Another important aspect of tuning is spark advance or timing. Pistons are moving very quickly inside the cylinders and though it might make sense that the explosion is happening instantly at the moment of spark, the reality is quite different. Not only is the piston changing position and moving very quickly to the top of its stroke, and then very quickly away from it, but the flame front also takes time to propagate and does so at different speeds depending on AFR mixture. This is why we call it spark advance, we trigger the spark plugs before TDC (top dead center where the piston has reached the very top of its stroke and compressed the air/fuel mixture as much as it possibly can) in order to hopefully catch it 15 degrees past TDC. Much like AFR this is also different for every car and is very import to get exactly right. Too much spark advance and the combustion could happen while the piston is still moving up which can be very damaging to a motor. Too little spark advance and almost all efficiency and power are lost. Also much like AFR the best way to get the most accurate spark advance is by using a dynometer to measure power, increment spark and monitor the change. The best spark advance is the spark advance that provides the most power for that specific rpm.