The more densely packed the grass is, the faster it burns while sparse areas burn more slowly. The reality is the spark plug fires and it takes a generous period of time for the combustion gases to burn completely across the top of the piston, much like a prairie fire across a large valley. The common perception of the combustion process is as an explosion-the spark goes off and boom-combustion occurs like a bomb. With a minimum of air and fuel entering each cylinder, this means the mixture is not tightly packed. Engines with long duration cams or cars with tall overdrive gears in overdrive might transition into the main circuit but most mild street engines with high vacuum at cruise will actually be running on the idle circuit. A little known fact is that most mild street engines cruise down the freeway pulling fuel from the carburetor's idle circuit. As mentioned before, high vacuum means low load and a nearly closed throttle. The engine could be pulling anywhere from 12 to 18 inches of vacuum. Let's use the example of a typical carbureted small-block cruising down the freeway at 70 mph at 2,800 rpm on level ground. It's worth a peek down the rabbit hole of the combustion process to understand why load-based timing is important. The more enlightened way to look at vacuum advance is to view it as load-based timing. There's a popular yet misguided view among many enthusiasts that vacuum advance is only for bone stock and/or emissions-controlled engines. Now we can introduce vacuum advance into this system. Heavier springs delay the onset and slow the rate of advance. Lighter springs allow the advance to begin and achieve maximum advance at a lower rpm. The rpm at which the weights begin to move and the point of their maximum travel is mainly determined by the strength of the springs that hold the weights in place. On a typical Chevrolet distributor that spins clockwise, as the mechanical advance weights open, this moves the rotor in the same direction, advancing the timing. The distance the pin travels is the amount of mechanical advance, accomplished by advancing the position of the rotor. The weights are attached to a plate that locates a pin moving within a fixed slot. The typical centrifugal advance uses a pair of weights that pivot on pins. But even Watt admits that he borrowed the idea from an earlier design that appeared on a 1600s gristmill. Mechanical advance is determined by use of a centrifugal advance mechanism that was first used on James Watt's steam engines in the 1780s. Mechanical advance is tied directly to engine rpm. This initial timing is used as a starting point for our next step, which is mechanical advance. Twisting the distributor opposite to the direction of rotation advances the initial timing. This changes the relationship between the distributor body and the spinning rotor. Initial timing is set by loosening the distributor hold-down bolt and rotating the distributor body. This timing is checked with a timing light that compares the position of the Number One cylinder TDC mark on the harmonic balancer with a timing reference tab located most often on the timing chain cover. It's not unusual to input 14 to perhaps 18 degrees of initial timing for engines with big cams. Engines with longer duration camshafts and other modifications often demand more initial timing. Most stock street engines call for 6 to 8 degrees of initial advance, but this is not set in stone. This is the amount of advance at idle with the spark triggered Before Top Dead Center (BTDC). Our approach with this engine is to optimize the spark timing over the engine's entire operating range while minimizing the chance of detonation.Īll discussion of ignition timing starts with the initial timing. The next step is to separate ignition timing into three basic components: initial timing, mechanical advance, and vacuum advance. Most engines will pull roughly 0.5 inches of Hg of manifold vacuum at WOT. At wide-open throttle (WOT), manifold vacuum drops to near zero. As the throttle is opened, manifold vacuum begins to drop. A typical street car with a mild cam might idle at 12 to 16 inches of mercury (Hg) on a vacuum gauge. When the throttle is barely open, the engine demands more air than the throttle allows, creating manifold vacuum (low pressure). Load is determined by the throttle and is easily monitored with a vacuum gauge. But condensing this down to its simplest aspects: timing is dependent on engine speed and load. An engine's ignition timing requirements will vary depending upon dozens of variables like compression ratio, fuel octane, air-fuel ratio, combustion chamber shape, mixture motion, and inlet air temperature to name a few biggies.
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