The "best" air/fuel ratio for a particular vehicle is a matter of great debate and I will do my best to avoid that debate in this article. Simply put, there are a number of factors that one must consider in determining the best ratio, including power, safety, and fuel economy. Fuel economy is the easiest to understand, as a lower air/fuel ratio means more fuel and obviously lower fuel economy. As far as safety is concerned, richer is considered safer (to a point) as the extra fuel helps things run cooler. The lower temperatures help reduce the chance of autoignition and can literally keep engine components from melting. The safest air/fuel ratios are continuously being debated, but it is widely accepted that 13:1 is a good ratio for normally aspirated engines and 12:1 is good for forced induction engines. Many choose to go even richer, even 11.5:1. Autoignition (or "detonation" or "knocking") is considered a critical concern with rotary engines, and many tuners choose to go even richer than that. One must also keep in mind that these "safe" ratios are considered safe because they have been tried with many thousands of vehicles over many years by dyno operators that use the same equipment that most people are likely to encounter. Therefore, a safety margin that takes into account the accuracy of that equipment is inherently factored in. If it were common for turbocharged cars to blow up at 12:1 as measured on commonly used equipment, then the "safe" air/fuel ratio would have been lowered.

As far as power is concerned, I'll say only this: Every vehicle is different. If one wants to find the best air/fuel ratio for generating power, one should put the vehicle on a dyno and test it. Many believe that a particular ratio will result in the most power under any circumstances, and that belief is just too narrow-minded. There are far too many factors involved to make such blanket statements.

As an engine speeds up, the spark plug should fire earlier (timing should be advanced) for the simple reason that there is less time for the combustion to happen as the piston speeds increase. The engine tuner's job is to make that spark happen at just the right time throughout the rpm range. For those that want to get technical, it turns out that the spark should occur at a point so that the flame front travels through the mixture and burns it completely, and the heated, expanding combustion products reach a maximum pressure when the piston is about 15-20 degrees after TDC. With a dynamometer at his disposal, that job would be very easy if it weren't for all of the variables mentioned earlier. Fortunately, the effect those variables have is very well understood and they can be accounted for if the engine has the right management system.