Geez, this thread is going on long for such a simple question.
A couple more things:
The high voltage for the spark travels at the speed of light, which is 186,000 miles per second, so it coulc travel from LA to Phoenix and come back in time to have no effect on timing. The reason why you need to advance the spark with RPM is because the rate of combustion increases exponentially with time, so you have to initiate combustion earlier in order to maximize the work of the expanding gas on the piston down stroke as RPM increases.
Vacuum advance was the an early solution to getting the optimum timing under different operating conditions. Centrifugal advance is fine for just normal cruising, but the optimal timing also depends on the engine load. Under load, like going up hill, you need to back off the timing compared to the same RPM under no load, to avoid knock or preignition. Vacuum advance accomplishes this.
This level of control was sacrificed on the first generation smog control when reducing NOx was a more important goal than optimum efficiency, hence the vacuum retard distributor under electronic control.
Second generation emissions controlled both advance and retard and now we don't even have distributors and the spark is controlled bhy a computer and sensors.
A couple more things:
The high voltage for the spark travels at the speed of light, which is 186,000 miles per second, so it coulc travel from LA to Phoenix and come back in time to have no effect on timing. The reason why you need to advance the spark with RPM is because the rate of combustion increases exponentially with time, so you have to initiate combustion earlier in order to maximize the work of the expanding gas on the piston down stroke as RPM increases.
Vacuum advance was the an early solution to getting the optimum timing under different operating conditions. Centrifugal advance is fine for just normal cruising, but the optimal timing also depends on the engine load. Under load, like going up hill, you need to back off the timing compared to the same RPM under no load, to avoid knock or preignition. Vacuum advance accomplishes this.
This level of control was sacrificed on the first generation smog control when reducing NOx was a more important goal than optimum efficiency, hence the vacuum retard distributor under electronic control.
Second generation emissions controlled both advance and retard and now we don't even have distributors and the spark is controlled bhy a computer and sensors.
