The analogy holds true in the combustion space at part throttle. But if we take a portion of that powder and sprinkle it in a line across the floor and ignite one end, the powder burns much more slowly. When tightly packed in a shell and combusted, it explodes with great power over a very short period of time. A good analogy to this would be to examine how gun powder burns. This means the mixture (assuming a proper air-fuel ratio) will burn more slowly. This means that with the piston at TDC, the air and fuel is less densely packed. When the engine is throttled, this radically reduces the volume of air and fuel in the combustion space. But street engines operate almost entirely at part throttle. Up until this point, we’ve been discussing engine operation exclusively at WOT. Vacuum advance for street-driven engines is a topic that seems to confuse many enthusiasts, so it’s worth delving into here. By starting the timing later, less negative work is required and the engine makes more power. This is a positive step since initiating the spark 40 degrees BTDC creates a small but still significant amount of negative work-which means the engine must work against building cylinder pressure with greater ignition timing. Today’s modern engines often demand 30 degrees or less total timing. Not long ago, 40 degrees BTDC was the norm. It is possible that if we doubled the engine speed from 6,000 to 12,000 rpm that timing changes might be necessary, but the doubling of speed from 3,000 to 6,000 is not sufficient to demand major changes in timing.Īnother issue worth considering is that as combustion space (the entire area of the chamber and piston top) design improves, the need for advanced ignition advance (timing before TDC) is reduced. This is an arbitrary rpm point, but based on experience, a fixed timing number of 32 to perhaps 40 degrees seems to work for most normally aspirated, pump gas engines. You also asked why engines don’t need more timing above 2,800 rpm. That’s why engines with fixed timing are also configured with an ignition start retard to not abuse the starter motor. Plus, a fixed ignition advance of 36 degrees in an engine with high compression will make the engine difficult to start when hot. But for street engines, a curve is generally used to improve drivability and throttle response. These generally are single-purpose drag race engines that only operate at wide-open throttle (WOT) within a narrow rpm band so an advance curve is not necessary. You asked why some engine builders set up crank trigger ignitionswith fixed timing. This is why ignition curves were created to produce more power at higher engine speeds as additional timing is required. The thought might also occur that as engine speed increases, additional time will be required to allow this flame front and pressure to build, which is true.Īt low engine speeds-idle to around 2,000 rpm-not as much timing is needed since the engine is spinning relatively slowly. The ideal pistonposition to create maximum downward pressure on the crankshaft is between 10 to 20 degrees after top dead center (ATDC). By beginning the combustion process before top dead center (BTDC), this allows the flame front time to create pressure. This burn process is quick, but still takes time to occur. Instead, when the spark plugfires, a flame front is created around the tip which travels across the combustion space much like a fire burning across a field of dry grass. The reason for ignition timing is because the explosion that many think occurs in the combustion space when an fuel-air mixture is ignited is not what really occurs. Jeff Smith: You have a few very good questions here, so let’s get started. Why does the engine not need more timing as the rpm’s go above 2,800 rpm? A friend that runs a computer-based timing control says he can add timing as the rpm increases so why isn’t mechanical advance set up to add more? Another question I have concerns advance curves and how they might be used to tune a car for drag racing? Can you help me? Some people lock the timing at 32 or 34 degrees and just run their car that way. A question I have is why when I set the total mechanical advance on my race car I’m told I want it all in by somewhere close to 2,800 rpm. Do you know of or are you a good source for questions about timing? I’m not talking about the basics.
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