![]() This same pressure can also be expressed as 29.92 inHg. The most familiar atmospheric pressure standard is 14.7 PSI at sea level. Atmospheric pressure can be expressed as pounds per square inch (PSI), millibars, or inches of mercury (inHg). Part throttle restricts the volume of air into the engine, which creates a vacuum in the intake manifold at a pressure that is much less than atmospheric. Keep in mind that as engine load increases, vacuum is reduced, and vacuum advance will also be reduced. We need to know about vacuum because this is the force that will ‘pull’ on the diaphragm on the vacuum advance canister, which in turn moves the baseplate tied to the magnetic pickup in the distributor to advance the timing. Intake manifold vacuum is a very easy way to judge engine load. With a reduced density charge in the cylinder, more timing is required for the engine to run efficiently. Engines operating at part throttle require more ignition timing to light the fire in the chamber because the cylinder contains only a portion of the total charge that is present at wide-open throttle (WOT). ![]() Not all distributors are equipped with vacuum advance, but for a street engine it can be very useful. ![]() Vacuum advance is a separate system that uses a small diaphragm assembly attached to the side of the distributor. This additional timing is especially important because as engine speed increases, there is less time for combustion to occur. At peak power at high RPM, total timing numbers generally are 32 to 38 degrees BTDC. Engines run best with initial timing that fires the spark plug somewhere between 4 and 12-14 degrees BTDC. This occurs roughly at around 15 to 17 degrees After TDC (ATDC). Conventional wisdom holds that lighting the fire a few degrees BTDC even at idle allows the combustion process adequate time to create maximum cylinder pressure just as the piston and rod assembly apply maximum leverage on the crankshaft. When the spark occurs, the combustion process requires a short amount of time to create the flame front that builds pressure. Advance means the spark plug fires before the piston arrives at TDC, referred to as the number of degrees Before TDC (BTDC).Ī popular misconception is that when the spark jumps the gap on the plug, there is an instant explosion inside the combustion space. Engines tend to idle best with a small amount of ignition advance to begin the combustion process. This and all other timing specs refer to the position of the piston as it approaches the top of its stroke, called Top Dead Center (TDC). Sometimes called base timing refers to the position of the distributor that creates the crankshaft position where the spark plugs fires at idle. Let’s set some definitions so everyone understands the terms we will be using: So, if you see a spec on a distributor, it is worth asking if the specs are in crankshaft degrees. Some shops that use a distributor machine may report an ignition curve in distributor degrees, which is half of crankshaft degrees. In this discussion all the specifications will be given in crankshaft degrees. It is also important to note that all distributors spin at half of engine speed. The important part to remember is that all three work together on a street engine not only to maximize power but also to enhance drivability and mileage. Together, they form the overall spark curve the engine uses over its entire RPM and load range for optimal performance. We will look at three critical ignition timing areas: initial timing, mechanical advance, and vacuum advance. Late model EFI engines use complex mapping to implement proper timing before the days of electronic spark control, distributors performed this task. While all four play a part in performance, it is the combustion step that relies on the critical sequence of idealized ignition timing. Everyone knows the classic four-stroke engine cycle: intake, compression, combustion, exhaust.
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