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Ignition timing light with advance4/25/2024 Ensuring that the mixture has a complete burn before leaving through the exhaust port also helps the engine to run cooler at idle. Again, this means the mixture burns slower and needs an earlier spark to optimize the burn. Much like cruise conditions, engines run leaner at idle than they do under load. Plugging your vacuum advance into a direct source will allow it to engage at idle, which is good for a number of reasons. And it's not an opinion it's just a fact. Internet forums are rife with opinions on both sides of the argument. There's been a lot of debate whether or not the vacuum canister should be plugged into a ported or direct vacuum source. Where should the vacuum canister be routed? As you apply more throttle, air rushes through the carb, into the intake manifold increasing pressure and pushing the diaphragm in the vacuum can right back out, retarding timing back to wherever it would normally be, given engine RPM and mechanical advance. Lets say you encounter a hill or go to pass another car while cruising down the highway. This negative pressure exerts a pulling force on the diaphragm inside the vacuum advance can which has a mechanism linked to it to advance timing. With the engine turning highway rpms of between 2000-3000 rpm and the throttle cracked ever so slightly, manifold vacuum shoots way up. In that situation, what many might deem a radical amount of timing is actually quite beneficial to engine performance. If you were to put a timing light on a car going down the highway with the vacuum advance properly connected, you would be extremely surprised to see somewhere around 40-50 degrees of ignition timing. For that reason, it is best paired with another form of ignition advance: you guessed it, the vacuum canister. It cannot take into account engine load, fuel mixture or any of the many other variables that dictate ideal ignition timing. Mechanical advance relies on one input, and one input alone: RPM. We make mechanical advance sound pretty great- and in theory, it is - but there is a major problem with it as the only source of ignition timing compensation. It can be adjusted by changing the stiffness of the springs on the distributor's weights, and the amount of mechanical advance can be increased or decreased based on stop-bushings in the mechanism. This mechanical (also known as a centrifugal) advance is an extremely reliable and simplistic approach to controlling engine timing at given engine speeds. As the distributor spins faster and faster with engine RPM, centrifugal forces fling out weights inside the distributor housing, moving a cam mechanism and advancing the timing. For this reason, a mechanical advance is built into most distributors. What's the difference between mechanical and centrifugal advance?Īs an engine revs up, we need to allow even more of a head start for the spark plug in order for complete combustion to occur. Most engines have between 5-20 degrees of ignition advance at idle. You know this process as ignition advance. So, in order to give the fuel mixture adequate time to burn, we start the fire early, before top dead center (TDC) occurs. That would make for an engine that was horribly inefficient and made terrible power. If the spark plug were fired at true top dead center (0 degrees in crank revolution), the piston could be well on its way to bottom dead center - maybe even past it and onto the exhaust stroke - before combustion of the air and fuel was completed. In fact, every aspect of the ignition process takes time the ignition signal to travel from points or a magnetic pickup, the spark energy to travel from the distributor's rotor, to the terminal, through the wire and finally to the plug. What actually happens is that the air and fuel mixture take time to burn. While that is a pretty easy visualization to conjure up, in the real world, that isn't quite how things work. In a theoretical world, air and fuel in a combustion chamber burn instantaneously as the spark plug ignites them, sending the piston downward in the bore and producing horsepower.
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