1. I want more power and the engine is detonating! Do I need to run richer to eliminate the knock?
What is the optimum air-fuel ratio to run for best power?
I've heard racers say that we need to run a bit lean for best power - is this true?
What is the optimum air-fuel ratio to run for best power?
I've heard racers say that we need to run a bit lean for best power - is this true?
Maybe. It's somewhat true that running a tad bit richer will help quench detonation (knock), but only to a certain extent. The stochiometric AF ratio for gasoline is 14.7:1. Stochiometric means that at this point, the fuel is completely burned with no unburned hydrocarbons left, giving you best emissions. This is the AF ratio the PCM is shooting for under idle and cruise.
However, best power is achieved for gasoline at around 12.5:1, leaving excess unburned hydrocarbons, but no unused oxygen molecules! (This is why 12.5:1 is typically the best power point.) So under ideal circumstances, the best power AF ratio to shoot for is around 12:1 to 12.5:1. However, running a tad bit richer will throw excess fuel (gasoline) into the combustion chamber, and the extra unburnt fuel actually acts as a heat sink, absorbing heat and cooling the combustion temperature, hence reducing the tendency for detonation to develop. However, this only helps to a certain extent. If you're knocking severely, you can throw all the raw fuel you want into the mix and it won't quench the detonation.
When you run richer than 12:1 on gasoline, you're losing power. Between 11:1 and 12:1, the power curve is fairly flat and you're not losing very much power, much richer than about 10.5:1 and you're starting to lose power drastically, and probably not helping your detonation problem either.
So the answer is to achieve around 11:1 AF ratio for your best compromise, not going richer than about 10.5:1 at most. You won't be inherently down very much on power due to the AF ratio, but you're probably delaying the onset of detonation enough so that your spark advance isn't being severely retarded, thus helping your power - end result, you get the optimum power / knock control balance.
When racers speak of running on the "rich" side they're really referring to running around 10.5:1 area, whereas when they speak of running on the "lean" side, they're really referring to the 12.5:1 area, whether they know it or not. They're not actually running "lean" (which would be something greater than 14.7:1), they're just running on the "lean" side of best power.
