|
(Continued from prior page) elevations low octane gas is 85 or 86-octane. Travel to the coast or below four or five thousand feet and low octane gas is rated 87-octane. The lower octane, used at high elevations burns a little faster and compensates for the lower pressures and lower heat and slower burning that takes place at higher altitudes. Some vehicles have altitude compensation which automatically adjusts the fuel mixture and spark timing. More on this later. 4. Ambient Air Temperature - Higher ambient air temperature causes the air fuel mixture to be hotter even before it gets compressed. The result is higher heat leading to faster burning. This requires that the burning be started closer to TDC - retarded - because it takes less time to reach max pressure. However, to complicate matters, hotter air is less dense, consequently it burns slower. So we have two factors here that affect what the optimal timing should be. 5. The weight of your accelerator foot - As you press down on the go-pedal the throttle opens and more air and gas are admitted into the cylinder and combustion chamber. This increases pressure and consequently increases heat and faster burning. So the timing must retard, fire the plug closer to TDC. Any factor that causes the gas to burn faster must be compensated for by retarding the timing - delaying the spark plug firing. That is, starting the burning later in the cycle - closer to TDC. The reason is that MAXIMUM burning is now occurring sooner because it's faster. If the timing is too advanced, burning too early, it would try to force the crankshaft and piston back the opposite way they were going. As you can imagine, this puts tremendous pressure on the piston head, rods and crankshaft. The pinging or knocking that you hear is this pressure taking place in the cylinder, trying to turn the crankshaft backwards, and it can cause serious damage as well as a loss of power. Of course the reverse is true also. Any factor that causes the gas to burn slower must me compensated by for by starting the burning earlier to give it time to reach max pressure. This means advanced timing. If the timing is too far retarded, that is taking place too late in the cycle, max pressure takes place as the piston is part way down the cylinder on the power stroke causing a loss of power. In older cars the ignition timing was set at idle and there were only one or two automatic timing compensations for varying needs. Timing was changed relative to RPM and how hard you were trying to accelerate. Higher RPM required advanced timing and heavier acceleration required retarded timing because of higher cylinder pressures and faster burning. Modern cars use fuel injection and on board computers to monitor the factors that affect ignition timing and cause it to change. There are numerous sensors on the car measuring air temperature, throttle position, RPM, engine temperature, altitude and even if the engine is pinging. If the computer senses that there is a ping caused by the timing being advanced too much, or because the gas you are using is of too low an octane, it causes the timing to automatically retard. Older Miatas did not have this knock or ping sensor. So if you set the timing too far advanced or use too low octane fuel your ear will tell you, if you were sensitive to these sounds. Newer Miatas have more sophisticated computers and if you choose too low an octane fuel or set the timing too far advanced the engine will automatically compensate by retarding the timing and you'll never know. This protects your engine from the possible damage of pinging but it also robs you of some power. Most true blue (or red) Miata aficionados, who are technically inclined, prefer the older cars, in this respect, because their ears tell them what is going on and they can set the timing the way they like. This is probably more than you ever wanted to know about ignition timing. But, if you got this far, I hope you have a better understanding of what is happening and what some of the factors are that influence the need for timing variations. |
