![]() As a result, the timing moves to 32 degrees before top dead center which allows the fuel to be ignited early enough so that it is burning while the piston is rising and completely burnt when the piston reaches top dead center. In order to have the same result, the fuel must be ignited much sooner in the compression stroke. This would equate to 10 degrees of crankshaft rotation before reaching top dead center to give the fuel sufficient time for a complete burn.Īs the engine rpm increases to 3,000 rpm, the fuel, - still requiring the same time to burn - would never have sufficient time to burn if ignited at the same 10-degree timing. Using an engine idling at 900 rpm further illustration, the piston is moving upward at a speed that the fuel, given the time it takes to burn completely, is ignited 1/16-inch from the top. This would mean that the fuel is still igniting while the piston is descending in the power stroke and would result in a massive loss of power. It would not be efficient having a small percentage of the fuel consumed before the piston hits top dead center. As the piston rises, the ignition spark plug ignites the fuel and the process begins again.Ĭonsider that the fuel must be burnt as completely as possible before the piston reaches the top of the compression stroke in order to force the piston downward in the power stroke. This is the fourth or compression stroke. The crankshaft turns again and the piston begins to move upward, compressing the raw fuel and air in the process. Just prior to reaching the bottom of this stroke the intake valve closes. ![]() As the piston reaches top dead center and continues downward once again it creates a vacuum sucking more fuel into the cylinder. Just before the piston rises completely in the exhaust stroke the intake valve opens, using the vacuum produced by the rapidly exiting exhaust gases to help draw in more fuel from the intake valve. Can two degrees of advance on the timing increase the engine temp alot. The upward moving piston forces the burnt gases out of the cylinder. As the crankshaft turns, the piston begins to go back up and the camshaft opens the exhaust valve. The burning fuel expanding forces the piston downward. The piston begins all the way up at top dead center. The crankshaft turns two revolutions, which moves the pistons up and down to one turn of the camshaft that opens and closes the valves. i wanted to have the timing like 38 degrees BEFORE TDC total, but when i put the initial timing to 3 degrees, the vacuum advance. the car starts fine but as you can imagine it is VERY slow. i put a timing light on it and the car was 4 degrees AFTER tdc TOTAL timing. Let's use a single cylinder in an engine as an illustration to demonstrate how all four strokes work. HEI distributor, timing is all in at 2500 rpm. Hope that helps eliminate and not add to your confusion.All automotive engines today have four strokes. Nothing will live very long with that much advance. For example, if you start at 32* and the mechanical advance moves it another 15*, you now have 47* of total advance. If you were to do that you would not be far from a BBQ that you didn't want to have. It is very important that you understand this enough to NOT lock your distibutor at 32* without disabling the mechanical advance. Search his posts, and you will find a bunch of information about that scenario. Most folks, including myself do not use the later method. When you do this, you need to have a means to remove some timing (ignition control box, etc) so you do not have a problem getting it to start and/or idle especially when it is warm. Then you would set the timing at max advance (32*) at leave it. In this case, the mechanical advance is 'locked' or disabled so that it is not operational. Talk of 'locking' the distributor is another critter all together. Two completely different and independent issues. Springs and weights have nothing to do with vacume advance, they have to do with controling the mechanical advance. However quick the total comes in does not change the fact that you want to limit the timing to 32* total. By using the lightest springs in whatever kit you get, you enable the total advance to be achieved sooner (ie. ![]() The amount it advances and how quick it gets to total advance is controled by the springs and weights inside the distributor. That leaves mechanical (or centrifical) advance.
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