a definitive answer, yet both sides are strongly supported with opposing theoretical views. Many agree that its automotive equivalent is "Which is better for my Pontiac, manifold- or ported-vacuum advance?" And like the chicken/egg dispute, some hobbyists strongly favor one side over the other in every instance. But the truth is every combination is different, and rarely can any two be tuned identically.
Some hobbyists have no trouble grasping the concept of vacuum advance and the effects various vacuum sources have on it. Others, however, consider vacuum-advance tuning a form of black magic and oftentimes never fully realize its benefits. So what advantages does vacuum advance offer? Follow along as we test the different forms on our Pontiacs and discuss the results.
Spark Advance Basics
Typical engines generate torque by applying the expansion forces of combustion as leverage on the crankshaft. Maximum possible torque seems to occur when cylinder pressure peaks at a crank angle between 10-20 degrees after top dead center (ATDC). This allows the pressure to exert its force over the entire length of the crankshaft's stroke. But since the crank is in constant motion, ignition must begin before top dead center (BTDC) for peak pressure to occur ATDC.
A normal trip for most Pontiacs today includes a number of different throttle positions ranging from idle to full throttle. Rapid acceleration or heavy engine workloads typically require excessive throttle position to intensify combustion and increase output. Because the burn requires a fixed time, spark must start earlier as engine speed increases due to less available combustion time. Distributors mechanically advance spark lead in relation to TDC for maximum performance in these conditions.
The carburetor is designed to provide minimal amounts of fuel and air in light-load conditions such as part-throttle cruise. But because cylinder volume doesn't change, the molecules within the chamber are not as tightly compacted, resulting in a less combustible mixture. Not only is such a mixture more difficult to ignite, the flame spreads slower, delaying peak pressure. For maximum performance in these conditions, spark must occur even earlier, sometimes in excess of 50 degrees BTDC.
Vacuum Advance Basics
A heavily loaded engine produces little vacuum while the highest vacuum levels occur at sustained light-throttle cruise. But for maximum efficiency in either instance, peak pressure must still occur in the desired ATDC range. A heavily loaded engine, however, will likely detonate with the high levels of spark lead a lightly loaded engine can tolerate, so a load-sensing unit that uses engine vacuum to advance spark in light-load conditions is added to the distributor. As engine load increases and vacuum levels diminish, vacuum advance reduces spark lead to lessen the risk of engine-damaging detonation.
AC Delco produced a number of vacuum advance units for GM, each with unique vacuum actuation points and/or maximum spark advance. Those used in original applications matched the engine's vacuum levels and centrifugal advance curve. But because emissions were of great concern, some concessions were required.
Excessive spark lead in light-load conditions can generate the highest level of hydrocarbon (HC) emissions. In these instances, the flame front is quenched by a cooler surface, such as the cylinder wall, and the unburned and partially burned fuel and air molecules are emitted as HC. Engineers found that HC emissions could be limited by reducing spark lead in light-load conditions. This not only increased overall cylinder temperature, but also delayed the crank angle that the flame extinguished.