Total timing is a term used to describe the combination of initial ignition advance at which a distributor is set, plus the amount of centrifugal (mechanical) advance that takes place within a distributor as the engine RPM increases. The small-block Chevy engines vary in the amount of total timing based on a number of factors, but most SBC engines require between 32 to 36 degrees of total timing, with some older, less efficient engines needing as much as 38 degrees or more. While most engines use a vacuum advance mechanism, some performance SBCs do not.
Importance of Proper Timing
The proper amount of total timing is critical. Too little timing can cause the engine to be sluggish and unresponsive. This is because the air/fuel mixture is being ignited in the cylinder too late in the compression cycle for maximum power. The full energy from combustion is released too far after the piston has past top-dead centre (TDC). Additionally, the fuel burn is not as complete which increases emissions.
Conversely, too much ignition advance begins the air/fuel ignition too soon. The early release of energy means that the rising piston has to fight against the increased cylinder pressure. If fuel octane is not sufficient, detonation (pinging) will occur. This is a potentially destructive condition and can burn pistons, valves or spark plugs and can cause damage to the upper connecting rod-bearing inserts, as well.
Chevrolet has manufactured the SBC in displacements from 265 to 400 cubic inches for a variety of applications. The size of the engine impacts the total timing requirements. In addition to this, induction system, cylinder head design and camshaft timing play a great role in timing needs, as well. Newer engines with more efficient cylinder heads which promote high turbulence in a smaller combustion chamber reduce the need for timing advance.
Most unmodified production, daily-driven Chevy small blocks will need about 8 to 12 degrees of initial distributor advance at idle for ease of starting. Once started, a high engine vacuum signal connected to a dashpot on the distributor rotates the internal advance mechanism forward to introduce more timing for good off-idle and part-throttle engine response. As engine speeds increase above 3000 to 3500 RPM, the centrifugal force of the distributor mechanical advance weights overcomes the springs holding the advance mechanism in the retarded position and mechanical advance is fully introduced. This is also known as having the timing "all in."
The purpose of the vacuum advance mechanism is to offer more timing--typically 10 to 14 additional degrees--when full mechanical advance is not completely introduced. This allows the engine to run more efficiently at low speeds at part throttle with better throttle response and more complete fuel combustion. Under full throttle loads, there is reduced vacuum available to the vacuum advance mechanism, and its advance is eliminated to avoid too much timing.
As an example, at high RPMs with part throttle (high vacuum) positions, the total timing will be: 12 degrees initial advance, 10 degrees vacuum advance, and 22 degrees mechanical advance (total timing is 44 degrees). At high RPMs with full throttle application (no/low vacuum), the total timing may be: 12 degrees initial, 0 degrees vacuum, and 22 degrees mechanical advance, for a total timing of 34 degrees.
For performance applications, some modifications will be required to the advance "curve." Engines with aftermarket camshafts change valve timing events (not to be confused with ignition timing), and this increases the amount of initial timing that is required. When initial advance is increased, mechanical advance needs to be reduced. This can be done with performance kits which allow you to easily modify the rate and amount of mechanical advance.