Let's talk some simple tips guys...
As it seems, there has been a need for another technical discussion on tuning theory. I think more and more questions arise why you run 12:1 afr vs. 13:1 or 14:1 or this certain ignition timing. Anyone can just find threads where such and such ran this ignition timing and this AFR. I think very few people know why you would run this certain AFR or Ignition timing.
Until now, i didnt want to get in depth of the reasons why, but to justify the reasons I recommend certain AFR's and Ignition timing, we will discuss it.
In order to really under stand this discussion, you will need to know a lot of mechanics of the engine. IE. the cycles the motor takes. i really dont want to take the time to go into detail for now, maybe later.
First lets go over a few things. we will talk about ignition first.
The name of the game to making power is making the burn duration when the peak combustion pressure occur around 16 degrees after top dead center. The reason you want this is because it creates the most force on the crankshaft at the optimum angle to produce the most power. Sometimes people refer to this as MBT (maximum brake torque) but i dont really use that term much.
So in order to do that, we need to know a few things. Our boosted application allows the cylinder w/ more dense charges.
Charge density affects the burn rate.
We need to ignite our charge at the right moment that will trigger our peak combustion pressure around 16 degrees after top dead center. We also know that because the air is more dense and more dense the charge is, the less time needed to complete the burn. This all correlates with gas burn rate, however also know that the faster the engine is turning, the shorter the time for the crank angle to reach that 16 degrees ATDC. so we would need to advance the ignition timing accordingly to rpms. Most so called tuners dont know this.
Obviously the afr and charge density are some big factors affecting the burn rate. as mentioned before the higher charge density will burn faster because of the function of gas pressures and gas temps. chamber size, spark plug location and other factors will also affect burn right, but we wont really be concerned with that for now. Slower burn time reduces pumping efficiency, thats why you want to use the lowest octane that will not detonate. The pure fact of higher octane does not make the fuel burn slower, its actually the chemical composition, many people do not know this either.
Lets talk a bit about AFR's. Many people just see people running XX:1 Air Fuel Ratio, and thats what they go by. You have to look deeper into Heat Transfer theory to really understand what is going on here, and why you would run certain AFR's.
About 25 % of the air/fuel mixture energy is converted to work, and the remaining 75% must be transferred from the engine to the environment. .
From what I have read, there are three basic paths for energy flow: coolant, exhaust, and of course work. They are approximately equal, each about 1/3 of the energy of the incoming fuel/air mixture (I think). If you wanted to get extremely technical, there are three heat transfer mechanisms used to derive formulas. I dont really want to get that in depth however. In short, you use a combination of ignition timing, and also importantly AFR's to control heat within the engine.
When you combine all of this theory, it tells you, the more load on the motor, the richer the engine must be. You run richer AFR's on boosted motors to help absorb the heat. Combustion dynamics tells us that when charge density goes up, more heat is generated. If you run too lean, you can guess what happens. There are many arguments on the correct AFR. By theory, you shouldnt ever run over 12.5:1 on high boosted applications. Typically I wouldnt even go over 12:1. Its pushing the envelope.
If you were to compare the power differences between 12.5:1 and even 11.8:1 the gains are very minimal. It provides a much much safer condition running richer and losing, say 5-10 whp on a 500 or 600 whp car. Its just not worth it. many people do not grasp that concept. Also, many people dont realize...especially with systems like hondata, when u can not control the Air Temp
Correction Factor. You go and tune the car to a risk taking 12.5:1 AFR on a high boosted application and lets say your intake temps that day on the dyno are 90 degrees. You take it to the track on a cooler night say 65. I can almost promise you, you're going to be over 13:1. Thats another reason why its important to not only tune on a dyno for high boost, but to put the car in real life conditions, things change a bit from the dyno to the street.
Ok, I got down the reason we want a Rich AFR under boost. But what about out of boost?
well im sure you've all heard of Stoichiometric conditions. This is described as 14.7:1 AFR. This is an important number for manufactures because this AFR allows the catalytic converter to maximize its effect of reducing harmful exhaust gas emissions.
This isnt the best in terms of Fuel efficiency. you can lean the motor out as much as possible when you are at lower load levels (cruising). I typically run 15.0:1 or so.
Octane Levels
The octane number of a gasoline is NOT a measure of it's hotness or coolness in the burning process, and it is NOT a measure of how 'powerful' it is. The octane number is simply a measure of how good the gasoline resistance of detonation. This is a huge misconception. Higher octane fuels are better at controlling the decomposition into auto-ignition compounds than lower octane fuels. Many people think they can run higher octane fuel at leaner temps. well they are simply wrong.