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As engine rpm increases, it is necessary to advance the ignition timing. If not, the burning process in the combustion chamber would take longer than the speeding piston would permit, resulting in an incomplete burn. To give the burning process a head start, an ignition advance mechanism is provided within the distributor. It is activated by engine revs and by the centrifugal force generated through the weights and springs within the distributor. Consequently, as the engine revs increase so, too, does the advancement of the ignition.The ignition advance mechanism operates either as a mechanical/centrifugal advance only or as a mechanical/centrifugal advance with vacuum assistance. Mechanically operated distributors are usually associated with high-performance and racing engines; vacuum-assisted distributors are chosen for their improved fuel economy and exhaust emissions. Vacuum timing is controlled by the vacuum or depression in the intake manifold and flourishes under light loads. Under acceleration, however, the vacuum in the manifold weakens as the throttle plates of the carburetor open and the ignition advance reverts to the control of the mechanical/centrifugal mechanism. With either design, the timing advance mechanism should not begin advancing before 1,000 rpm and, usually, full advance will have occurred before 3,500 rpm. Too much advance while the engine speed is low can cause harmful pinging or detonation; too little advance as engine revs increase will demonstrate a lack of power. The rate at which the timing advance occurs is referred to as the advance curve.

The advance curve used on most production engines and on aftermarket distributors is designed to allow an engine to operate under a wide range of conditions (flat terrain, hilly terrain, towing, fluctuations in the quality of fuel, etc). As a consequence, ignition advance curves tend to be conservative for general use. But for hot rods and other specialized uses, the advance curve can be easily altered and tuned by specialists like John Bishop's Hot Rod Tuning of Burlingame, California. According to Bishop, the best way of checking the advance curve of either the vacuum or mechanical distributor is on a distributor tester. These test machines are not as fashionable in tuning shops today as they previously were, but, nonetheless, they're still in use and perform an invaluable service for hot rod owners. In the 1960s, when most tuning shops possessed a distributor tester, hot rodders gained most of their power advantages by having their ignition advance curves modified. Today, the ignition advance curves on most hot rods remain unchecked and are taken for granted; they may function close to their optimum, but, then again, they may not. Obviously, checking the advance curve on a distributor tester is a lot less stressful than on an engine. The tester can check the distributor at 6,000 engine rpm, or more, without involving the engine. Note: The advance readings provided by a distributor tester have to be doubled as the distributor turns at half the engine speed; thus, a reading of 12 degrees advance @ 1,750 distributor rpm equates to 24 degrees @ 3,500 engine rpm.

If you're going to run a race-inspired intake manifold or camshaft on the street, you'll need a carb such as the Mighty Demon, which supplies a finer atomized air/fuel mixture at part-throttle to allow the engine to run properly.

The alternative method of checking the mechanical and vacuum advance curve, according to Bishop's Tuning Shop, is with the distributor fitted in the engine. If the engine has a degreed harmonic balancer, the amount of timing advance can be observed by using a standard timing light. If not, use a dial-back style of timing light or an MSD tape. The MSD timing tapes adhere to the harmonic balancers and are available in diameters of 5.250 to 8.00 inches.Begin the procedure by disconnecting the distributor's vacuum hose from the carburetor and capping the open port. Then, observe the amount of advance of the mechanical advance mechanism in 250-rpm increments from idle until it stops advancing. To vary the vacuum supplied to the vacuum advance mechanism, Bishop's uses a hand vacuum pump and an OTC/SPX Advance Timing Light. This allows them to read the amount of advance generated by different amounts of vacuum from 1 to 23 inches. When choosing an ignition advance curve, the specialist will consider such factors as the octane of the fuel, compression ratio, design of the combustion chamber, engine speed, engine load, engine temperature, air temperature, the weight of the vehicle, and the driving style of the operator. When the amount of advance timing from both the mechanical and vacuum advance mechanisms is added to the initial timing, we have total timing.
As cylinder head designs progressed, the amount of total ignition timing necessary decreased. The earlier style of the small-block Chevrolet engine, for example, needs 36 to 40 degrees of total ignition timing; in contrast, modern small-block Chevrolet engines operate between 28 and 32 degrees. Of course, initiating a spark 28 or 30 degrees before the piston reaches top dead center wastes a lot less energy than igniting the charge at 40 degrees before TDC. Yet, while cruising at light throttle loads, igniting the charge early is exactly what occurs on a fuel-efficient hot rod with vacuum advance ignition timing. Leaner air/fuel mixtures, those that occur during idle and light throttle loadings, have a shorter burn time than rich air/fuel mixtures.These leaner, low-load mixtures are the result of slower air velocities, which cause lower volumetric efficiencies (poorer cylinder filling). In addition, the scavenge effect in the exhaust port needed to expel the spent exhaust gases from the combustion chamber dilutes the oxygen levels with unburned fuel and slows the burn rate. To exploit this leaner condition, the vacuum advance mechanism "adds timing." Typically, early small-block Chevy and most small-block Ford engines with a compression ratio of 9.5:1 will, in most cases, respond well to 36 degrees of ignition timing (initial timing plus the mechanical advance), plus a further 10 degrees from the vacuum advance mechanism, making a total timing of 46 degrees.

Checking the air/fuel ratios at the tailpipe will also give you valuable info when fine-tuning the ignition timing. A good tune-up shop should have the equipment necessary to get these readings.

Adding timing with a vacuum advance is also used to overcome a fault in hot rods with street engines fitted with race-designed induction equipment and performance cylinder heads. Air gap-style intake manifolds are not designed to transfer heat from the cylinder heads, but instead their high-rise runners feed and thrive on high-revving race engines. Naturally, when used in an incompatible setting, the absence of heat and the consequent poor vaporization of the air/fuel mixture cause hesitation and poor drivability at lower engine rpm.One way of addressing these troubles is to use an ignition advance curve that allows an initial timing of 16 to 18 degrees combined with a highly tunable race-inspired carburetor, such as a Mighty Demon. This carburetor supplies a finer atomized air/fuel mixture at part-throttle and allows an engine with a race-inspired intake manifold or camshaft to run properly. An engine with a performance camshaft will also respond well to more initial timing because, at lower engine speeds, the air velocities are reduced due to valve overlap and the air/fuel mixture is poor. Therefore, advancing the initial timing provides longer time for this poorer mixture to burn in the cylinder; the same applies to supercharged applications. When the initial timing is altered, the amount of total ignition advance must be checked and adjusted on both mechanical and vacuum distributors to ensure the maximum setting is not exceeded. If not, engine damage will result if the total advance is excessive for the engine.
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