This discussion will be mostly aimed at optimizing the part-throttle timing curve on a street engine. We’re not talking about a drag race engine – we must emphasize this. So let’s take a look at a 10.5:1 compression small block Chevy street engine. Total max timing on it is 28 degrees and it has 10.5:1 compression. Look up the timing on a 2002 Z06 Corvette. If you have a high performance engine you don’t want to ever see more then 38 degrees.
#ADVANCE TIMING SBC CRACKED#
I have a cracked cylinder in my garage to show you what happens when you allow the advance to reach 48 degrees or more on a high compression engine. On pump gas and 10.5:1 compression you can’t have a max total timing more than 28 degrees. If you’re running 10.5:1 compression you will detonate with vacuum advance hooked up. Here is the link to the original question and answer dated March 27, 2015. But since this gentleman brought it up, we’ll take that ball and run with it! Nowhere in my answer did we talk about a 10.5:1 compression engine. The original question and my answer related to a mild small block Chevy with low static compression. So I thought this required us to go a little deeper to shed more light on vacuum advance. This may have resulted from my answer not being complete enough. The following comment reveals some confusion about how vacuum advance works as opposed to mechanical advance. Some comments are right on target – others not so much. All this activity indicates there is still quite a bit of confusion about ignition timing and how it works. The original post was roughly a year ago and has resulted in over 20 comments on the answer. One reader posted a comment (below) in response to my answer about ignition timing curves. 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.Our tech column seems to be gathering quite a following of guys who want to know more about their cars and engines. 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. 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. Let's use a single cylinder in an engine as an illustration to demonstrate how all four strokes work. All automotive engines today have four strokes.
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